Critical Reviews in Food Science and Nutrition ISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/bfsn20 Comparative bioavailability of vitamins in human foods sourced from animals and plants Sylvia M. S. Chungchunlam & Paul J. Moughan To cite this article: Sylvia M. S. Chungchunlam & Paul J. Moughan (31 Jul 2023): Comparative bioavailability of vitamins in human foods sourced from animals and plants, Critical Reviews in Food Science and Nutrition, DOI: 10.1080/10408398.2023.2241541 To link to this article: https://doi.org/10.1080/10408398.2023.2241541 © 2023 The Author(s). Published with license by Taylor & Francis Group, LLC. Published online: 31 Jul 2023. 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S. Chungchunlam and Paul J. Moughan riddet institute, Massey university, Palmerston north, new Zealand ABSTRACT Vitamins are essential components of enzyme systems involved in normal growth and function. The quantitative estimation of the proportion of dietary vitamins, that is in a form available for utilization by the human body, is limited and fragmentary. This review provides the current state of knowledge on the bioavailability of thirteen vitamins and choline, to evaluate whether there are differences in vitamin bioavailability when human foods are sourced from animals or plants. The bioavailability of naturally occurring choline, vitamin D, vitamin E, and vitamin K in food awaits further studies. Animal-sourced foods are the almost exclusive natural sources of dietary vitamin B-12 (65% bioavailable) and preformed vitamin A retinol (74% bioavailable), and contain highly bioavailable biotin (89%), folate (67%), niacin (67%), pantothenic acid (80%), riboflavin (61%), thiamin (82%), and vitamin B-6 (83%). Plant-based foods are the main natural sources of vitamin C (76% bioavailable), provitamin A carotenoid β-carotene (15.6% bioavailable), riboflavin (65% bioavailable), thiamin (81% bioavailable), and vitamin K (16.5% bioavailable). The overview of studies showed that in general, vitamins in foods originating from animals are more bioavailable than vitamins in foods sourced from plants. Introduction Recent international reports suggest that some forms of ani- mal food production may be environmentally unsustainable, and that the current consumption of animal-sourced foods should be lowered in favor of plant-based foods (Adesogan et  al. 2020; Beal et  al. 2023; FAO et  al. 2020; Pimentel and Pimentel 2003; Springmann et  al. 2018; WHO and FAO 2019; Willett et  al. 2019). However, the nutritional quality of animal- and plant-sourced foods must be considered in the formulation of affordable, sustainable dietary patterns (Ambikapathi et  al. 2022; FAO et  al. 2020; Herforth et  al. 2020; Hirvonen et  al. 2020; Springmann et  al. 2018). Our previously reported modeling studies, using Linear Programming, showed that dietary patterns that met the rec- ommended minimum intake requirements for essential nutrients of an average adult in the United States of America or New Zealand, formulated at the lowest dietary cost, relied on foods sourced from both plants and animals (Chungchunlam et  al. 2020, Chungchunlam, Garrick, and Moughan 2021). Modeled diets that included animal-sourced foods were relatively 30 to 45% cheaper than modeled diets that consisted exclusively of plant-based foods, and the prices of animal-sourced foods had to be increased by two to eleven times to be excluded from the least-cost dietary patterns. It was also highlighted that the first-limiting nutri- ents for adults in mixed modeled diets were not the macronutrients but rather mostly the vitamins and minerals, particularly vitamin A, B group vitamins, calcium, iron, potassium, and zinc (Chungchunlam et  al. 2020, Chungchunlam, Garrick, and Moughan 2021). While the amount and form of essential nutrients may differ among their main dietary sources, their inherent bioavailability is often overlooked. These essential nutrients generally occur in animal-derived foods in higher concentrations and appar- ently with greater bioavailability, compared to plant-based foods (Adesogan et  al. 2020; Murphy and Allen 2003). However, there is a paucity of published data on the com- parison of the overall availability of vitamins and minerals between animal- and plant-sourced foods. The bioavailability of minerals and trace elements will be the subject of a future review from our research group. This review focuses on the bioavailability of vitamins. Vitamins are essential components of enzyme systems that assist chemical reactions within the body, and are important for cell membrane integrity, nerve and muscle function, bone formation, and normal growth and overall good functioning of the human body (WHO and FAO 2004). The chemically active forms of vitamins are some- times referred to as vitamers. There are two categories of vitamins, based on their solubility and the extraction method used to isolate them. The fat-soluble vitamins include vita- mins A, D, E, and K. For the determination of the content © 2023 the author(s). Published with license by taylor & Francis Group, llC. CONTACT sylvia M. s. Chungchunlam sylvia.lawrence.17@gmail.com this article has been corrected with minor changes. these changes do not impact the academic content of the article. https://doi.org/10.1080/10408398.2023.2241541 this is an open access article distributed under the terms of the Creative Commons attribution-nonCommercial-noderivatives license (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. the terms on which this article has been published allow the posting of the accepted Manuscript in a repository by the author(s) or with their consent. KEYWORDS bioavailability; vitamin; digestibility; absorption; availability; utilization http://orcid.org/0000-0002-9838-7359 http://orcid.org/0000-0001-9644-9133 mailto:sylvia.lawrence.17@gmail.com https://doi.org/10.1080/10408398.2023.2241541 http://creativecommons.org/licenses/by-nc-nd/4.0/ http://www.tandfonline.com 2 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN of fat-soluble vitamins in food, an organic solvent is used to remove the vitamins dissolved in food fat. The extraction process for water-soluble vitamins in food involves the use of an aqueous solution. The water-soluble vitamins comprise vitamin C, and the B group vitamins, namely biotin (vita- min B-7), folate (vitamin B-9), niacin (vitamin B-3), panto- thenic acid (vitamin B-5), riboflavin (vitamin B-2), thiamin (vitamin B-1), vitamin B-6, and vitamin B-12. The water-soluble compound choline is recognized strictly as not a vitamin, but is related to the B vitamins, and will be con- sidered in this review. Animal-sourced foods are the almost exclusive natural sources of dietary vitamin B-12 (Scott 1997; Watanabe 2007) and choline (Zeisel et  al. 2003; Zeisel and da Costa 2009), and plant-based foods are the main sources of natural dietary vitamin C (Bates 1997a; Olson and Hodges 1987). While most vitamins are widely distrib- uted in foods (Ball 1998, 2006; WHO and FAO 2004), vita- min deficiencies are prevalent in human populations and may even occur with apparently sufficient dietary intakes (Bailey, West, and Black 2015; Beal et  al. 2017; Beal and Ortenzi 2022; Passarelli et  al. 2022; WHO and FAO 2004). This may in part be attributable to inadequate absorption and inadequate utilization of vitamins present in the human diet, which in turn may be dependent on food source (Ball 1998, 2006; Beal and Ortenzi 2022; Beal et  al. 2023; Melse-Boonstra 2020; Passarelli et  al. 2022; WHO and FAO 2004). Bioavailability may be defined as the proportion of an ingested nutrient that is released during digestion, absorbed via the gastrointestinal tract (GIT), transported and distrib- uted to target cells and tissues, in a form that is available for utilization in metabolic functions or for storage (Baker 1995; Ball 1998, 2006; Bates and Heseker 1994; Gibson 2007; Godber 1990; Heaney 2001; WHO and FAO 2004). However, the unifying term “bioavailability” has been employed gener- ically and a clear consensus definition of bioavailability is needed. Various terms, such as absorbability, digestibility, availability, utilizability, and utilization, have been used when discussing the metabolic processes involved in transforming a specific nutrient present in food to a utilizable form (Table 1). Humans consume a combination of different foods, and digestion is the process that breaks down the food materials, releasing nutrients that can be absorbed by the human body (Singh and Gallier 2014). Mechanical action and saliva in the mouth, acid and enzymes released in the stomach, and bile and enzymes in the small intestine, all function together to break down the food macromolecules and release the nutrients (Singh and Gallier 2014). Absorption is the process by which the released nutrients pass from the GIT lumen, either pas- sively or actively, through the intestinal epithelial cells, the enterocytes, into the systemic circulation or lymphatic sys- tem (Baker 1995; Baker and Stein 2013). Digestibility refers here to the disappearance of a nutrient during its passage through the GIT; this nutrient disappearance is sometimes assumed to equate with absorption (Baker 1995; Baker and Stein 2013; Bates and Heseker 1994). The bioavailability of dietary protein and amino acids is often expressed as digest- ibility and provides an indication of the quality of protein and amino acid supplied by a particular food (FAO 2013; Moughan 2021; Moughan and Wolfe 2019; Sarwar 1987). Availability refers to the proportion of absorbed nutrient that is physiologically available for anabolic utilization, mostly transported in the systemic circulation, and supplied to tar- get body tissues (Baker 1995; Baker and Stein 2013; Gibson 2007; Godber 1990). Utilization is defined as the proportion of absorbed and available nutrient that has been used by the body for metabolic or physiological functions (Baker 1995; Baker and Stein 2013; Heaney 2001). Some of these terms are used interchangeably, but each has a precise meaning and this needs to be taken into account when comparing the bio- logical availability of dietary nutrients. Several in vitro and in vivo approaches have been used to study digestibility and bioavailability. In the context of in vitro digestion approaches, the term bioaccessibility refers to the fraction of a nutrient found in food that has been con- verted by digestion into potentially accessible soluble forms for absorption, while the term bioactivity applies to the events that occur during transport, assimilation by the target tissue, interaction with other biomolecules, metabolism or biotrans- formation, and the physiological responses (Fernández-García, Carvajal-Lérida, and Pérez-Gálvez 2009; Marze 2017). Most in vitro methods are based on a simulated digestive process followed by measurement of the released test nutrient to assess bioaccessibility of nutrients. However, the bioactivity of nutrients is not adequately taken into account in in vitro studies and the responses are not as robust as those based on measures made in humans or animal models. In this review, “bioavailability” values were collected from in vivo studies conducted in humans or the pig as an animal model for the adult human (Baker 1995; Ball 1998, 2006; Bates and Heseker 1994; Godber 1990; WHO and FAO 2004). The gastrointes- tinal tract of humans and pigs has been shown to be similar (Guilloteau et  al. 2010; Moughan et  al. 1994). It should be recognized that bioavailability estimates are representative of the response of the human or animal model to a specific food or diet and therefore, may be dependent on the nutri- tional and health status of the subject (Gibson 2007; Godber 1990; WHO and FAO 2004). In addition to host-related factors, the quantification of bioavailability is influenced by dietary factors, such as the amount and form of the nutri- ents that are consumed in the diet, the dietary matrix, food processing and treatment, and the presence of other food components that may enhance or inhibit nutrient digestion, absorption, and utilization (Dave et  al. 2023; Gibson 2007; Gibson, Perlas, and Hotz 2006; Godber 1990; Melse-Boonstra 2020; Murphy and Allen 2003; Platel and Srinivasan 2016). Table 1. terms used to define the processes pertaining to nutrient bioavailability. definition digestion the process during which food is broken down and nutrients released into the lumen of the gastrointestinal tract (Git) absorption the proportion of nutrient released in the Git that is absorbed from the Git digestibility the amount of nutrient disappearing from the Git during the transit of food availability the proportion of nutrient that is absorbed in a form that can be used for anabolic processes utilization the proportion of absorbed nutrient that is used by the body for metabolic or physiological functions Git, gastrointestinal tract CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 3 The most common approach to measuring vitamin bio- availability is the balance study, whereby the difference between ingestion of a vitamin and its excretion is evalu- ated (Bates and Heseker 1994; Godber 1990; Heaney 2001; WHO and FAO 2004). Ileal digestibility is assessed by the difference between the ingested amount of vitamin and the amount of vitamin found in ileal effluent, and may be used as a robust proxy for apparent absorption (Baker 1995; Baker and Stein 2013; Bates and Heseker 1994). Fecal digestibility is assessed as the difference between vitamin intake and its fecal excretion, where the assumption is that unabsorbed vitamins are released in the feces (Baker 1995; Baker and Stein 2013; Bates and Heseker 1994). In some cases, this assumption is not valid, as large intestinal bacte- ria can both degrade and synthesize some vitamins (Baker 1995; Baker and Stein 2013; Bates and Heseker 1994; Said 2013; WHO and FAO 2004; Yoshii et  al. 2019). Another potentially more accurate method involves the use of stable isotopes or radioactive isotopes to label foods either intrin- sically or extrinsically, and monitoring the route and fate of the isotopically labeled dietary vitamin (Godber 1990; Heaney 2001). Other methods include the measurement of responses in blood, tissues, organs, body pools, or the urine (Godber 1990; Heaney 2001). Bioavailability values may also be obtained in relation to that of the test vitamin given in a purified form or in a standard ingredient. When the absorption efficiencies of the reference vitamin are known, the absolute apparent estimates of vitamin bioavailability may be calculated from the relative bioavailability values (Baker and Stein 2013). “True” bioavailability values are obtained either by correcting for endogenous losses with the balance method, or the comparison of the physiological response between oral doses of a dietary vitamin and the vitamin intravenously administered by injection (Baker 1995; Baker and Stein 2013; Bates and Heseker 1994; Dainty et  al. 2007; WHO and FAO 2004). Nonetheless, the bio- availability data of most vitamins in human foods remain limited and fragmentary (Baker 1995; Ball 1998, 2006; Bates and Heseker 1994; Dave et  al. 2023; Melse-Boonstra 2020; WHO and FAO 2004). There is an even wider knowledge gap when comparing vitamin bioavailability amongst human foods sourced from animals or plants. The objective of this conspectus was to provide a review of studies summarizing the multiple methodologies used to determine vitamin bio- availability and estimating the bioavailability of the fat-soluble vitamins A, D, E, and K, and the water-soluble biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, vitamin B-6, vitamin B-12, vitamin C, and choline. Comparable relevant bioavailability data are brought together to allow a comparison of vitamin bioavailability in animal- and plant-sourced foods. Bioavailability of vitamins in foods sourced from animals and plants This section presents a general introduction of each specific vitamin, with a common description of biochemical forms, physiological functions, dietary sources, absorption and uti- lization, and the experimental approaches used to determine bioavailability of each vitamin in food. An important ques- tion remains as to what extent the bioavailability of vitamins differs amongst human foods sourced from animals or plants. It is difficult to provide such comparative assessment as bioavailability estimates have often been observed using different definitions of bioavailability, different methodolo- gies to measure bioavailability and different study popula- tions. Special care has been applied in this section, where these considerations are taken into account, to only use rel- evant and comparable data for comparisons of bioavailability of each vitamin in foods originating from animals or plants (Table 2 and Figure 1). Vitamin A Vitamin A absorption and utilization Vitamin A (retinol) is a fat-soluble vitamin present in food as preformed vitamin A, primarily as retinol derivatives, and provitamin A carotenoids (carotenes) that are the precursors for retinol (Bates and Heseker 1994; IOM (Institute of Medicine, US) 2000, 2001; WHO and FAO 2004). In addi- tion to its central role as a constituent of a visual pigment to maintain eye vision, vitamin A has many other roles in the human body, such as in growth and development, cell surface integrity, immune function, and reproduction (Biesalski 1997; de Pee and West 1996; Wolf 2002). Preformed vitamin A is found mostly in animal-sourced products, including meat, liver, milk, eggs, and fish (Biesalski 1997). Provitamin A carotenoids, mainly β-carotene and α-carotene, are commonly found in colored (orange, yellow, red, green, or purple) vegetables and fruits (Castenmiller and West 1998; de Pee and West 1996; Parker 1997; Parker et  al. 1999; van Het Hof et  al. 2000). The carotenoids exist mostly as the all-trans isomers, but can also occur as cis isomers (Castenmiller and West 1998; de Pee and West 1996; Parker 1997; Parker et  al. 1999; van Het Hof et  al. 2000). Some carotenoids, such as lycopene, do not exhibit vitamin A bio- logical activity, but act as anti-oxidants (Castenmiller and West 1998; de Pee and West 1996; Parker 1997; Parker et  al. 1999). Fatty acyl esters of retinol (retinyl palmitate, retinyl acetate), and β-carotene are often used as fortification agents of vitamin A to enrich milk, margarine, oils, condiments, and cereal products (Biesalski 1997; Castenmiller and West 1998; de Pee and West 1996; Parker 1997; Parker et  al. 1999). On a cautionary note, vitamin A can be toxic when ingested in large amounts daily over a long period of time, with daily intake levels of more than 30 mg by pregnant women thought to have detrimental effects on the fetus (WHO and FAO 2004). Ingested vitamin A is released from food by the action of gastric pepsin and intestinal proteolytic enzymes, as pre- formed vitamin A in the form of retinyl esters, and as pro- vitamin A carotenoids (Biesalski 1997; Blomhoff et  al. 1991; Goodman et  al. 1966; Reboul 2013). In the duodenum, bile salts and pancreatic enzymes hydrolyze retinyl esters and free carotenoids, so that they may be solubilized into micelles and absorbed in the intestinal enterocyte. The presence of dietary fat is essential in forming these micelles and enhances the absorption of retinol and carotenoids (Blomhoff et  al. 4 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN Ta bl e 2. s um m ar y of r es ul ts o f st ud ie s in h um an s an d pi gs t ha t es tim at ed t he b io av ai la bi lit y of v ita m in s in a ni m al - an d pl an t- ba se d fo od s. n ut rie nt Fo od so ur ce Fo od it em in ta ke st ud y po pu la tio n Pr oc es s of b io av ai la bi lit y Bi oa va ila bi lit y re fe re nc e Vi ta m in A ( re tin ol ) an im al Be ef li ve r, co ok ed 50 m g re 10 f em al e hu m an s, m ea n ag e 26 y av ai la bi lit y 84 % Bu ss e t  al . 1 99 4 Be ef li ve r, co ok ed 15 0 m g re 10 f em al e hu m an s, m ea n ag e 26 y av ai la bi lit y 75 % Bu ss e t  al . 1 99 4 li ve r pa st e 3. 0 m g re 35 f em al e hu m an s, m ea n ag e 27 y av ai la bi lit y 43 % va n vl ie t et  a l. 20 01 li ve r pa st e 7. 5 m g re 35 f em al e hu m an s, m ea n ag e 27 y av ai la bi lit y 62 % va n vl ie t et  a l. 20 01 li ve r pa st e 15 .0 m g re 35 f em al e hu m an s, m ea n ag e 27 y av ai la bi lit y 98 % va n vl ie t et  a l. 20 01 Vi ta m in A (β -c ar ot en e) Pl an t Ca rro ts , r aw , c ho pp ed 2. 50 m g re 8 hu m an s, ag e ra ng e 38 –7 5 y av ai la bi lit y 41 % li vn y et  a l. 20 03 Ca rro ts , c oo ke d, p ur ee d 2. 50 m g re 8 hu m an s, ag e ra ng e 38 –7 5 y av ai la bi lit y 65 % li vn y et  a l. 20 03 Ca rro ts , c oo ke d, sh re dd ed 2. 00 m g re 10 m al e hu m an s, m ea n ag e 25 y av ai la bi lit y 13 .2 % Hu an g et  a l. 20 00 Ca rro ts , c oo ke d fro m fro ze n 4. 83 m g re 30 m al e hu m an s, ag e ra ng e 20 –4 5 y av ai la bi lit y 6. 9 % Br ow n et  a l. 19 89 Ca rro ts , c oo ke d fro m fro ze n 4. 83 m g re 30 m al e hu m an s, ag e ra ng e 20 –4 5 y av ai la bi lit y 7. 3 % M ic oz zi e t  al . 1 99 2 sw ee t po ta to es , c oo ke d, m as he d 2. 00 m g re 10 m al e hu m an s, m ea n ag e 25 y av ai la bi lit y 14 .8 % Hu an g et  a l. 20 00 sp in ac h, c oo ke d, w ho le le af 1. 67 m g re 2 m al e an d 5 fe m al e hu m an s, m ea n ag e 51 y av ai la bi lit y 26 % Fa ul ks e t  al . 2 00 4 sp in ac h, c oo ke d, fi ne ly ch op pe d 1. 67 m g re 2 m al e an d 5 fe m al e hu m an s, m ea n ag e 51 y av ai la bi lit y 23 % Fa ul ks e t  al . 2 00 4 sp in ac h, c oo ke d, w ho le le af 3. 97 m g re 31 m al e an d 38 f em al e hu m an s, m ea n ag e 42 y av ai la bi lit y 7. 3 % va n He t Ho f et  a l. 19 99 b sp in ac h, c oo ke d, m in ce d 4. 10 m g re 31 m al e an d 38 f em al e hu m an s, m ea n ag e 42 y av ai la bi lit y 7. 4 % va n He t Ho f et  a l. 19 99 b sp in ac h, c oo ke d, w ho le le af 1. 73 m g re 5 m al e an d 7 fe m al e hu m an s, m ea n ag e 20 y av ai la bi lit y 2. 0 % Ca st en m ill er e t  al . 1 99 9 sp in ac h, c oo ke d, m in ce d 1. 47 m g re 5 m al e an d 7 fe m al e hu m an s, m ea n ag e 21 y av ai la bi lit y 2. 6 % Ca st en m ill er e t  al . 1 99 9 sp in ac h, c oo ke d, liq ue fie d 1. 50 m g re 5 m al e an d 7 fe m al e hu m an s, m ea n ag e 21 y av ai la bi lit y 3. 8 % Ca st en m ill er e t  al . 1 99 9 sp in ac h, c oo ke d, liq ue fie d, w ith a dd ed fib er 1. 47 m g re 5 m al e an d 7 fe m al e hu m an s, m ea n ag e 21 y av ai la bi lit y 3. 7 % Ca st en m ill er e t  al . 1 99 9 w at er s pi na ch , c oo ke d, w ho le le af 2. 00 m g re 10 m al e hu m an s, m ea n ag e 25 y av ai la bi lit y 10 .4 % Hu an g et  a l. 20 00 Vi ta m in K (p hy llo qu in on e) Pl an t Ka le , c oo ke d, w ith a dd ed fa t 11 9 µg p er 2 00 0 kc al ( 8. 4 M J) 4 m al e an d 3 fe m al e hu m an s, m ea n ag e 46 y av ai la bi lit y 4. 7 % n ov ot ny e t  al . 2 01 0 sp in ac h, r aw 49 5 µg 11 h um an s, ag e ra ng e 22 –3 0 y av ai la bi lit y 13 .9 % Ga rb er e t  al . 1 99 9 sp in ac h, c oo ke d 10 00 µ g 3 m al e an d 2 fe m al e hu m an s, ag e ra ng e 25 –4 5 y av ai la bi lit y 3. 3 % Gi jsb er s, Jie , a nd v er m ee r 19 96 sp in ac h, c oo ke d w ith ad de d fa t 10 00 µ g 3 m al e an d 2 fe m al e hu m an s, ag e ra ng e 25 –4 5 y av ai la bi lit y 10 .6 % Gi jsb er s, Jie , a nd v er m ee r 19 96 Br oc co li, c oo ke d 37 7 µg 36 h um an s, m ea n ag e 51 y av ai la bi lit y 50 % Bo ot h, l ic ht en st ei n, a nd d al la l 2 00 2 Bi ot in an im al M ea t m ea l 50 µ g 6 m al e ile os to m iz ed p ig s ile al d ig es tib ili ty 82 % Ko pi ns ki , l ei bo ho lz, a nd B ry de n 19 89 b M ilk c as ei n 22 µ g 6 m al e ile os to m iz ed p ig s ile al d ig es tib ili ty 95 % Ko pi ns ki , l ei bo ho lz, a nd B ry de n 19 89 b Bi ot in Pl an t so ya be an m ea l 11 0 µg p er k g dr y m at te r 6 m al e ile os to m iz ed p ig s tr ue ( co rre ct ed ) ile al d ig es tib ili ty 55 % sa ue r, M os en th in , a nd o zi m ek 1 98 8 so ya be an m ea l 13 0 µg 6 m al e ile os to m iz ed p ig s ile al d ig es tib ili ty ( co rre ct ed ) 25 % Ko pi ns ki , l ei bo ho lz, a nd B ry de n 19 89 b w he at 12 3 µg p er k g dr y m at te r 6 m al e ile os to m iz ed p ig s tr ue ( co rre ct ed ) ile al d ig es tib ili ty 22 % sa ue r, M os en th in , a nd o zi m ek 1 98 8 w he at ( va r. Ba nk s) 14 4 µg 6 m al e ile os to m iz ed p ig s ile al d ig es tib ili ty ( co rre ct ed ) 18 % Ko pi ns ki , l ei bo ho lz, a nd B ry de n 19 89 b Ba rle y 12 1 µg p er k g dr y m at te r 6 m al e ile os to m iz ed p ig s tr ue ( co rre ct ed ) ile al d ig es tib ili ty 4. 8 % sa ue r, M os en th in , a nd o zi m ek 1 98 8 Ba rle y 18 8 µg 6 m al e ile os to m iz ed p ig s ile al d ig es tib ili ty ( co rre ct ed ) 27 % Ko pi ns ki , l ei bo ho lz, a nd B ry de n 19 89 b M ai ze 54 µ g pe r kg d ry m at te r 6 m al e ile os to m iz ed p ig s tr ue ( co rre ct ed ) ile al d ig es tib ili ty 4. 0 % sa ue r, M os en th in , a nd o zi m ek 1 98 8 Ca no la m ea l 51 3 µg p er k g dr y m at te r 6 m al e ile os to m iz ed p ig s tr ue ( co rre ct ed ) ile al d ig es tib ili ty 3. 9 % sa ue r, M os en th in , a nd o zi m ek 1 98 8 CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 5 Fo la te an im al Be ef li ve r 0. 67 –1 .6 1 m g d Fe 12 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 50 % ta m ur a an d st ok st ad 1 97 3 Go at li ve r 1. 00 m g d Fe 5 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 70 % Ba bu a nd s rik an tia 1 97 6 w ho le e gg s, co ok ed 0. 89 –1 .0 3 m g d Fe 7 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 72 % Ba bu a nd s rik an tia 1 97 6 eg g yo lk s, ha rd -b oi le d 0. 28 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 82 % ta m ur a an d st ok st ad 1 97 3 eg g yo lk s, ha rd -b oi le d 1. 03 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 59 % ta m ur a an d st ok st ad 1 97 3 Fo la te Pl an t Ch ic kp ea ( Be ng al g ra m ) 0. 99 m g d Fe 6 hu m an s, ag e ra ng e 20 –2 2 y Bi oa va ila bi lit y 71 % d ev ad as , P re m ak um ar i, an d M oo rt hy 19 79 Ch ic kp ea ( Be ng al g ra m ) 0. 96 m g d Fe 7 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 69 % Ba bu a nd s rik an tia 1 97 6 M un g be an ( gr ee n gr am ) 0. 98 m g d Fe 6 hu m an s, ag e ra ng e 20 –2 2 y Bi oa va ila bi lit y 71 % d ev ad as , P re m ak um ar i, an d M oo rt hy 19 79 M un g be an ( gr ee n gr am ) 0. 99 m g d Fe 7 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 55 % Ba bu a nd s rik an tia 1 97 6 li m a be an s, gr ee n, co ok ed f ro m f ro ze n 1. 02 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 96 % ta m ur a an d st ok st ad 1 97 3 li m a be an s, m at ur e, dr ie d, c oo ke d 0. 84 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 70 % ta m ur a an d st ok st ad 1 97 3 sp in ac h 0. 99 m g d Fe 8 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 63 % Ba bu a nd s rik an tia 1 97 6 sp in ac h 0. 36 –0 .4 4 m g d Fe 12 h um an s, ag e ra ng e 48 –5 6 y av ai la bi lit y 69 % Ko ni ng s et  a l. 20 02 sp in ac h 0. 36 m g d Fe 10 m al e an d 10 f em al e hu m an s, m ea n ag e 27 y av ai la bi lit y 77 % Pr in z- la ng en oh l e t  al . 1 99 9 sp in ac h 0. 20 m g d Fe 74 m al e hu m an s, m ea n ag e 31 y av ai la bi lit y 31 % Ha nn on -F le tc he r et  a l. 20 04 ro m ai ne le tt uc e 0. 75 m g d Fe 13 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 25 % ta m ur a an d st ok st ad 1 97 3 Gr ee n ca bb ag e, r aw 0. 83 –1 .0 9 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 47 % ta m ur a an d st ok st ad 1 97 3 Gr ee n ca bb ag e, c oo ke d 0. 67 –1 .0 3 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 47 % ta m ur a an d st ok st ad 1 97 3 to m at oe s 0. 98 m g d Fe 7 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 37 % Ba bu a nd s rik an tia 1 97 6 Ye as t 0. 20 m g d Fe 74 m al e hu m an s, m ea n ag e 31 y av ai la bi lit y 53 % Ha nn on -F le tc he r et  a l. 20 04 Br ew er ’s ye as t 0. 98 m g d Fe 8 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 10 % Ba bu a nd s rik an tia 1 97 6 Br ew er ’s ye as t 1. 40 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 60 % ta m ur a an d st ok st ad 1 97 3 Br ew er ’s ye as t ex tr ac t 0. 75 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 63 % ta m ur a an d st ok st ad 1 97 3 Ba na na s 0. 85 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 82 % ta m ur a an d st ok st ad 1 97 3 Ba na na s 0. 87 –0 .9 3 m g d Fe 8 m al e hu m an s, ag e ra ng e 25 –3 5 y Bi oa va ila bi lit y 46 % Ba bu a nd s rik an tia 1 97 6 o ra ng e ju ic e 0. 84 m g d Fe 14 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 31 % ta m ur a an d st ok st ad 1 97 3 w he at g er m 1. 33 m g d Fe 6 m al e hu m an s, ag e ra ng e 20 –3 5 y Bi oa va ila bi lit y 30 % ta m ur a an d st ok st ad 1 97 3 Fi ng er m ill et ( ra ji) 0. 98 m g d Fe 6 hu m an s, ag e ra ng e 20 –2 2 y Bi oa va ila bi lit y 50 % d ev ad as , P re m ak um ar i, an d M oo rt hy 19 79 Pe ar l m ill et ( Ba jra ) 0. 98 m g d Fe 6 hu m an s, ag e ra ng e 20 –2 2 y Bi oa va ila bi lit y 54 % d ev ad as , P re m ak um ar i, an d M oo rt hy 19 79 N ia ci n an im al Be ef , c oo ke d 45 .2 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 69 % ro th -M ai er e t  al . 2 00 0 Po rk , c oo ke d 66 .5 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 65 % ro th -M ai er e t  al . 2 00 0 N ia ci n Pl an t Po ta to es , b oi le d 39 .5 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 69 % ro th -M ai er e t  al . 2 00 0 w he at 54 .1 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 59 % ro th -M ai er e t  al . 2 00 0 w he at 54 .1 –5 4. 4 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 61 % w au er e t  al . 1 99 9 w ho le -m ea l b re ad 20 .6 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 40 % ro th -M ai er e t  al . 2 00 0 Pa nt ot he ni c ac id an im al Be ef , c oo ke d 3. 25 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 65 % ro th -M ai er e t  al . 2 00 0 Po rk , c oo ke d 5. 05 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 74 % ro th -M ai er e t  al . 2 00 0 sk im m ed m ilk p ow de r 24 .8 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 90 % ro th -M ai er a nd K irc hg es sn er 1 99 6 Pa nt ot he ni c ac id Pl an t Po ta to es , b oi le d 10 .9 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 70 % ro th -M ai er e t  al . 2 00 0 w he at 14 .1 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 81 % ro th -M ai er e t  al . 2 00 0 w he at 11 .1 –1 4. 1 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 78 % w au er e t  al . 1 99 9 w ho le -m ea l b re ad 4. 03 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 28 % ro th -M ai er e t  al . 2 00 0 w he at 10 .7 –1 5. 0 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 40 % w au er e t  al . 1 99 9 w he at b ra n 9. 92 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 47 % ro th -M ai er a nd K irc hg es sn er 1 99 6 M ai ze 3. 87 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 20 % ro th -M ai er a nd K irc hg es sn er 1 99 6 Ri bo fla vi n an im al Be ef , r oa st ed 1. 25 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 31 % ro th -M ai er e t  al . 1 99 8 Po rk , r oa st ed 1. 86 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 58 % ro th -M ai er e t  al . 1 99 8 sk im m ed m ilk p ow de r 12 .0 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 94 % ro th -M ai er a nd K irc hg es sn er 1 99 6 Ri bo fla vi n Pl an t w he at b ra n 1. 43 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 62 % ro th -M ai er a nd K irc hg es sn er 1 99 6 M ai ze 1. 50 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 67 % ro th -M ai er a nd K irc hg es sn er 1 99 6 (C on tin ue d) 6 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN Th ia m in an im al Be ef , r oa st ed 0. 71 m g 3 fe m al e ee v pi gs ile al d ig es tib ili ty 79 % ro th -M ai er e t  al . 1 99 8 Po rk , r oa st ed 8. 54 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 96 % ro th -M ai er e t  al . 1 99 8 M ilk p ow de r 1. 23 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 88 % ro th -M ai er e t  al . 1 99 9 w ho le e gg s, bo ile d 1. 40 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 82 % ro th -M ai er e t  al . 1 99 9 Fi sh , s te w ed 0. 97 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 73 % ro th -M ai er e t  al . 1 99 9 Be ef , r oa st ed 0. 93 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 51 % ro th -M ai er e t  al . 1 99 8 Po rk , r oa st ed 7. 56 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 93 % ro th -M ai er e t  al . 1 99 8 sk im m ed m ilk p ow de r 2. 60 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 96 % ro th -M ai er a nd K irc hg es sn er 1 99 6 Th ia m in Pl an t so ya be an s, bo ile d 4. 74 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 94 % ro th -M ai er e t  al . 1 99 9 Po ta to es , b oi le d 2. 51 m g 4 fe m al e ee v pi gs ile al d ig es tib ili ty 84 % ro th -M ai er e t  al . 1 99 8 w hi te c ab ba ge 1. 38 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 81 % ro th -M ai er e t  al . 1 99 9 Br ew er ’s ye as t 2. 54 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 91 % ro th -M ai er e t  al . 1 99 9 Ba na na s 1. 33 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 77 % ro th -M ai er e t  al . 1 99 9 w he at 5. 18 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 87 % ro th -M ai er e t  al . 1 99 8 w he at 6. 26 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 87 % ro th -M ai er e t  al . 1 99 8 w he at b ra n 2. 88 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 92 % ro th -M ai er e t  al . 1 99 9 w ho le -w he at b re ad 2. 59 m g 4 fe m al e ee v pi gs ile al d ig es tib ili ty 75 % ro th -M ai er e t  al . 1 99 8 Ba rle y 4. 72 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 94 % ro th -M ai er e t  al . 1 99 9 ry e 4. 17 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 84 % ro th -M ai er e t  al . 1 99 9 M ai ze 3. 47 m g 2 m al e ee v pi gs ile al d ig es tib ili ty 81 % ro th -M ai er e t  al . 1 99 9 ri ce , b oi le d 2. 88 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 94 % ro th -M ai er e t  al . 1 99 9 Po ta to es , b oi le d 2. 51 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 63 % ro th -M ai er e t  al . 1 99 8 w he at 4. 80 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 75 % ro th -M ai er e t  al . 1 99 8 w he at 4. 95 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 75 % ro th -M ai er e t  al . 1 99 8 w he at b ra n 2. 88 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 91 % ro th -M ai er a nd K irc hg es sn er 1 99 6 w ho le -w he at b re ad 2. 59 m g 3 fe m al e es v pi gs ile al d ig es tib ili ty 60 % ro th -M ai er e t  al . 1 99 8 M ai ze 3. 96 m g 6 fe m al e es v pi gs ile al d ig es tib ili ty 87 % ro th -M ai er a nd K irc hg es sn er 1 99 6 Vi ta m in B -6 an im al Be ef , r oa st ed 3. 15 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 89 % ro th -M ai er e t  al . 1 99 8 Po rk , r oa st ed 4. 02 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 89 % ro th -M ai er e t  al . 1 99 8 M ilk p ow de r 2. 01 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 84 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 w ho le e gg s, bo ile d 1. 71 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 67 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Fi sh , s te w ed 2. 61 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 85 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Vi ta m in B -6 Pl an t so ya be an s, bo ile d 3. 34 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 76 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Po ta to es , b oi le d 5. 60 m g 5 fe m al e ee v pi gs ile al d ig es tib ili ty 87 % ro th -M ai er e t  al . 1 99 8 w hi te c ab ba ge 6. 23 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 91 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Br ew er ’s ye as t 2. 42 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 78 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Ba na na s 9. 10 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 86 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 w he at 2. 74 m g 6 fe m al e ee v pi gs ile al d ig es tib ili ty 69 % ro th -M ai er e t  al . 1 99 8 w he at 2. 77 m g 4 fe m al e ee v pi gs ile al d ig es tib ili ty 69 % ro th -M ai er e t  al . 1 99 8 w he at b ra n 3. 30 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 56 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 w ho le -g ra in b re ad 2. 18 m g 4 fe m al e ee v pi gs ile al d ig es tib ili ty 71 % ro th -M ai er e t  al . 1 99 8 Ba rle y 3. 91 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 63 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 ry e 1. 81 m g 3 m al e ee v pi gs ile al d ig es tib ili ty 51 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Ta bl e 2. C on tin ue d. n ut rie nt Fo od so ur ce Fo od it em in ta ke st ud y po pu la tio n Pr oc es s of b io av ai la bi lit y Bi oa va ila bi lit y re fe re nc e CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 7 M ai ze 3. 53 m g 2 m al e ee v pi gs ile al d ig es tib ili ty 67 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Br ow n ric e, b oi le d 1. 62 m g 6 m al e ee v pi gs ile al d ig es tib ili ty 16 % ro th -M ai er , K et tle r, an d Ki rc hg es sn er 20 02 Vi ta m in B -1 2 an im al Ch ic ke n m ea t (1 00 g ) 0. 42 –0 .6 4 µg 3 hu m an s Fe ca l d ig es tib ili ty 65 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 8 Ch ic ke n m ea t (2 00 g ) 0. 84 –1 .2 8 µg 3 hu m an s Fe ca l d ig es tib ili ty 62 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 8 Ch ic ke n m ea t (3 00 g ) 1. 26 –1 .9 2 µg 3 hu m an s Fe ca l d ig es tib ili ty 61 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 8 sh ee p m ut to n m ea t (1 00 g ) 0. 95 µ g 7 m al e hu m an s av ai la bi lit y 65 % He ys se l e t  al . 1 96 6 w ho le e gg s, fri ed 0. 50 –0 .5 6 µg 18 h um an s Fe ca l d ig es tib ili ty 76 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 5 w ho le e gg s, so ft- bo ile d 0. 50 –0 .5 6 µg 18 h um an s Fe ca l d ig es tib ili ty 76 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 5 w ho le e gg s, sc ra m bl ed 0. 50 –0 .5 6 µg 18 h um an s Fe ca l d ig es tib ili ty 73 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 5 eg g yo lk s, sc ra m bl ed 0. 50 –0 .5 6 µg 18 h um an s Fe ca l d ig es tib ili ty 64 % d os ch er ho lm en , M cM ah on , a nd ri pl ey 1 97 5 Fi sh fi lle ts ( 50 g ) 1. 95 –2 .1 8 µg 3 hu m an s Fe ca l d ig es tib ili ty 42 % d os ch er ho lm en , M cM ah on , a nd ec on om on 1 98 1 Vi ta m in C Pl an t M us ta rd g re en s, co ok ed 57 m g 3 m al e an d 9 fe m al e hu m an s av ai la bi lit y 73 % Ho lli ng er 1 94 8 Br oc co li, r aw 10 9 m g 23 m al e hu m an s, m ea n ag e 40 y av ai la bi lit y 58 % M an ge ls et  a l. 19 93 Br oc co li, c oo ke d 10 8 m g 23 m al e hu m an s, m ea n ag e 40 y av ai la bi lit y 78 % M an ge ls et  a l. 19 93 Gr ee n ca bb ag e, r aw 85 m g 3 m al e an d 1 fe m al e hu m an s av ai la bi lit y 97 % Cl ay to n an d Bo rd en 1 94 3 Po ta to es , b ak ed 75 m g 3 m al e an d 1 fe m al e hu m an s av ai la bi lit y 81 % Cl ay to n an d Bo rd en 1 94 0 Po ta to es , m as he d 50 m g 5 m al e hu m an s, m ea n ag e 24 y av ai la bi lit y 54 % Ko nd o et  a l. 20 12 Po ta to c hi ps 50 m g 5 m al e hu m an s, m ea n ag e 24 y av ai la bi lit y 53 % Ko nd o et  a l. 20 12 to m at o ju ic e 85 m g 3 m al e an d 1 fe m al e hu m an s av ai la bi lit y 89 % Cl ay to n an d Bo rd en 1 94 3 o ra ng e se gm en ts 10 9 m g 22 m al e hu m an s, m ea n ag e 40 y av ai la bi lit y 66 % M an ge ls et  a l. 19 93 o ra ng e ju ic e 11 0 m g 22 m al e hu m an s, m ea n ag e 40 y av ai la bi lit y 74 % M an ge ls et  a l. 19 93 o ra ng e ju ic e 50 m g 5 fe m al e hu m an s av ai la bi lit y 82 % to dh un te r, ro bb in s, an d M ci nt os h 19 42 st ra w be rr ie s 50 m g 5 fe m al e hu m an s av ai la bi lit y 86 % to dh un te r, ro bb in s, an d M ci nt os h 19 42 re d ra sp be rr ie s 60 m g 7 fe m al e hu m an s, m ea n ag e 31 y av ai la bi lit y 77 % to dh un te r an d Fa tz er 1 94 0 Pa pa ya s 75 m g 3 m al e an d 3 fe m al e hu m an s, ag e ra ng e 29 –3 4 y av ai la bi lit y 86 % Ha rt zl er 1 94 5 Gu av a ju ic e 75 m g 3 m al e an d 3 fe m al e hu m an s, ag e ra ng e 29 –3 4 y av ai la bi lit y 90 % Ha rt zl er 1 94 5 Go ld k iw ifr ui t 20 0 m g 9 m al e hu m an s, m ea n ag e 24 y av ai la bi lit y 78 % Ca rr, B oz on et , a nd v iss er s 20 13 re , r et in ol e qu iv al en t, w he re by 1 m g of r e = 1 m g of r et in ol = 6 m g of β -c ar ot en e. d Fe , d ie ta ry F ol at e eq ui va le nt , w he re by 1 m g d Fe = 1 m g of f oo d fo la te = 0 .6 m g of f ol ic a ci d fro m f or tifi ed f oo d or a s a di et ar y su pp le m en t co ns um ed w ith f oo d. ee v, e nd -t o- en d ile o- re ct al a na st om os is. es v, e nd -t o- sid e ile o- re ct al a na st om os is. 8 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN 1991; Goodman et  al. 1966; Reboul 2013). Retinol absorp- tion mainly occurs via a carrier-mediated, saturable trans- porter, and carotenoids are absorbed mostly by passive diffusion (Blomhoff et al. 1991; Goodman et al. 1966; Reboul 2013). Within the enterocyte, some of the absorbed β-carotene may be cleaved and converted to retinol and ret- inyl esters or β-apo-carotenals. Retinol, retinyl esters, β-apo-carotenals, and β-carotene, are incorporated into chy- lomicrons to be transported by the lymphatic system to the liver for retinol metabolism and storage (Blomhoff et  al. 1991; Goodman et  al. 1966; Reboul 2013). Retinol-binding proteins are involved in the release of free retinol from hepatic retinyl esters for distribution to target tissues (Blomhoff et  al. 1991; Goodman et  al. 1966; Reboul 2013). Vitamin A not absorbed from the gastrointestinal tract is eliminated in feces as biliary metabolites, and retinol metab- olites are also excreted in the urine (Biesalski 1997; Blomhoff et  al. 1991; Castenmiller and West 1998; de Pee and West 1996; Goodman et  al. 1966; Parker 1997; Parker et  al. 1999). Vitamin A content in food Vitamin A is generally expressed as retinol equivalents (REs) to account for the different forms of vitamin A and the lower absorption efficacy of provitamin A carotenoids and their bioconversion to retinol (FAO 1998; WHO and FAO 1967). Based on rat growth bioassays, one unit of RE is defined as the equivalent amount of retinol that can be obtained from the provitamin A carotenoids, namely 6 units of retinol from β-carotene, or 12 units of retinol from α-carotene, γ-carotene, β-cryptoxanthin and other provita- min A carotenoids (Biesalski 1997; FAO 1988; Olson 1987a; WHO and FAO 1967, 2004). Vitamin A was previously expressed as International Units (IU) of retinol, whereby 1 IU retinol is equivalent to 0.3 unit of RE (Biesalski 1997; WHO and FAO 2004). In the United States of America, where the intake of provitamin A carotenoids is higher and where their biological activities are assumed to be two-fold lower, vitamin A is usually expressed as retinol activity equivalents (RAEs), whereby 1 unit of RAE is equivalent to 1 unit of retinol, 12 units of β-carotene, or 24 units of other provitamin A carotenoids (IOM (Institute of Medicine, US) 2001). In this review, the dietary intakes of vitamin A are expressed in terms of RE, using the conversion factors of 1:1 for retinol, 1:6 for β-carotene, and 1:12 for other provitamin A carotenoids. The retinol equivalence of different forms of carotenoids from different foods remains contentious. Determination of the bioavailability of dietary vitamin A and supplementary vitamin A in purified form The bioavailability of dietary vitamin A has been determined through the measurement of retinol, retinyl esters, β-carotene, α-carotene, and other provitamin A carotenoids in the blood, ileal effluent, and feces. Several studies have compared the relative blood response of dietary vitamin A to synthetic vitamin A. Pure preformed vitamin A (retinol, retinyl esters) given with dietary oil to healthy human subjects, has been estimated to be 80% bioavailable (Edwards et al. 2001, 2002; Goodman et  al. 1966; IOM (Institute of Medicine, US) 2001; Olson 1987a; WHO and FAO 1967, 2004; Wolf 2002), and this value is often used as a baseline to calculate absolute bio- availability. There was a wide variability (3 to 100%) reported for the bioavailability of purified provitamin A carotenoids, particularly β-carotene (Castenmiller and West 1998; de Pee and West 1996; Faulks et  al. 1997; Haskell 2012; IOM (Institute of Medicine, US) 2001; Olson 1987a; Figure 1. estimated bioavailability (%) of vitamins in foods sourced from animals and plants. CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 9 Parker 1997; Parker et  al. 1999; van Lieshout, West, and van Breemen 2003; WHO and FAO 2004), which may pos- sibly be attributable to the presence of dietary fat and intake doses (Blomhoff et  al. 1991; Goodman et  al. 1966; Reboul 2013). For a usual optimal intake of 1–4 mg of β-carotene per meal, 6 units of β-carotene are equivalent to 1 unit of retinol or RE. However, at lower intakes (<1 mg of β-carotene per meal) when β-carotene is absorbed more efficiently, 4 units of β-carotene may yield 1 unit of retinol, while at higher intakes (>4 mg of β-carotene per meal) when the system may be saturated, 10 units of β-carotene is needed to give 1 equivalent unit of retinol (de Pee and West 1996; FAO 1988). The bioavailability estimates for provitamin A carotenoids reflect both the absorption effi- ciency of carotenes and the proportion of carotenes con- verted to retinol or retinyl esters. Based on the assumption that the absorption and conversion efficiencies of pure all-trans-β-carotene consumed in oil, is half that of pure retinol (de Pee and West 1996; Goodman et  al. 1966; IOM (Institute of Medicine, US) 2001; Olson 1987a; WHO and FAO 1967, 2004), pure β-carotene is considered to be 40% bioavailable. The factors influencing the bioavailability of dietary pro- vitamin A carotenoids have been identified under the acro- nym “SLAMANGHI” or “SLAMENGHI”, where the letters stand for: Species of carotenoid, molecular Linkage, Amount of carotene consumed in a meal, dietary Matrix in which the carotenoid is incorporated, Absorption modifiers or Effectors of absorption and bioconversion, Nutrient status of the subject, Genetic factors, Host-related factors, and Interactions (de Pee and West 1996). Each factor of SLAMENGHI also affects the efficiency of absorption of vitamin D (Borel, Caillaud, and Cano 2015) and vitamin E (Borel, Preveraud, and Desmarchelier 2013). Data estimates of the bioavailability of vitamin A in food depend on the form of the vitamin A, the dietary matrix, food processing, and the experimental method used. Bioavailability of vitamin A (β-carotene) in mixed diets In a human study measuring β-carotene in plasma in response to ingested β-carotene, the availability of β-carotene in a mixed vegetable and fruit diet (5.10 mg β-carotene, 0.85 mg RE), relative to supplementary purified β-carotene, was 14% (van Het Hof et  al. 1999a). Based on the assump- tion that purified β-carotene is 40% bioavailable, an absolute availability of β-carotene of 5.6% was calculated. Bioavailability of vitamin A (retinol) in animal-sourced foods Two human studies measured the amount of retinol and ret- inyl esters in blood plasma and determined the availability of vitamin A in food liver (3–150 mg RE), relative to that of supplementary vitamin A (Buss et  al. 1994; van Vliet et  al. 2001). The relative availability of vitamin A in 50 and 150 mg of fried beef calf liver (50–150 mg RE) was 105 and 94%, respectively (Buss et  al. 1994). Consumption of 3.0, 7.5, and 15.0 mg of vitamin A in liver paste (3–15 mg RE) resulted in relative vitamin A availability of 54, 77, and 123%, respectively (van Vliet et  al. 2001). Based on the assumption that purified preformed vitamin A retinol is 80% bioavail- able, the absolute mean apparent availability of vitamin A was calculated to be 68% for liver paste and 80% for cooked beef liver (Table 2). A human study showed that the absolute apparent bio- availability of retinol following the consumption of whole (3.6% fat) milk (0.14–0.26 mg RE) was 16.4% (Herrero-Barbudo et  al. 2006). The data were highly variable and it is unclear from the study that all retinol in the triglyceride-rich lipo- protein fraction was determined. Nor did the study include an external control (pure retinol) and the values are thus likely to underestimate absolute availability. Consequently, the absorption values were considered to be more qualitative than quantitative. Interestingly, the plasma response of ret- inyl esters following consumption of whole (3.6% fat) milk fortified with vitamin A (0.63–1.19 mg RE) and skimmed (0.2% fat) milk fortified with vitamin A (0.31–0.57 mg RE) was lower than that for the non-fortified whole milk, despite the higher vitamin A content in the vitamin A-fortified milks (Herrero-Barbudo et  al. 2006). Bioavailability of vitamin A (β-carotene) in plant-based foods Livny et  al. (2003) determined the mass balance (food intake corrected for blood and ileal effluent excretion) of β-carotene in ileostomates, and found that the apparent availability of β-carotene in raw chopped carrots (15.00 mg β-carotene, 2.50 mg RE) was 41%, and 65% for cooked pureed carrots (15.00 mg β-carotene, 2.50 mg RE) (Table 2). In another study of similar design, Faulks et  al. (2004) demonstrated that the apparent availability of β-carotene in cooked whole leaf spinach (10.00 mg β-carotene, 1.67 mg RE) and cooked finely chopped spinach (10.00 mg β-carotene, 1.67 mg RE) was 26 and 23%, respectively (Table 2). Several studies compared the blood β-carotene response to dietary β-carotene with that following the consumption of similar doses of supplementary purified β-carotene (Brown et al. 1989; Castenmiller et  al. 1999; Huang et  al. 2000: Micozzi et  al. 1992; van Het Hof et  al. 1999b). The relative availability of β-carotene was found to be 17.2% for cooked from frozen carrots (29.00 mg β-carotene, 4.83 mg RE) (Brown et al. 1989), 18.2% for cooked from frozen carrots (29.00 mg β-carotene, 4.83 mg RE) (Micozzi et  al. 1992), 33% for cooked stir-fried shredded carrots (12.01 mg β-carotene, 2.00 mg RE) (Huang et  al. 2000), 37% for cooked deep-fried mashed sweet potato balls (12.01 mg β-carotene, 2.00 mg RE) (Huang et  al. 2000), 26% for cooked stir-fried whole water spinach (water convolvulus) leaves (12.01 mg β-carotene, 2.00 mg RE) (Huang et  al. 2000), 18.2% for cooked whole leaf spinach (23.80 mg β-carotene, 3.97 mg RE) (van Het Hof et  al. 1999b), 18.5% for cooked chopped minced spinach (24.60 mg β-carotene, 4.10 mg RE) (van Het Hof et  al. 1999b), 5.1% for cooked whole leaf spinach (10.40 mg β-carotene, 1.73 mg RE) (Castenmiller et  al. 1999), 6.4% for cooked minced spinach (8.80 mg β-carotene, 1.47 mg RE) (Castenmiller et  al. 1999), 9.5% for cooked liquefied spinach (9.00 mg β-carotene, 1.50 mg RE, 28.2 g dietary fiber) 10 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN (Castenmiller et  al. 1999), and 9.3% when dietary fiber was added to cooked enzymatically liquefied spinach (8.80 mg β-carotene, 1.47 mg RE, 31.3 g dietary fiber) (Castenmiller et  al. 1999). Based on the assumption that supplementary purified β-carotene is 40% bioavailable, the absolute appar- ent availability of β-carotene was estimated to be 9.1% for cooked carrots, 14.8% for sweet potatoes, 4.5% for spinach, and 10.4% for water spinach (Table 2). Using the human oro-fecal balance of β-carotene, the apparent absorption of β-carotene was found to be 14% for a mixed vegetable and fruit diet (Van Loo-Bouwman et  al. 2009, 2010), 46% for sweet potatoes (James and Hollinger 1954), 81% for carrots (Rao and Rao 1970), 76% for Amaranth green leafy vegetables (Rao and Rao 1970), and 90% for papaya fruit (Rao and Rao 1970). These absorption values based on fecal collection may be misleading and need to be interpreted with caution due to the potential interfer- ence from the microbial metabolism of carotenoids by intes- tinal bacteria. Comparison of vitamin A bioavailability between animal- and plant-based foods The bioavailability of vitamin A in animal- and plant-based foods is difficult to compare due to differences in the form of vitamin A found in animal- and plant-sourced foods. The mean bioavailability of preformed vitamin A retinol in liver was found to be 74%, and the mean bio- availability of provitamin A carotenoid β-carotene in veg- etables was 15.6%, and ranged from 2.0 to 65%. In general, β-carotene in plant foods appears to be less bioavailable than retinol in animal foods (Figure 1), though the num- ber of observations, especially in animal-sourced foods, is low. It has been recognized that there are differences in the efficiency of conversion from provitamin A carot- enoids (carotenes) to preformed vitamin A retinol. An attempt to take this information into account is to express vitamin A in terms of retinol equivalent (RE), whereby the biological activity of β-carotene is one-sixth of that of retinol, and one-twelfth for the other provitamin A carot- enoids. Using RE must be interpreted with caution. Using RE to interpret the bioavailability (absorption and biocon- version) of vitamin A in foods may be misleading. The bioavailability estimates of vitamin A in foods sourced from animals and plants cannot be directly compared and it may be more meaningful to use retinol and β-carotene values separately, to provide comparable bioavailability data in animal- and plant-based foods. Vitamin D Vitamin D absorption and utilization Vitamin D is a fat-soluble vitamin and plays an important role in bone, muscle, and nervous functions, and modu- lates the absorption and re-absorption in the kidney of cal- cium and phosphorus (Bates and Heseker 1994; IOM (Institute of Medicine, US) 2011; van den Berg 1997d; WHO and FAO 2004). Cereal grains, vegetables, fungi (e.g., mushrooms), and fruits naturally contain ergosterol, that is converted to vitamin D2 (ergocalciferol), upon exposure to ultraviolet (UV) radiation (Borel, Caillaud, and Cano 2015; Cashman 2012; Holmes and Kummerow 1983; Jäpelt and Jakobsen 2013; van den Berg 1997d). The main form of vitamin D obtained from the diet is vitamin D3 (cholecal- ciferol), which is the irradiation product of the steroid 7-dehydrocholesterol and mostly occurs in animal-derived foods, such as fatty fish, meat, milk and dairy products, and eggs (Borel, Caillaud, and Cano 2015; Cashman 2012; Holmes and Kummerow 1983; Jäpelt and Jakobsen 2013; Schmid and Walther 2013; van den Berg 1997d). The amount of vitamin D naturally found in milk relies on the level of exposure of the animal to solar UV radiation and the content of vitamin D in the diet of the animal (Jakobsen and Saxholt 2009; Kurmann and Indyk 1994; Schmid and Walther 2013). Most vitamin D in milk is attributed to for- tification. The amount of vitamin D naturally present in eggs may be influenced by the concentration of supplemen- tary vitamin D consumed by the laying hens (Mattila, Valkonen, and Valaja 2011; Mattila et  al. 1999). Vitamin D3 produced in the human skin following adequate sun- light exposure may sufficiently meet vitamin D require- ments (Cashman 2012; Holmes and Kummerow 1983; Jäpelt and Jakobsen 2013; van den Berg 1997d). Under low exposure to UV sunlight, it has been recommended that humans consume at least 10 µg of vitamin D (Cashman 2012; Holmes and Kummerow 1983; IOM (Institute of Medicine, US) 2011; WHO and FAO 2004). However, nat- ural food sources provide less than 1 µg of naturally occur- ring vitamin D, so dietary supplements or foods commonly fortified with vitamin D, such as milk and margarine, are common sources of vitamin D (Cashman 2012; Holmes and Kummerow 1983; van den Berg 1997d). Dietary vitamin D is solubilized within mixed micelles in the duodenum and this process is stimulated by the pres- ence of dietary fat. The micelles are absorbed in the jejunum into chylomicrons, which transport the vitamin D in the lymph to the systemic circulation (Borel, Caillaud, and Cano 2015; Holmes and Kummerow 1983; Reboul 2015). The major circulating form of vitamin D is the biologically active metabolite 25-hydroxy-vitamin D (25(OH)D), which can be stored in the adipose tissue, muscle, and liver (Borel, Caillaud, and Cano 2015; Holmes and Kummerow 1983; Jäpelt and Jakobsen 2013; Reboul 2015; Seamans and Cashman 2009; van den Berg 1997d; Whyte et  al. 1979). It is unclear if vitamin D2 (ergocalciferol) is as effective as vitamin D3 (cholecalciferol) in increasing and maintaining circulating levels of the bioactive 25(OH)D (Cashman 2012; Heaney et  al. 2011; Holick et  al. 2008; Lehmann et  al. 2013; Rapuri, Gallagher, and Haynatzki 2004; Seamans and Cashman 2009; Trang et  al. 1998; Tripkovic et  al. 2012; van den Berg 1997d; Whyte et  al. 1979). Vitamin D content in food Vitamin D can be expressed in international units (IU), whereby 1 IU of vitamin D is 0.025 µg of vitamin D or 0.005 µg of 25(OH)D (IOM (Institute of Medicine, US) 2011; van den Berg 1997d; WHO and FAO 2004). CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 11 Determination of the bioavailability of dietary vitamin D and supplementary vitamin D in purified form and in food The bioavailability of vitamin D in natural or fortified food sources has been determined by comparing the response of circulating vitamin D with that from a vitamin D supple- ment, which is assumed to be 75–85% bioavailable (Thompson, Lewis, and Booth 1966). Studies have determined the uptake of supplementary vitamin D following the consumption of fortified cheese (Wagner et  al. 2008), fortified breads (Natri et  al. 2006), and fortified orange juice (Biancuzzo et  al. 2010), relative to a vitamin D supplement. Wagner et  al. (2008) found that the relative availability of supplementary vitamin D given with fortified regular-fat (33% fat) and low-fat (7% fat) Cheddar cheese (100 µg vitamin D3 cholecalciferol, 4000 IU vitamin D) was 114%. The mean relative availability of supplemen- tary vitamin D in breads fortified with vitamin D, low-fiber (3 g per 100 g) wheat bread (10.8 µg vitamin D, 432 IU vita- min D) and high-fiber (12 g per 100 g) sour dough rye bread (12.3 µg vitamin D, 492 IU vitamin D), was 80% (Natri et  al. 2006). In a human clinical trial over 11 wk, Biancuzzo et  al. (2010) studied which form of supplementary vitamin D would be more bioavailable following the daily consumption of a glass of orange juice fortified with water-soluble vitamin D (25 µg, 1000 IU). When incorporated in orange juice, sup- plementary vitamin D3 (cholecalciferol) had a relative avail- ability of 98%, and supplementary vitamin D2 (ergocalciferol) a relative availability of 108%, with an average estimated rel- ative availability of supplementary vitamin D of 103% (Biancuzzo et  al. 2010). Based on the assumption that sup- plementary purified vitamin D is 80% bioavailable (Thompson, Lewis, and Booth 1966), the absolute apparent availability of supplementary vitamin D in vitamin D-fortified foods was estimated to be 79%. Bioavailability of vitamin D (ergocalciferol) in animal- sourced foods In a review, van den Berg (1997d) reported that the relative availability of naturally occurring vitamin D2 (ergocalciferol) in pig meat (10 µg vitamin D2, 400 IU vitamin D) was 60% that of a vitamin D2 supplement, based on the increase in serum 25-hydroxy-vitamin D2 (25(OH)D2) over a 6 week period in human participants. Assuming that the bioavail- ability of 100 µg of supplementary vitamin D is 77% (Thompson, Lewis, and Booth 1966), the absolute apparent availability of vitamin D in pork was estimated to be 46%. Bioavailability of vitamin D (ergocalciferol) in plant-based foods Brown button mushrooms cultivated in the dark contain very low concentrations of naturally occurring vitamin D2 (ergocalciferol) (0.18 µg per 100 g fresh weight), and can be enriched with more vitamin D when irradiated with ultravi- olet (UV) light (491 µg per 100 g fresh weight) (Urbain et  al. 2011). When a soup (4.2% fat) containing the UV-irradiated vitamin D-enriched mushrooms (700 µg vitamin D2 ergocalciferol, 28000 IU vitamin D) was consumed, the serum concentrations of the biologically active form of ergo- calciferol (25(OH)D2) increased to a similar extent to those associated with the ingestion of 700 µg of vitamin D2 sup- plement. The relative availability of vitamin D from mush- rooms was determined to be 83%, and based on the assumption that 500–1000 µg doses of pure vitamin D are 80% bioavailable (Thompson, Lewis, and Booth 1966), the absolute apparent availability of vitamin D from UV-irradiated mushrooms was calculated to be 66%. Comparison of vitamin D bioavailability between animal- and plant-based foods Vitamin D is synthesized in the human skin upon exposure to sunlight and this synthesis may in some cases, under ample solar exposure, be sufficient to meet the daily requirements for vitamin D. Studies comparing the relative bioavailability of naturally occurring vitamin D in food to that of supplemen- tary vitamin D are lacking. Although the bioavailability of vitamin D in pork was estimated to be 46% (van den Berg 1997d), this data needs to be interpreted with caution as the review provided few details of the clinical study. It was esti- mated that the bioavailability of vitamin D in vitamin D-enriched brown button mushrooms exposed to ultraviolet irradiation was 66% (Urbain et  al. 2011). There are insuffi- cient data to make a comparison of the bioavailability of nat- urally occurring vitamin D in animal- and plant-based foods. Vitamin E Vitamin E absorption and utilization Vitamin E is a fat-soluble vitamin, that includes four tocoph- erols (α, β, γ, δ) and four tocotrienols (α, β, γ, δ) (Bates and Heseker 1994; Borel, Preveraud, and Desmarchelier 2013; Cohn 1997; IOM (Institute of Medicine, US) 2000; Jiang 2014; WHO and FAO 2004). Most can be converted to the most biologically active form of vitamin E, α-tocopherol, that has a methyl group attached to an asymmetric carbon atom at positions 2, 4ꞌ, and 8ꞌ (Borel, Preveraud, and Desmarchelier 2013; Cohn 1997; Jiang 2014). This d-α-to- copherol or RRR-α-tocopherol is mostly found in the sys- temic circulation and stored in the body (Borel, Preveraud, and Desmarchelier 2013; Cohn 1997; Reboul 2017; Reboul et  al. 2006). Vitamin E, as α-tocopherol, mainly functions as an anti-oxidant by protecting polyunsaturated fatty acids present in body cells from being broken down by oxidation (Borel, Preveraud, and Desmarchelier 2013; Cohn 1997; Jiang 2014). Food sources that are naturally rich in vitamin E include cereal grains, nuts, seeds, vegetable oils, and some vegetables and fruits (Bauernfeind 1977; Chun et  al. 2006; Cohn 1997; Jiang 2014; McLaughlin and Weihrauch 1979; Piironen et  al. 1986). Margarines that are high in polyunsat- urated fats are often fortified with vitamin E. Vitamin E is also found in small amounts in meat, dairy products, eggs, and fatty fish, but this is mostly dependent on the amount of vitamin E or supplementary vitamin E in the animal’s diet (Bauernfeind 1977; McLaughlin and Weihrauch 1979; Piironen et  al. 1985). 12 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN The absorption of α-tocopherol requires the presence of dietary fat and involves emulsification with food fats in the stomach. Dietary α-tocopherol is incorporated in mixed micelles to be solubilized across the unstirred water layer coat- ing the brush border membrane of the small intestine, and absorbed either by passive diffusion or with the help of cho- lesterol transporters in the small intestine. Absorbed α-tocopherol enters the blood via the lymphatic system incor- porated with chylomicrons (Borel, Preveraud, and Desmarchelier 2013; Cohn 1997; Jiang 2014; Reboul 2017; Reboul et al. 2006). Vitamin E content in food Vitamin E is often described in terms of α-tocopherol equiv- alents (α-TEs), whereby 1 mg of α-TE equals 1 mg of RRR-α- tocopherol, 0.5 mg of β-tocopherol, 0.1 mg of γ-tocopherol, 0.3 mg of α-tocotrienol, or 0.74 mg of synthetic α-tocopherol from fortified foods and dietary supplements (IOM (Institute of Medicine, US) 2000; WHO and FAO 2004). It has been argued that vitamin E should be given in units of α-tocopherol only, and that 1 unit of α-TE can be converted to α-tocopherol using a factor of 0.8 (IOM (Institute of Medicine, US) 2000; WHO and FAO 2004). Determination of the bioavailability of dietary vitamin E and supplementary vitamin E in purified form and in food Isotope labeling was mostly used to determine the bioavail- ability of vitamin E, and most studies assessed the amount of α-tocopherol circulating in blood. Human studies have evaluated the uptake of supplementary vitamin E, when con- sumed with oil (Kelleher and Losowsky 1968; MacMahon and Neale 1970; Novotny et  al. 2012) and when added to collard greens (Traber et  al. 2015), apples (Bruno et  al. 2006), and ready-to-eat wheat-based breakfast cereals (Leonard et  al. 2004). The mean bioavailability of supple- mentary purified vitamin E, given with oil, was estimated to be 75%, based on a multi-compartmental kinetic study and two balance studies, involving normal human participants who provided blood, urine, and fecal samples (Kelleher and Losowsky 1968; McMahon and Neale 1970; Novotny et  al. 2012). While consumption of vitamin E-fortified breakfast cereals (400 mg α-tocopherol or α-TE) resulted in a higher plasma response of isotopically labeled (d9) α-tocopheryl acetate than that after ingestion of a purified vitamin E cap- sule, there was only a small amount of fat co-ingested with the supplement and fortified breakfast cereal, and the study did not report the percentage availability (Leonard et  al. 2004). It was estimated that isotopically labeled (2H) supple- mentary vitamin E (α-tocopheryl acetate) consumed in the presence of dietary fat (1.6 g) was 24% available from the ingestion of fortified collard greens (9.2 mg α-tocopherol or α-TE) (Traber et  al. 2015). The estimated availability of sup- plementary deuterium labeled α-tocopherol added to apples increased from 10% when no fat was consumed with the α-tocopherol-enriched apples (8.0 mg α-tocopherol or α-TE), to 20% with 2.4 g of fat (7.3 mg α-tocopherol or α-TE), and 33% with 11 g of fat (6.5 mg α-tocopherol or α-TE) (Bruno et  al. 2006). Supplementary vitamin E consumed in the pres- ence of dietary fat with fortified collard greens (Traber et  al. 2015) and fortified apples (Bruno et  al. 2006), was about 26% bioavailable. Bioavailability of vitamin E in plant-based foods In a pilot ileal digestibility study involving two female human ileostomy patients, Mandalari et  al. (2008) found that the apparent availability of naturally occurring vitamin E in almonds (5.4 mg of vitamin E) was 54%. Comparison of vitamin E bioavailability between animal- and plant-based foods In a human oro-ileal balance study, the bioavailability of naturally occurring vitamin E in almonds was estimated to be 54% (Mandalari et  al. 2008), but this estimate was based on two female ileostomates only, and further studies are needed. The concentration of vitamin E in animal-sourced foods is dependent on the amount of vitamin E consumed by the animal (Bauernfeind 1977; McLaughlin and Weihrauch 1979; Piironen et  al. 1985). To the best of our knowledge, there are no reported studies in human subjects or pig as an animal model for adult humans, on the bioavailability of vitamin E in animal-based foods. A paucity of clinical stud- ies means that the bioavailability of naturally occurring vita- min E in food remains unknown, and it is not possible to provide a comparison of vitamin E bioavailability between animal- and plant-based foods. Vitamin K Vitamin K absorption and utilization Vitamin K is a fat-soluble vitamin essential for its role in blood clotting and bone metabolism (Bates and Heseker 1994; IOM (Institute of Medicine, US) 2001; Olson 1987b; Shearer, Fu, and Booth 2012; WHO and FAO 2004). All forms of vitamin K comprise the 2-methyl-1,4-naphthoquinone nucleus attached at C-3 to a naturally occurring phytyl group for phylloquinone (vitamin K1), or a side chain com- posed of 6–10 unsaturated isoprene units for menaquinone (vitamin K2) (Basset et  al. 2017; Beulens et  al. 2013; Booth 2012; Shearer 1992; Shearer, Fu, and Booth 2012; Shearer, McBurney, and Barkhan 1974). Menadione (vitamin K3) is a fat-soluble synthetic compound that contains the 2-methyl-1,4-napthoquinone structure with no side chain, and can be converted to biologically active menaquinones in body tissues (Beulens et  al. 2013; Booth 2012; Olson 1987b; Shearer 1992; Shearer, Fu, and Booth 2012). While most animal-sourced foods contain both phylloquinone and men- aquinone, plant-based foods, mainly green leafy vegetables and vegetable oils, provide mainly phylloquinone, and bacte- ria in the human large intestine can also synthesize men- aquinone (Basset et  al. 2017; Beulens et  al. 2013; Booth 2012; Booth and Suttie 1998; Hollander, Muralidhara, and Rim 1976; Olson 1987b; Parrish, Sheppard, and Sheppard 1980; Shearer 1992; Shearer, Fu, and Booth 2012). The absorption of vitamin K requires the presence of dietary fat and the action of pancreatic juices in the duode- num for solubilization within mixed micelles (Hollander, Rim, and Muralidhara 1977; Olson 1987b; Shearer, Fu, and CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 13 Booth 2012; Shearer, McBurney, and Barkhan 1974). Following absorption mainly through the jejunum and ileum (Hollander, Rim, and Muralidhara 1977) and minimally from the colon (Hollander, Muralidhara, and Rim 1976), vitamin K is transported via the lymphatic route into the systemic circulation (Olson 1987b; Shearer, Fu, and Booth 2012; Shearer, McBurney, and Barkhan 1974). Vitamin K is mobilized in the liver before being distributed to body tis- sues or converted to water-soluble metabolites, which are either secreted into the bile for fecal excretion, or excreted in the urine (Olson 1987b; Shearer, Fu, and Booth 2012; Shearer, McBurney, and Barkhan 1974). Determination of the bioavailability of dietary vitamin K and supplementary vitamin K in purified form The bioavailability of vitamin K (phylloquinone) from food sources was assessed by determining the circulating levels of phylloquinone, based either on appearance in the blood of isotopically labeled phylloquinone or the plasma levels of ingested dietary phylloquinone relative to purified phylloqui- none. Synthetic purified phylloquinone (vitamin K1) is con- sidered to be 80% bioavailable (Shearer, McBurney, and Barkhan 1974). Bioavailability of vitamin K (phylloquinone) in mixed diets The availability of isotopically labeled (13C) phylloquinone (vitamin K1) from three meals was measured from area under the plasma curve values over 8 h in twelve healthy human participants (Jones et  al. 2009). The three test meals, representing the commonly consumed dietary patterns in the United Kingdom, were beef lasagna for an animal- and dairy-oriented meal (higher than average consumption of red meat and saturated fats, 33.0 μg phylloquinone), fish- and dairy-oriented pie meal for a cosmopolitan meal (higher than average consumption of vegetables, fruits, whole grains, fish, and dairy products, 26.3 μg phylloquinone), and chicken and dairy meal provided with baked beans and potato chips for a convenience meal (higher than average consumption of fast foods and refined cereals, and lower than average con- sumption of vegetables, fruits, and whole-grain cereals, 19.9 μg phylloquinone). The study showed statistically signif- icant differences in the effects of meal food matrix and meal composition of the food material, on the uptake of phyllo- quinone, relative to the absorption of 20 μg of tracer phyllo- quinone consumed together with the three meals. The overall relative bioavailability of the meal and matrix effects was 20% for the animal-oriented meal (beef lasagna), 46% for the cosmopolitan meal (fish pie), and 100% for the con- venience meal (chicken pie with beans and chips) (Jones et  al. 2009). Bioavailability of vitamin K (phylloquinone) in plant-based foods Several studies have investigated the availability of phylloqui- none (vitamin K1) in vegetables. In a kinetic study using a three-compartment (gastrointestinal tract, blood plasma, body tissue pool) model of the estimated fraction of labeled (13C) phylloquinone, Novotny et  al. (2010) found that the mean availability of phylloquinone in cooked kale (119 µg phylloquinone per 2000 kcal or 8.4 MJ) ingested with 30 g of peanut or safflower oil, was 4.7% (Table 2), and ranged from 1.0 to 14.0%. Comparing the blood response to food phyllo- quinone with that of supplementary purified phylloquinone, the relative availability of phylloquinone ranged from 4.1% for boiled spinach (1000 µg phylloquinone) (Gijsberg, Jie, and Vermeer 1996), 13.3% for boiled spinach consumed with 25 g of butter (1000 µg phylloquinone) (Gijsberg, Jie, and Vermeer 1996), 17.4% for raw spinach (495 µg phyllo- quinone, 25% of energy from fat) (Garber et  al. 1999), and 63% for microwaved broccoli (377 µg phylloquinone) (Booth, Lichtenstein, and Dallal 2002). The higher relative availabil- ity of phylloquinone from cooked broccoli may be partly because of its higher extraction and loose association with the thylakoid membrane in chloroplasts (Booth, Lichtenstein, and Dallal 2002). With regards to the study of Garber et  al. (1999), that used a reference dose of 500 µg of pure phyllo- quinone, the amounts of phylloquinone from raw spinach (50 g), raw Romaine lettuce (200 g), and raw and cooked broccoli (159 g) were too low (165–214 µg phylloquinone) to be considered for calculating the relative availability of phyl- loquinone from these test vegetables. Based on the assump- tion that the bioavailability of supplementary purified phylloquinone is 80% (Shearer, McBurney, and Barkhan 1974), the absolute apparent availability of vitamin K was determined to be 13.9% for raw spinach, 3.3% for cooked spinach, 10.6% for cooked spinach consumed with fat, and 50% for cooked broccoli (Table 2). Comparison of vitamin K bioavailability between animal- and plant-based foods The bioavailability of phylloquinone (vitamin K1) in dark green vegetables has been estimated (Figure 1), though the results that ranged from 3.3 to 50% were conflicting. Some vegetable oils are also rich sources of phylloquinone. Animal-sourced food products contain low levels of phyllo- quinone, but liver can supply great amounts of menaqui- nones (vitamin K2). Measures of bioavailability of vitamin K in vegetable oils and animal-sourced foods have not been reported. A comparison of the bioavailability of vitamin K in animal- and plant-based foods cannot be made. Biotin Biotin absorption and utilization Biotin (vitamin B-7) is a water-soluble vitamin whose main essential function is as a coenzyme required for fat synthe- sis, branched-chain amino acid catabolism and gluconeogen- esis (Bates and Heseker 1994; IOM (Institute of Medicine, US) 1998; van den Berg 1997a; WHO and FAO 2004). Biotin is found in various natural food sources, and most of the biotin obtained from meat, cereals, yeast, legumes, and nuts, is bound to protein, and biotin occurring in vegetables, fruits, rice bran, and milk, is in the free form (Bonjour 1977; van den Berg 1997a; Zempleni and Mock 1999). A glycoprotein present in raw egg white, avidin, has a high 14 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN binding affinity to biotin and prevents biotin from being absorbed. However, avidin has no effect on biotin when the egg is cooked (Bonjour 1977; van den Berg 1997a; Zempleni and Mock 1999). Biotin is largely found bound to food protein. After its hydrolytic release from its protein-bound form, ingested free biotin is mainly absorbed in the jejunum through a satura- ble, sodium-dependent transporter (Said 1999, 2011). Absorbed biotin is transported in the blood to the liver, which is the major site of biotin utilization and metabolism (Said 1999, 2011). It has been suggested that free unbound biotin is also synthesized by microflora mainly in the colon, potentially by the stimulatory effect of fiber present in plant foods, and can be absorbed by the host (Bonjour 1977; Kopinski, Leiboholz, and Bryden 1989a; Said 1999, 2011, 2013; Scholtissek et  al. 1990; Yoshii et  al. 2019; Zempleni and Mock 1999). This is based on the observation that when biotin is directly injected into the colonic lumen, the plasma concentrations of biotin increase considerably (Kopinski, Leiboholz, and Bryden 1989a; van den Berg 1997a). Furthermore, the excretion of biotin in the urine and feces often exceeds the dietary intake of biotin by three to six times, indicating a significant degree of large intestinal microbial biotin synthesis (Bonjour 1977; Kopinski, Leiboholz, and Bryden 1989a, 1989b; Sauer, Mosenthin, and Ozimek 1988; Scholtissek et  al. 1990). Determination of the bioavailability of dietary biotin and supplementary biotin in purified form The bioavailability of biotin from dietary sources has been mainly determined based on ileal digestibility measures using the growing pig as an animal model for adult humans. In a pig study involving cannulated growing pigs, Sauer, Mosenthin, and Ozimek (1988) determined the endogenous biotin present in ileal digesta, and found that the true (corrected) ileal digestibility of biotin for synthetic purified biotin was 94%. Bioavailability of biotin in animal-sourced foods A pig study conducted by Kopinski, Leiboholz, and Bryden (1989b) found that meat meal (50 µg biotin) had an apparent ileal digestibility of biotin of 82% (Table 2), and milk casein (22 µg biotin) an apparent biotin ileal digestibility of 95% (Table 2). It is likely that the digestibility of biotin in these animal-sourced foods would be higher if correction was made for the GIT endogenous biotin, but this has not been reported. In the same study, based on the amount of biotin excreted in the urine and correcting for the 35 µg of biotin lost daily in the urine in biotin-deficient pigs of similar weight, the true utilization of biotin in meat meal was reported to be 92% (Kopinski, Leiboholz, and Bryden 1989b). Bioavailability of biotin in plant-based foods The apparent ileal digestibility of biotin in soyabean meal (130 µg biotin) was 12% in a pig digestibility study (Kopinski, Leiboholz, and Bryden 1989b). By accounting for an endogenous ileal biotin loss of 11 µg per kg dry matter, Sauer, Mosenthin, and Ozimek (1988) reported that the true (corrected) ileal digestibility of biotin in soyabean meal (110 µg biotin per kg dry matter) was 55% (Table 2). Misir and Blair (1988) used the regression method to relate doses of synthetic biotin to plasma responses in biotin-depleted newly-weaned pigs. Although plasma-based biotin values are difficult to interpret as they are con- founded by the uptake of microbially synthesized biotin, the data do suggest that the biotin digestibility value for soyabean meal may also be applicable to soya protein iso- late, commonly consumed by humans. Therefore, the ileal digestibility values for soyabean meal will be used in the current comparison of biotin bioavailability between animal and plant foods. A pig digestibility study found that the apparent ileal digestibility of biotin was 6% for wheat (var. Banks, 144 µg biotin), −3% for wheat (var. Egret, 125 µg biotin), 18% for barley (188 µg biotin), and −123% for sorghum (355 µg bio- tin) (Kopinski, Leiboholz, and Bryden 1989b). These low or sometimes negative values for the apparent ileal digestibility of biotin in plant-based foods may indicate that there is lit- tle uptake of biotin from these plant foods, or that there are relatively large amounts of endogenous biotin present in ileal digesta. Similarly, in the pig digestibility study of Sauer, Mosenthin, and Ozimek (1988), generally low apparent ileal and true (corrected) ileal digestibilities of biotin were found for plant-based foods. Based on the assumption that 11 µg per kg dry matter of endogenous biotin is excreted in ileal digesta, the true (corrected) ileal digestibility of biotin was 22% for wheat (123 µg per kg dry matter), 4.8% for barley (121 µg per kg dry matter), 4% for maize (54 µg per kg dry matter), and 3.9% for canola meal (513 µg per kg dry mat- ter) (Sauer, Mosenthin, and Ozimek 1988) (Table 2). By accounting for the urinary excretion of 35 µg of endogenous biotin, the true utilization of biotin in soybean meal was 30%, and 12, 2, 20, and −1, for wheat (var. Banks), wheat (var. Egret), barley, and sorghum, respectively (Kopinski, Leiboholz, and Bryden 1989b). Comparison of biotin bioavailability between animal- and plant-based foods The bioavailability of biotin in food sources has not been studied directly in humans, but there is some evidence for the ileal digestibility of biotin in studies involving ileal can- nulated pigs. By correcting the results of Kopinski, Leiboholz, and Bryden (1989b) for gut endogenous biotin (Table 2) and taking the data of Sauer, Mosenthin, and Ozimek (1988) (Table 2), together the ileal digestibility of biotin in meat and milk products appears high (82–95%), but in legumes, cereals and seed products much lower val- ues (4–55%) are found. It is concluded that in general, bio- tin is more bioavailable in animal-based foods (89%) than in plant-based foods (20%) (Figure 1). Nonetheless, esti- mates of biotin digestibility determined in pigs should be interpreted with caution as the absorption of colonic microbial synthesized biotin may influence the amount of biotin measured in the ileal digesta. CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 15 Folate Folate absorption and utilization Folate (folacin, vitamin B-9) belongs to the water-soluble group of B vitamins and is mainly biologically active as the mono-glutamate, with the chemical name pteroyl mono-glutamic acid (PteGlu) (Bates and Heseker 1994; Gregory 1989; IOM (Institute of Medicine, US) 1998; WHO and FAO 2004). Folate functions as a coenzyme involved in metabolic pathways associated with other B vitamins, partic- ularly as a methyl donor substrate with vitamin B-12 (Gregory 1989, 1997a, 2001; Krishnaswamy and Nair 2001; McNulty and Pentieva 2004; Ohrivk and Witthoft 2011; Witthöft et  al. 1999). Both vitamins are involved in the con- version of homocysteine to methionine (Gregory 1989, 1997a, 2001; Krishnaswamy and Nair 2001; McNulty and Pentieva 2004; Ohrivk and Witthoft 2011; Witthöft et  al. 1999). Folate also plays a role in the formation and normal functioning of red blood cells, the methylation of nucleic acids and the metabolism of amino acids (Gregory 1989, 1997a, 2001; Krishnaswamy and Nair 2001; McNulty and Pentieva 2004; Ohrivk and Witthoft 2011; Witthöft et  al. 1999). Folate, in the form of mono-glutamates or poly-glutamates, occurs naturally in a wide variety of foods, including meat, liver, milk, vegetables, and fruits (Gregory 1989, 1997a, 2001; Krishnaswamy and Nair 2001; McNulty and Pentieva 2004; Ohrivk and Witthoft 2011; Witthöft et  al. 1999). Some folate is lost in the heat treatment of foods (Gregory 1989; Krishnaswamy and Nair 2001; Witthöft et  al. 1999). As a public health measure aimed at children, girls and women of childbearing age, and pregnant women, who are more at risk of folate deficiency, cereals and grain food products are generally enriched with the synthetic form of folate, referred to as folic acid (IOM (Institute of Medicine, US) 1998; Krishnaswamy and Nair 2001; WHO and FAO 2004). The synthetic folic acid normally used in fortified foods and dietary supplements, occurs in the form of the chemically stable oxidized mono-glutamate. When food folate is ingested in the form of poly-glutamates, it is hydrolyzed by intestinal conjugase enzymes to mono-glutamates (Said 2011; Witthöft et  al. 1999). The presence of conjugase inhibitors and folate-binding proteins (FBP) in some foods may reduce the absorption of folate (Said 2011; Witthöft et  al. 1999). Mono-glutamates are primarily actively absorbed by a pH-dependent saturable mechanism in the jeju- num, and by passive diffusion in the ileum at higher intakes (Said 2011, 2013; Witthöft et  al. 1999). It is also thought that bacteria in the large intestine synthesize mono-glutamates but these are incompletely absorbed (Gregory 1989, 2001; Ohrivk and Witthoft 2011; Said 2011, 2013; Witthöft et  al. 1999; Yoshii et al. 2019). The major circulating form of folate is 5-methyl-tet- rahydrofolate and the main metabolic product of folate is acy- lated mono-glutamate, which is excreted in the urine (Gregory 1997a; Said 2011, 2013). Folate and folic acid content in food Dietary folate equivalents (DFEs) have been established to adjust for the lower bioavailability of food folate compared with that of supplementary folic acid consumed as part of a mixed diet, whereby 1 mg of DFE equals 1 mg of food folate, or 0.6 mg of folic acid from fortified food or as a dietary supplement consumed with food (IOM (Institute of Medicine, US) 1998; WHO and FAO 2004). Determination of the bioavailability of dietary folate and supplementary folate (folic acid) in purified form and in food There is much uncertainty associated with the forms (mono-glutamates or poly-glutamates) of folate present in food, the presence and concentrations of dietary components that may inhibit intestinal absorption of folate (conjugase enzymes, folate-binding proteins), folate analysis methods (high performance liquid chromatography or microbiological folate assay), and the end-point criteria used to determine bioavailability (plasma or serum folate, folate in red blood cells, urinary folate). The bioavailability of food folate and the synthetic folic acid has been determined in human stud- ies using isotopic labeling and the measurement of folate in the circulating blood and in the urine. Several human studies have determined the availability of food folate based on incremental folate excretion in the urine of human subjects who were in a folate saturated condition (Babu and Srikantia 1976; Devadas, Premakumari, and Moorthy 1979; Tamura and Stokstad 1973). It was assumed that for the folate-saturated individuals, absorbed incremental food folate would be com- pletely excreted, and folate bioavailability values were given. Often, the bioavailability of naturally occurring folate in food sources is derived in relative terms to a reference dose of supplementary synthetic purified folic acid. It has been assumed that the bioavailability of folic acid ingested as a dietary supplement under fasting conditions is 100% (Gregory 1997a; IOM (Institute of Medicine, US) 1998). Witthöft et  al. (2006) determined plasma concentrations of folate (5-methyl-tetrahydrofolate) and found that when a dose of the vitamin was given by intramuscular injection, plasma values were higher (but not statistically significantly so) than when the supplementary vitamin was given per os via a gelatin capsule after fasting. Based on measures of ileal folate excretion and plasma folate, the folate given in capsule form had an absorption of around 90% in healthy ileosto- mates, who were given folic acid supplements prior to the study and had optimal baseline levels of plasma folate (Witthöft et  al. 2006). This may indicate that even when supplementary folate is administered on an empty stomach, that it is not fully absorbed. When the supplementary vita- min was given along with food, the uptake was considerably lower (Witthöft et  al. 2006). When added to fermented milk, the plasma folate content was around 80% that of the sup- plement given after fasting (Witthöft et  al. 2006). The bio- availability of a folic acid supplement consumed with a light meal (0.40 mg folic acid and 0.14–0.15 mg food folate) was estimated to be 85% (Pfeiffer et  al. 1997). The relative bio- availability of folate in food (0.02–0.30 mg food folate) was found to be no more than 50% in a depletion-repletion human study (Sauberlich et  al. 1987). It is assumed here that the bioavailability of synthetic folate (folic acid) added to a food is 85%. 16 S. M. S. CHUNGCHUNLAM AND P. J. MOUGHAN Bioavailability of dietary folate in mixed diets In a human study measuring folate in serum as a percentage of ingested folate, Winkels et  al. (2007) determined the availability of folate following the consumption of a non-vegetarian mixed diet (0.37 mg food folate or DFE, with liver paste contributing 20%, boiled vegetables 29%, and fruits and fruit juices 38% of the total food folate content), relative to that of added synthetic folic acid. The relative availability of folate was 78% based on serum isotopically labeled (13C11) folate, and 85% based on changes in absolute serum folate concentrations. The overall availability of folate in the non-vegetarian mixed diet relative to that of folic acid from capsules taken with the meal, was around 82%. If it is assumed that the added folic acid was itself 85% bioavail- able, an absolute value for folate availability in the non-vegetarian mixed diet of 70% can be calculated. In a study by Brouwer et  al. (1999), the availability of folate in human participants provided with a mixed diet rich in folate (0.56 mg food folate or DFE) from vegetables (spin- ach, green peas, broccoli, brussels sprouts, green beans) and citrus fruits (oranges, tangerines), was compared to folate availability from a supplementary folic acid capsule con- sumed with a low-folate diet. The relative availability of folate in the vegetable and fruit mixed diet was estimated to be 78% based on changes in plasma concentrations of folate, and 98% based on folate levels in red blood cells. If a mean overall value of 88% is taken, and it is assumed that the supplementary folic acid is 85% bioavailable, an absolute value for folate bioavailability in the vegetable and citrus fruit mixed diet of 75% can be calculated. In another human study, Vahteristo et  al. (2002) found that the relative folate availabilities after consumption of a diet containing natural folates from rye-based products and orange juice (0.18 mg food folate or DFE) compared to a folic acid supplemented wheat diet, were 84% based on plasma folate, and 55% based on red blood cell folate. If a mean overall value of 70% is assumed, then an absolute value for bioavailability of folate in the rye and orange mixed diet of 60% can be calculated. Bioavailability of dietary folate in animal-sourced foods Using the measurement of folate in the urine of folate-saturated human subjects, Babu and Srikantia (1976) found that the bioavailability of folate in goat liver