Protein quality in the New Zealand vegan diet : challenges and solutions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutritional Sciences at Massey University, Palmerston North, New Zealand

Abstract

The global shift toward plant-based dietary patterns has intensified interest in the nutritional adequacy of vegan diets, particularly regarding protein quality. Plant proteins are often considered poorer sources of several indispensable amino acids (IAAs), which are essential for metabolic function in the human body, and must be obtained from the diet. The lower digestibility and less balanced amino acid profiles of many plant-based foods compared to animal-sourced foods raise concerns regarding the adequacy of utilisable protein in individuals consuming fully plant-based diets. Although total protein intake in vegan populations is frequently reported to be sufficient, protein intake and protein quality are rarely assessed using digestibility-adjusted measures. Consequently, uncertainty remains regarding the capacity of vegan diets to meet protein requirements. Overall, this thesis aimed to 1) evaluate protein intake and protein quality in habitual vegan diets in New Zealand (NZ), and 2) develop practical and data-driven strategies to improve protein and IAA adequacy within realistic dietary constraints. Baseline protein intake and protein quality were assessed with four-day food diaries collected from 193 adult vegans in NZ. Protein content of each food was derived from the NZ FOODFiles™ food composition database, while IAA content for each food was obtained from the most appropriately matched food in the USDA food composition database. To reflect the portion of food that provides utilisable protein and IAA, the true-ileal digestibility coefficients for available foods and available IAA were applied. Although total protein intake met minimum daily requirements for most participants, lysine and leucine were identified as the most limiting IAAs, with nearly half of the cohort at risk of inadequate intake when digestibility was considered. To further characterise dietary patterns, time-series and clustering analyses were applied to examine meal-level protein intake. Uneven protein distribution was observed across the day, with the emergence of distinct dietary clusters stratified by protein quantity and protein quality. Further meal composition analysis was undertaken to examine how the combination of different plant food groups influences protein quality, as measured by the Digestible Indispensable Amino Acid Score (DIAAS). Eight distinct meal clusters with varying protein quality were observed. Overall, the baseline diets of vegans indicate risks of deficiency in protein quality, particularly for lysine, leucine and the sulphur-amino acids. Dietary interventions will require the use of diverse plant foods and ingredients in complementary proportions to meet individual IAA requirements, while balancing energy consumption and dietary acceptability. These findings informed the development of diet optimisation frameworks designed to explore food combinations at both the daily and meal level to address individual shortfalls in protein and IAA intake. Most plant foods contain all IAAs in varied amounts. However, legumes and pulses (including legume-based protein isolates), nuts and seeds were particularly important contributors to protein quality, with many foods from these groups providing higher IAAs at a lower caloric cost than other food groups, such as grain-based foods, fruits and starchy vegetables. When plant protein sources were combined to achieve high protein quality, as assessed by the DIAAS, the optimisation models prove that vegan diets can theoretically meet protein quality requirements without necessitating excessive food intake or energy consumption, thus countering the assumption that plant-based diets are intrinsically limited in protein quality. Dietary patterns influencing protein quality may also affect the overall nutritional adequacy of vegan diets. Considering that some micronutrient requirements may be even more difficult to achieve in vegan diets without the use of fortified foods and supplementation, future research in plant-based diets should examine strategies that jointly optimise protein quality and micronutrient adequacy.