Journal Articles

This review discusses different protein sources and their role in human nutrition, focusing on their structure, digestibility, and bioavailability. Plant-based proteins, such as those found in legumes, nuts, and seeds, may contain anti-nutritional factors that impact their bioavailability apart from structural and compositional differences from animal proteins. Animal proteins are generally highly digestible and nutritionally superior to plant proteins, with higher amino acid bioavailability. Alternative protein sources are also processed in different ways, which can alter their structure and nutritional value, which is also discussed.

Both Hayward (green) and SunGold (gold) kiwifruit varieties contain a proteolytic enzyme, actinidin, that has been reported to enhance the upper tract digestion of animal proteins. Unlike the other gold varieties, which do not contain any actinidin, the SunGold variety contains significantly higher actinidin activity, but its activity is still much lower than that present in the green (Hayward) fruit. The objective of this study was to determine the effectiveness of actinidin in Hayward and SunGold kiwifruit in digesting alternative proteins, including pea protein, almonds, tofu, and quinoa. The protein sources were digested using a three-stage in vitro oral-gastro-small intestinal digestion model. The findings showed that both kiwifruit extracts enhanced the breakdown (observed through SDS-PAGE) for all the studied protein sources, particularly during gastric digestion, possibly due to higher actinidin activity at gastric pH. The increase in the rate of protein breakdown was probably due to the broader specificity of actinidin compared to pepsin. For many protein sources, most of the intact proteins disappeared within the first few minutes of gastric digestion with added kiwifruit extract. Green kiwifruit extract, due to its higher actinidin activity, had a higher effect on protein breakdown than the SunGold extract. However, for some proteins and under certain digestion conditions, SunGold extract resulted in higher protein breakdown. The latter, in the absence of any digestive enzymes, also led to some protein breakdown during the small intestinal digestion phase, which was not the case for the green kiwifruit extract. The green kiwifruit extract led to the greater breakdown of polypeptide chains of Pru-du 6, a major allergen in almonds. The results, for the first time, suggest that both Hayward and SunGold kiwifruit can lead to improved breakdown and digestion of alternative proteins when consumed as part of a meal; and therefore, have the potential to be used as a digestive aid in population groups looking to achieve faster and greater protein digestion such as athletes, elderly and people with the impaired digestive system.

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