Insights and gaps on protein digestion
Introduction
Foods are increasingly associated with different health outcomes. Some foods are associated with the prevention of disease while others are associated with increasing risk factors for the onset of disease. In particular, there has recently been a movement away from highly processed foods towards so-called natural food [1]. This has been driven, at least in part, by recent reports that consumption of highly processed foods is associated with non-communicable diseases such as type 2 diabetes and cardiovascular disease [2,3] or appetite and obesity [4•]. Using a survey of over 9000 people, results suggested that ultra-processed foods are less satiating because they contain less protein, which might partially explain the increase seen in energy intake. In particular, the NOVA classification system for food processing based on the nature, extent and purpose of food processing led to the development of the term ultra-processed. This was defined as ‘industrial formulations manufactured from substances derived from foods or synthesized from other organic sources. They typically contain little or no whole foods, are ready-to-consume or heat up, and are fatty, salty or sugary and depleted in dietary fibre, protein, various micronutrients and other bioactive compounds.’ The reason that food processing is important in relation to non-communicable disease (NCD) is because it alters the composition and most importantly the multi-scale structure of the food. This in turn affects bioaccessibility and bioavailability of the macronutrients and bioactives within those foods. More specifically, food structure effects digestion and absorption kinetics and it is largely the kinetics, that represents the risk factor for NCD. However, because processing effects the release kinetics, this includes release of molecules that are also important for palatability and regardless of the health implications of a food; palatability is one of the primary drivers of food consumption.
Of the three macronutrients protein, fat and carbohydrate, protein appears the most nutritionally benign. From a nutritional perspective, both lipid and now increasingly sugars have been associated with non-communicable diseases (NCDs), while protein is still seen as healthy. There have been more than 300 articles published on ‘protein digestion’ in the last two years alone. These articles have tended to focus on some very specific issues related to either potential health benefits, or detriments or issues associated with digestibility more specifically. The public perception of protein is that it is healthy and is linked to sport. Perhaps this explains why searches for the word protein on Google© show such periodicity (Figure 1). There is a consistent peak immediately after Christmas and then a steady decline over the year and a marked drop in late December. Finally, although there is still significant research on animal based proteins as they are currently the mainstay of the food industry, there is a rise in the amount of research looking at plant proteins. This trend is only likely to increase in the future as the need to lower carbon emissions increases.
In this review, I will endeavour to provide some insights into the research that is currently being undertaken in protein digestion and also comment on some of the gaps where research is still to be undertaken.
Section snippets
Factors affecting protein digestion
Events at many different length-scales affect protein digestion as indicated in Figure 2. At larger scales, gastric behaviour such as the clotting of proteins or the phase separation of lipid can alter digestion patterns and kinetics [5,6•]. The latter article shows the effect of conventional milk processing on digestion kinetics using a validated semi-dynamic in vitro simulation of upper gastrointestinal tract digestion, with similar effects seen Ye et al. [7]. Gastric emptying is one of the
Food allergy and digestion
There is a hypothesis that one of the properties of protein that increases its propensity to be an allergen is its resistance to digestion or at least its structural stability [28]. Although this hypothesis has had doubt cast upon it in a well-argued article from the Infogest and impARAS networks [29••], it has become common to assess the digestibility of allergenic proteins under a range of conditions [30]. Given that both primary sequence and protein folding are involved, studies have looked
Health and protein digestion
The general recommendation for protein consumption is that people involved in a general fitness program, should be ingesting 0.8–1.0 g/kg/day of protein while older individuals may benefit from a higher protein intake (e.g. 1.0–1.2 g/kg/day) in order to help prevent sarcopenia. However, this is only part of the story. In two recent articles by Corrochano et al., the authors look at specific health benefits of bovine whey proteins [41,42]. Using an in vitro model of digestion and absorption, they
Bioactive peptides
An extension to the health effects of proteins is the bioactivity of specific peptides or hydrolysates. Bioactive peptides have been a topic of intensive investigation for a number of years and there is sufficient literature to warrant a review on its own. A number of recent reviews highlight particular issues such as the need to find cost-effective ways to produce the peptides at industrial scale. Additionally and perhaps more important, is the lack of well-designed clinical trials to provide
Appetite and the role of protein
The role of protein in appetite suppression is often thought to be well understood. However, this is often not the case when tested in vivo [59] as this study by Nguo et al. shows. However, the use of a small dose of whey protein co-ingested with meals to improve postprandial glycaemia and suppress appetite has been shown to be effective in men with type 2 diabetes [60]. A similar study was able to show that whey proteins could reduce appetite and stimulate anorexogenic gastrointestinal
Gaps
Although there are some interesting areas of research relating to dietary protein that are driven by the wider strategic drivers for food research, namely safety, health and sustainability, there are still some areas where our knowledge is still incomplete. The readout from digestion studies of proteins is often LC–MS, and the statistical analysis associated with the large data sets generated is still problematic. The use of artificial intelligence (AI) and machine learning to process large
Conclusions
Although this brief review cannot hope to cover all of the research concerning protein digestion, I have attempted to give some insights. An area that I have not included because there are many other good reviews available already is the use of protein for encapsulation in functional foods and nutraceuticals. Despite this omission, it is clear that protein digestion is a vibrant topic of research with significant effort being made to make protein-based foods healthier and more sustainable. Our
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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Comparison of tropomyosin released peptide and epitope mapping after in vitro digestion from fish (Larimichthys crocea), shrimp (Litopenaeus vannamei) and clam (Ruditapes philippinarum) through SWATH-MS based proteomics
2023, Food ChemistryCitation Excerpt :It also has been reported that in comparison with allergenic epitopes from shrimp-TM, those from clam-TM showed high cross-reactivity (>80 %) and those from fish-TM showed low cross-reactivity (<20 %) based on several shrimp-allergenic patients’ sera and shrimp-TM immunized mouse sera (Xu, Chen, et al., 2020). It is well known that the immunogenicity of protein allergens could be influenced by gastrointestinal (GI) digestion (Mackie, 2020; Zhang, Zhang, Chen, & Zhou, 2018). After being cut by various enzymes (salivary amylase, pepsin, trypsin, etc.), numerous protein fragments have generated and those with potential antigenicity could pass intestinal epithelium and be taken up by immune cells, including dendritic cells, lymphocytes, mast cells, etc., to trigger anaphylactic reactivity (Guo et al., 2021; Pekar, Ret, & Untersmayr, 2018).
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