The investigation of protein-rich whole-food sources within protein metabolism

Abstract

All tissues within the human body are made up of proteins, which are in a constant state of turnover to maintain protein homeostasis. From the body’s protein pools, skeletal muscle provides the largest reservoir of amino acids, which in the absence of nutrition, serves as one of the main regulators of whole-body metabolism. Whilst this is crucial for the maintenance of protein homeostasis, during a fasted state, it comes at an expense of a loss of muscle mass, which is of concern when skeletal muscle is a major dictator of overall quality of life, being critical for human locomotion and postural maintenance. Therefore, understanding how the food we eat influences whole-body and skeletal muscle metabolism is of great importance to regulate and maintain muscle mass. Our current understanding of how nutrition regulates whole-body protein metabolism and skeletal muscle metabolism has been primarily derived from the investigation of isolated (supplemental) protein. However, what is less well known, is the effect of protein in whole-food form. This is concerning, when the majority of daily food and protein intake is consumed through cooked whole-foods, although as yet, this has not been well documented. In addition, the relatively little data we have on whole-foods has suggested non-protein components within the whole-food matrix may play a role in potentiating the anabolic response to feeding. Granted this has only been suggested in a few whole-food sources, with supporting data largely determined by in vitro work or animal models. Accordingly, the purpose of this thesis is to provide a comprehensive investigation into the role of protein-rich whole-foods in protein metabolism. This approach involves characterising the consumption of protein-rich whole-foods in habitual diets. It considers potential anabolic non-protein components within whole-foods and investigating the effect that whole-food ingestion has on post resistance exercise whole-body protein turnover and muscle protein synthesis rates. Firstly, this thesis characterised how dietary protein is consumed in habitual diets of recreationally active and resistance trained young adults. With a focus primarily on the source (animal or non-animal) and form (whole-food or supplemental [isolated]) of protein consumed throughout the day and at different meal moments. Furthermore, this thesis demonstrates that the vast majority of protein is consumed in whole-food form, with the largest contribution from animal derived sources. This is true for both recreationally active, and resistance trained individuals. Although resistance trained individuals are more likely to consume supplemental protein at snack moments. With this in mind, more mechanistic studies are required to investigate protein-rich whole-foods from a range of sources and their effect on protein metabolism, given current dietary protein guidelines are underpinned primarily from studies investigating isolated (supplemental) animal derived proteins. As such, this subsequently transitions to applying a targeted metabolomic approach to determine non-protein components within a range of protein-rich whole-foods, that have been considered to have an anabolic potential. The foods included within this analysis were; whole egg, pork, salmon, lentils, mycoprotein and egg whites (more isolated less nutrient dense). With an interest in the effect of cooking, this thesis investigated how the metabolomic profile of foods differ between their raw and cooked forms, using a water bath to cook each food source. From the targeted metabolites detected, 22 out of 24 (in the raw form; 92%) and 23 out of 24 metabolites (in the cooked form; 96%) differ between foods. The number of metabolites that either increase or decrease with cooking varies across foods. Large differences at the nutritional and metabolite level between food sources indicate the diverse range of additional components, other than protein, within a whole-food matrix. The identification of potential anabolic non-protein components within a range of whole-foods provides the necessary step in order to examine why certain whole-foods may provide a more (or less) robust anabolic potential. This work is developed further by demonstrating a novel finding that the ingestion of a variety of whole-food sources (0.25 g protein per kg of body mass) stimulates postexercise whole-body protein and myofibrillar protein synthesis to a similar extent compared to a more isolated protein (egg whites). This is in spite of a large variability in plasma essential amino acid kinetics, specifically leucine, between food sources. These data contribute to the growing narrative that the key factors determining postprandial handling of protein, to produce a robust protein synthetic response, are not considered as important when ingesting protein in whole-food form. Collectively this thesis demonstrates that the majority of protein in young adults is consumed in whole-food form, which possess potential anabolic non-protein components within their matrices, and when consumed, have the ability to robustly stimulate acute postprandial whole-body and muscle protein synthesis rates. Contained within this thesis, is a novel body of work utilising three unique approaches to provide a comprehensive analysis of the role of whole-foods in protein metabolism.