Regulation of postprandial protein metabolism after food ingestion and exercise

Whole body and skeletal muscle mass is regulated by counterbalanced fluctuations between protein synthesis and breakdown rates. Protein ingestion has been shown to stimulate protein synthesis rates and thus is an important variable within the net protein balance equation dictating a positive balance. However, postprandial protein metabolism has been almost exclusively characterized on the ingestion of isolated protein fractions (e.g., dairy-based whey and casein and plant-based soy) in liquid beverages, while the majority of protein in the diet is obtained from whole food sources with other macronutrients available. The purpose of this dissertation was to improve the understanding of postprandial protein metabolism after the ingestion of protein-dense whole foods during resting and post-exercise recovery conditions in both healthy and diseased individuals. Previous work suggested a short lived stimulation of postprandial muscle protein synthesis rates after protein ingestion. However, these findings were made after ingestion of fast-digested whey. In study 1, we showed that ingestion of a natural matrix of milk protein (20 % whey/80 % casein), resulted in a sustained release of dietary amino acids in circulation that coincided with prolonged postprandial elevations in intramuscular anabolic signaling and rates of muscle protein synthesis. In study 2, we demonstrated that production of labeled food proteins can be done in a cost-effective manner by supplementing the feed of laying hens with an amino acid tracer. The labeled eggs were used as a research tool in studies 3 and 4 to assess protein digestion and absorption kinetics after nutritional stimuli in health and disease. In study 3, we found that the ingestion of egg whites resulted in more rapid appearance of dietary amino acids in circulation when compared to isonitrogenous whole egg ingestion, with no differences in total dietary amino acid availability in circulation between the egg conditions. However, we did observe that whole egg ingestion resulted in a more potent postprandial muscle protein synthetic response when compared with egg white ingestion. In study 4, we assessed the postprandial protein metabolic responses to mixed meal ingestion in maintenance hemodialysis (MHD) patients. We observed that basal rates of muscle protein synthesis were ~ 2.5 fold elevated in MHD patients when compared to controls; suggesting prolonged post-dialysis hypercatabolism. Furthermore, protein-dense mixed meal ingestion failed to stimulate postprandial rates of muscle protein synthesis in MHD patients. This observation may be attributed to reduced dietary amino acid availability after eating a meal. Overall, the studies performed as part this dissertation show that protein dense food ingestion is effective to stimulate the postprandial muscle protein synthetic response in healthy individuals. Moreover, it seems that clinical strategies to improve skeletal muscle health should involve a combination of increased physical activity and protein-dense whole food ingestion to maximize the muscle anabolic response with health and disease.