Proteins differ based on their source, amino acid profile, and the methods of processing or isolating the protein undergoes [11]. These differences influence the availability of amino acids and peptides, which may possess biological activity (e.g., α-lactalbumin, ß-lactoglobulin, glycomacropeptides, immunoglobulins, lactoperoxidases, lactoferrin, etc.). Additionally, the rate of digestion and/or absorption and metabolic activity of the protein also are important considerations [91]. For example, different types of proteins (e.g., casein, whey, and soy) are digested at different rates, which may affect whole body catabolism and anabolism and acute stimulation of muscle protein synthesis (MPS) [91,92,93,94,95,96]. Therefore, care should be taken not only to make sure the athlete consumes enough protein in their diet but also that the protein is high quality. The best dietary sources of low fat, high quality protein are light skinless chicken, fish, egg whites, very lean cuts of beef and skim milk (casein and whey) while protein supplements routinely contain whey, casein, milk and egg protein. In what is still an emerging area of research, various plant sources of protein have been examined for their ability to stimulate increases in muscle protein synthesis [77, 97] and promote exercise training adaptations [98]. While amino acid absorption from plant proteins is generally slower, leucine from rice protein has been found to be absorbed even faster than from whey [99], while digestive enzymes [100], probiotics [101] and HMB [102] can be used to overcome differences in protein quality. Preliminary findings suggest that rice [98] and pea protein [103] may be able to stimulate similar changes in fat-free mass and strength as whey protein, although the reader should understand that many other factors (dose provided, training status of participants, duration of training and supplementation, etc.) will ultimately impact these outcomes and consequently more research is needed.
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