Dietary Protein to Support Anabolism with Resistance Exercise in Young Men

Resistance exercise is fundamentally anabolic and as such stimulates the process of skeletal muscle protein synthesis (MPS) in an absolute sense and relative to skeletal muscle protein breakdown (MPB). However, the net effect of resistance exercise is to shift net protein balance (NPB ϭ MPS Ϫ MPB) to a more positive value; however, in the absence of feeding NPB remains negative. Feeding stimulates MPS to an extent where NPB becomes positive, for a transient time. When combined, resistance exercise and feeding synergistically interact to result in NPB being greater than with feeding alone. This feeding-and exercise-induced stimulation of NPB is what, albeit slowly, results in muscle hypertrophy. With this rudimentary knowledge we are now at the point where we can manipulate variables within the system to see what impact these interventions have on the processes of MPS, MPB, and NPB and ultimately and perhaps most importantly, muscle hypertrophy and strength. We used established models of skeletal muscle amino acid turnover to examine how protein source (milk versus soy) acutely affects the processes of MPS and MPB after resistance exercise. Our findings revealed that even when balanced quantities of total protein and energy are consumed that milk proteins are more effective in stimulating amino acid uptake and net protein deposition in skeletal muscle after resistance exercise than are hydrolyzed soy proteins. Importantly, the finding of increased amino acid uptake would be independent of the differences in amino acid composition of the two proteins. We propose that the improved net protein deposition with milk protein consumption is also not due to differences in amino acid composition, but is due to a different pattern of amino acid delivery associated with milk versus hydrolyzed soy proteins. If our acute findings are accurate then we hypothesized that chronically the greater net protein deposition associated with milk protein consumption post-resistance exercise would eventually lead to greater net protein accretion (i.e., muscle fiber hypertrophy), over a longer time period. In young men completing 12 weeks of resistance training (5d/wk) we observed a tendency (P ϭ 0.11) for greater gains in whole body lean mass and whole as greater muscle fiber hypertrophy with consumption of milk. While strength gains were not different between the soy and milk-supplemented groups we would argue that the true significance of a greater increase in lean mass that we observed with milk consumption may be more important in groups of persons with lower initial lean mass and strength such as the elderly. Key teaching points: • Resistance exercise is fundamentally anabolic; as such it stimulates MPS which pushes muscle NPB in a more positive direction. • Muscle NPB becomes positive, only when amino acids are provided to muscle (i.e., protein or amino acids are consumed). • After resistance exercise, the consumption of protein results in an increase in MPS that is greater than consumption of protein alone; this is due to synergistic stimulation of MPS by amino acids and exercise, which appear to be acting through different signalling pathways. • Over time the synergistic combination of amino acid feeding and resistance exercise results in accretion of muscle proteinsmuscle hypertrophy. • Protein source acutely affects muscle amino acid uptake and NPB following resistance exercise in a manner that appears to be related not to amino acid composition but to the pattern of amino acid delivery to peripheral tissues. In this regard, milk proteins are more effective at supporting protein accretion than are soy proteins. • The ability of milk to support muscle protein accretion may have greater relevance in populations with compromised muscle mass