The Doctor's Corner

Q. How does Soy Protein stack up against Whey Protein?

A. It has been conclusively demonstrated that the augmentation of muscle protein synthesis that can be recorded after an acute bout of high-intensity resistance exercise can be synergistically augmented by consuming a nutritionally complete source of protein (especially one enriched with essential amino acids and leucine in particular) near the time of the workout. Although reports exist that demonstrate such augmentation when a maximally stimulatory dose of protein is ingested before training, the vast majority of existing literature suggests that the optimal timing for delivery of the protein dose is immediately after the workout. The nuance of these responses has to do with the following: Recognition of the fact that the intersection of the loading (or stretch) stimulus and the nutritional input most likely occurs at a control node in muscle cells called the mammalian target of rapamycin (mTOR) (so named because of early studies demonstrating the ability of rapamycin to disrupt this signaling cascade) and that the amino acid leucine plays a pivotal role in gating the responses, leading to enhanced synthesis of muscle protein. Both effects appear within the first hour after exercise and continue for at least three hours after the exercise session is completed. The response is truly synergistic; that is, the total response is greater than the simple arithmetic sum of the two independent stimuli. Although leucine is believed to interact with specific components of the mTOR complex to activate downstream pathways that enhance the efficiency of the protein synthetic machinery of muscle cells, other factors are brought to bear on the duration of the response. And it has been clearly demonstrated that leucine in isolation isn’t sufficient to realize actual production of stable new muscle proteins in the absence of additional amino acid protein precursors. Hence, leucine must be delivered within the envelope of nutritionally “complete” protein sources with PDCAA scores greater than 1.0. As discussed above, the post-workout metabolic responses to supplemental protein (in this case fractional rates of muscle protein synthesis) depend critically on the absorption kinetics of the ingested protein source. A multitude of congruent clinical studies have shown that protein that delivers rapid assimilation kinetics (and thus bumps in the plasma levels of leucine in particular) produces higher amplitude responses with respect to the fractional rate of muscle protein synthesis in human subjects. Because soya and milk proteins share these common attributes, both dietary sources would qualify as bona fide candidates for post-workout supplementation strategies. However, currently available literature clearly defines better results with dairy-based proteins than with soya when comparing isonitrogenous doses of proteins head to head with one another in a post-workout administration protocol. Averaged over 12 weeks of training, these differences in newly acquired muscle tissue are significant. The reasons for the superiority of dairy-based protein supplements to soya haven’t been precisely defined but may be related to higher concentrations of leucine in these proteins than in soya, or perhaps the known inhibitory effect of soya phytoestrogens (particularly genistein) on IGF-1, a known myogenic regulatory factor.

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