Macro partitioning
In elderly men, 20g of soy protein isolate fails to stimulate myofibrillar protein synthesis (MPS) at rest or after resistance exercise, whereas 40g only stimulates MPS post-exercise, while equivalent doses of whey protein stimulate MPS in both conditions.
For elderly men, 20g of soy protein is insufficient to build muscle, whether you exercise or not. You need 40g of soy protein, and even then, it only works if you perform resistance exercise. If you want to maximize muscle growth with less protein or without exercising, whey protein is a more effective source due to better leucine availability and lower oxidation.
Rates of MPS for S20 were less than W20 (P = 0.02) and not different from 0 g (P = 0.41) in both exercised and non-exercised leg muscles. For S40, MPS was also reduced compared with W40 under both rested and post-exercise conditions (both P < 0.005); however S40 increased MPS greater than 0 g under post-exercise conditions (P = 0.04).
Why this rating
Randomized controlled trial with isotope tracing (gold standard for MPS measurement) in a specific, well-characterized elderly cohort.
Source
Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men
Yifan Yang et al. · Nutrition & Metabolism · 2012
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