Whey's edge over soy disappears once the leucine is matched
Nine months matched on total protein: whey put on 3.3 kg of lean mass against soy's 1.8 kg. Twelve weeks matched on leucine, and the two were indistinguishable.
What the study found
Whey beat soy on grams, and tied it on matched leucine
Nine months of supervised training, matched on total protein: whey put on 3.3 kg of lean mass against soy's 1.8 kg. Twelve weeks matched on leucine, the amino acid that triggers muscle building, and the two were indistinguishable. Whey's edge looks like a leucine edge.
The caveat · Beginners gain from training alone
Both trials used people who were not training beforehand, who gain readily from the training alone and so make group differences hard to see. In the nine-month trial everyone ate 1.1 to 1.4 g per kg a day, below the level where gains flatten, so it may be measuring a shortfall corrected. Leucine-matching meant unequal grams, 26 g of soy against 19 g of whey. Whey-industry funding is common across this literature.
What we don't know
Whey beat soy matched on grams, tied matched on leucine, pointing at leucine. That stays open because in the grams trial soy gained less lean mass than a carbohydrate control, an unlikely result, so its gaps may be noisier than they look.