Whey protein supplementation during resistance training augments lean body mass 

This randomized, controlled trial looked to compare the effects of whey protein or soy protein or carbohydrates on muscle mass. Both men and women were randomized into groups to receive either carbohydrate-based supplementation, whey protein or soy proteins, while all completing a supervised, whole-body, periodized resistance training program consisting of 96 workouts for 9 months.   

The results show that lean body mass gains were significantly greater in the whey supplementation group, compared to the carbohydrate and soy supplemented groups. Fasting leucine concentrations appeared to be positively correlated with lean body mass responses. Whey protein is generally higher in leucine content than soy protein, therefore it is not surprising that the whey protein supplemented group showed a significant improvement in lean body mass responses to resistance training.

References:

Volek, J. S., Volk, B. M., Gómez, A. L., Kunces, L. J., Kupchak, B. R., Freidenreich, D. J., Aristizabal, J. C., Saenz, C., Dunn-Lewis, C., Ballard, K. D., Quann, E. E., Kawiecki, D. L., Flanagan, S. D., Comstock, B. A., Fragala, M. S., Earp, J. E., Fernandez, M. L., Bruno, R. S., Ptolemy, A. S., Kellogg, M. D., … Kraemer, W. J. (2013). Whey protein supplementation during resistance training augments lean body mass. Journal of the American College of Nutrition, 32(2), 122–135. https://doi.org/10.1080/07315724.2013.793580