Daily protein intake of 1.4 g/kg body mass from whey or soy supplementation during 9 months of resistance training results in greater increases in lean body mass compared to 1.1 g/kg from carbohydrate supplementation, and total protein intake above 1.1 g/kg is necessary for optimal muscle adaptation.

When someone consumes whey protein after lifting weights, the body breaks it down quickly and releases a large amount of leucine into the blood. This leucine signals muscle cells to turn on a key growth pathway that tells the cells to build more proteins. Over time, this causes muscle fibers to get bigger and increases the total amount of lean tissue in the body.