Dietary protein, especially leucine-rich sources like whey, can induce muscle protein synthesis without exercise.

“dietary protein, in particular, dietary proteins that contain high amounts of leucine, like whey protein, will help induce so-called muscle protein synthesis, which is generally good for us, again, and it's occurring even if we're not exercising, if we are exercising, especially if we're exercising very hard, and in particular, if we're doing a lot of resistance training, or frankly, any amount of resistance training that's taken near or to failure.”