High-protein meals stimulate protein synthesis, even when you are not weight training. Including protein with each meal is important for athletes and for older adults— who are more susceptible to sarcopenia (muscle loss).
Dutch researchers found that whey protein stimulated post-meal protein synthesis better than casein or casein hydrolysate in older men. Whey protein is digested and absorbed faster than other proteins and has a high leucine content. Leucine activates the mTOR pathway, which is a series of chemical reactions in cells that initiates protein synthesis. Whey protein is an important dietary component in athletes trying to increase muscle hypertrophy and in older adults trying to maintain muscle mass. (American Journal of Clinical Nutrition, 93: 997-1005)