The amino acid leucine and whey protein consumed following weight training trigger increases in muscle protein synthesis that promote muscle mass and strength. Endurance training does not cause increases in muscle mass, but it does cause tissue damage that must be repaired during recovery. David Rowlands from Massey University in New Zealand, and colleagues, in a very sophisticated study, found that consuming five grams of leucine and 25 grams of whey protein after 100 minutes of intense cycling promoted muscle protein synthesis, which would help speed recovery and promote adaptation to exercise and improved fitness. Supplements containing leucine and protein consumed after exercise are good for bodybuilders and endurance athletes.
Source: Medicine Science Sports Exercise, 47: 547-555, 2015