Protein is important after exercise to switch protein balance from negative to positive. Which protein is best at turning the switch on is the question. Scientists pitted protein sources against each other in the same study.
Canadian researchers compared the most popular sources of protein—whey, casein and soy—on their ability to promote muscle protein synthesis after exercise. Trained men consumed 22g of whey isolate, casein or soy isolate following a bout of resistance exercise. The results were clear that the rates of muscle protein synthesis over the three-hour post-exercise period were about two-fold higher after taking whey compared to casein, whereas soy ranked between whey and casein. The authors attributed the superiority of whey to its greater content of leucine—2.3g—compared to casein and soy—1.8g. Leucine levels in the blood over the three-hour period were 73% greater than soy and 200% greater than casein.
These findings indicate that whey maximizes muscle protein synthesis during the three-hour period after exercise because of its higher leucine content, and rapid digestion and absorption. Past three hours, casein and soy may be better due to their slower digestion rate and anti-catabolic (muscle-sparing) qualities.