Protein is essential for muscle protein synthesis and improvements in lean body mass and strength. An excellent literature review by Christine Rosenbloom from Georgia State University concluded that the best protein sources contain the essential amino acids (amino acids only available in foods). Proteins vary in the rate that they enter the bloodstream, taste, expense, and accompanying substances. Milk protein contains fast (whey) and slow (casein) proteins, so it triggers protein synthesis rapidly and provides amino acids for post-exercise repair.
Whey is the fastest protein source, and it may improve immune and antioxidant function and decrease the risk of cardiovascular disease and high blood pressure. Soy is a high-quality protein, but contains plant estrogens that can influence the metabolism of testosterone and estrogen. Egg protein contains all the essential amino acids and 13 other essential nutrients, but is high in cholesterol. Raw eggs can contain salmonella that can cause severe illness or death.
Specific amino acids can be valuable for boosting nitric oxide to increase blood flow (arginine), improving immune function (glutamine), and supplying fuels during endurance exercise (branched-chain amino acids). Active people need 1.2 to 1.7 grams of protein per kilogram bodyweight. Protein appears to promote lean muscle mass best when taken shortly after workouts. (Nutrition Today, 44: 204-210, 2009)