Whey protein is a very high quality source of protein that,
along with casein, are the major proteins in milk. It has all
the essential amino acids and therefore scores very high on all
methods used to score protein quality. Whey protein is rich in
the amino acid leucine that promotes protein synthesis. Whey
also contains a healthy dose of cysteine that increases
synthesis of the antioxidant glutathione, a major fighter of
free radicals. Other research indicates that whey can decrease
appetite and improve body composition by enhancing lean muscle
tissue and decreasing fat. A recent report found that whey
protein increased glycogen levels in both the liver and skeletal
muscle of exercise-trained rats more so than other types of
protein. Glycogen is a crucial energy source to perform high
intensity exercise.