Calorie restriction has been demonstrated to increase the life span and decrease the age-associated morbidity of many experimental animals. Increases in longevity or reductions in age-associated morbidity have also been shown for model systems where protein or specific amino acids have been reduced. In particular, experiments in model systems in rats, mice, and Drosophila fruit flies have shown increases in life-span with reduced protein intake comparable to that for calorie restriction. Restriction of the amino acid methionine, which is required to initiate protein synthesis, is sufficient to extend lifespan. Restriction of the branched-chain amino acids is sufficient to the extend the lifespan of Drosophila fruit flies and male mice.