The present invention relates to dietary compositions comprising reduced levelof methionine, tryptophan, allamino acids, or protein, dietary compositions comprising glycerol as asubstitute for monosaccharides, disaccharides, andpolysac-charides, and hypocaloric or calorie free diets with reduced level ofenergy, carbohydrates, or protein. Also disclosed are methodsof using these compositions and diets, as well as fasting, to protect subjectsagainst chemotherapy, radiotherapy, oxidative stress,or aging.