Amino acid metabolism in health and disease Signal Transduction and Targeted Therapy Figure 3 from Glutathione synthesis. Semantic Scholar Our ancestors ate whole animals. Skin, cartilage, tendons, bones, organs. That meant they consumed massive amounts of glycine from collagen rich tissues. Modern humans eat almost exclusively muscle meat, which is extremely high NAC [N AcetylCysteine] is a precursor of Glutathione. PICTURE: Glutathione is made from three amino acids and cysteine is one of them