Protein restriction may trigger some of the same protective responses without requiring a reduction in total calories. Protein restriction also raises FGF21 in people, but researchers have not yet shown that this response extends human lifespan. The review identifies reduced mTORC1 activity and increased cellular stress defenses as two possible reasons protein restriction benefits animals. Certain Amino Acids May Matter MostNot every part of protein appears to have the same effect. Animal experiments suggest that restricting individual amino acids can sometimes reproduce many of the metabolic and longevity benefits seen with broader protein restriction.