The FGF21 hormone link to lower protein has solid backing, but the claim that less protein is better for aging gets pushback.

Other research backs the FGF21 part of the story: several studies found that when people ate less protein, their FGF21 levels rose, and mice engineered to make more FGF21 lived longer. But the broader claim that lower protein improves metabolism and lowers inflammation is on shakier ground—one direct study found that people on higher-protein diets lost more weight and fat and had better triglyceride levels, which cuts against the idea. The amino acid angle is also mixed: some mouse and cancer studies hint that valine or branched-chain amino acids might be harmful, but another study found that people who ate fewer amino acids had a higher risk of dying, especially older adults. FGF21's effects on energy expenditure and blood sugar are suggested by some research, though often indirectly or in animals. We couldn't find studies in our databases on whether FGF21 directly reduces inflammation or on the new US protein guidance itself.