The Proposed MechanismThe central pathway the authors describe involves fibroblast growth factor 21, a hormone that rises when protein intake falls. Protein restriction and targeted amino acid restriction extend lifespan and improve metabolic measures across yeast, fruit flies, and rodents. Pregnant women, growing children, people recovering from injury, anyone not eating enough calories overall, and older adults whose protein intake is already low. A hormone that rises when protein intake falls, linked to greater energy expenditure, better glucose control, and lower inflammation. Any change to protein intake should be discussed with a clinician, particularly for older adults and people with medical conditions.