The Caloric Restriction Question: Does Fasting Actually Extend Human Life?

01 The Most Robust Intervention

Caloric restriction (CR) — reducing food intake by 30-40% while maintaining adequate nutrition — is the most robust lifespan intervention known. It works in yeast (+50%), worms (+50%), flies (+45%), mice (+35%), and rats (+40%). The mechanism is evolutionarily conserved: when nutrients are scarce, cells shift from growth to repair, activating autophagy (cellular cleanup), reducing mTOR signaling, and increasing stress resistance. The question is whether it works in humans.

02 The Primate Puzzle

Two major primate studies gave conflicting results. The NIA study (started 1987) found no lifespan extension in calorie-restricted rhesus monkeys. The Wisconsin study (started 1989) found significant extension. The difference? Diet quality. The NIA monkeys ate a healthier diet, so CR had less room to help. The Wisconsin monkeys ate a typical lab diet closer to the Western diet, so CR had more benefit. The implication: CR works, but mainly when starting from a bad diet.

03 The Human Evidence

The CALERIE trial (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) put humans on 25% caloric restriction for 2 years. Results: improved blood pressure, cholesterol, insulin sensitivity, and inflammatory markers. But adherence was difficult — participants achieved only about 12% CR, not 25%. No lifespan data exists for humans (the trial would take 50+ years). The practical alternative: intermittent fasting (16:8, 5:2, OMAD) which provides some CR benefits with better adherence. But no fasting protocol has proven lifespan extension in humans either.