Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism

Summary

This is one of the foundational human alternate-day fasting trials, and — importantly — the actual source of the famous "57 percent insulin drop" claim that circulates widely in popular fasting content. Sixteen nonobese adults (8 men, 8 women) fasted every other day for 22 days. The protocol alternated full fasting days with normal eating days. Body weight dropped 2.5 percent and fat mass dropped 4 percent over the three weeks. Resting metabolic rate did not change significantly through day 21, but respiratory quotient fell on day 22 — indicating a clear shift toward fat oxidation, with daily fat oxidation rising by 15 grams or more. Glucose and ghrelin remained essentially stable, but fasting insulin dropped 57±4 percent. Hunger on fasting days remained elevated throughout the protocol, suggesting that adaptation to alternate-day hunger patterns does not happen quickly. The paper concluded that alternate-day fasting is feasible in nonobese adults and produces substantial fat-oxidation and insulin-sensitivity shifts, but adherence is challenging.