Alternate-day fasting helps fat loss, may cut muscle too, Study Finds - Video

Alternate-day fasting promises quick fat loss, but does it secretly steal your hard-earned muscle too? A new study in Nutrients tested four weeks of this popular intermittent fasting style on young Asian men, revealing it trims weight and fat effectively-yet also shrinks muscle mass, even with added protein shakes.

Alternate-day fasting (ADF) flips between full eating days and "fasting" days with just one small 400-600 calorie meal around noon, typically making up 16-20 hours without big eats. Worldwide, 44% of adults are overweight and 16% obese, with Singapore close behind at 41% and 14%-sparking huge interest in ADF for slashing body fat, BMI, cholesterol, blood pressure, and triglycerides while boosting metabolic health.

Researchers ran two trials on non-smoking Asian males aged 21-35 with BMI over 23 kg/m², excluding those on meds or with fasting risks.

Study 1 had 20 men follow ADF for four weeks: free eating on feed days, water/zero-cal drinks plus a ~525 kcal meal (38% carbs, 37% fat, 25% protein) on fast days. Body composition used bioelectrical impedance; weekly checks tracked blood pressure, fasting glucose, activity via Global Physical Activity Questionnaire, and diet logs.

Study 2 randomized 26 similar men into control (533 kcal fast meals: 46% carbs, 33% fat, 22% protein) or protein groups (495 kcal with 25g whey shake: 34% carbs, 26% fat, 40% protein). After dropouts, 37 men (age 25, BMI 26.7) completed, with combined analysis showing no group interaction.

Post-ADF, body mass, fat mass, and fat-free (muscle) mass all dropped significantly. Physical activity dipped in weeks 2-3; diastolic blood pressure fell briefly at week 3, fasting glucose at week 1—but neither lasted to endpoint. Systolic pressure stayed steady. Hydration held consistent.

Low-dose protein failed to protect muscle, likely because fast-day intake stayed below daily needs, group differences were tiny, and feed-day protein went unmonitored.

Future research eyes higher leucine/protein dosing across all days plus resistance training to safeguard muscle during ADF weight loss.

REFERENCE: Pang BW, Yang Y, Rashiqah N, Huang CB, Sim DW (2025). Effects of Four Weeks of Alternate-Day Fasting with or Without Protein Supplementation - A Randomized Controlled Trial. Nutrients, 17(23), 3691. DOI: 10.3390/nu17233691. https://www.mdpi.com/2072-6643/17/23/3691