Alternate-Day Fasting Triggers Muscle Loss in Older Men Despite Protein Supplements
Older men practicing alternate-day fasting experience weight loss over a four-week period, and adding body mass-scaled protein and leucine supplements fails to prevent lean mass reduction, News-Medical reported. Researchers evaluating 80 participants aged 60 to 89 found that a dietary regimen alone cannot spare muscle mass during weight loss, pointing toward resistance training as a necessary addition for older adults.
The randomized controlled trial, published in the journal Nutrients by B. W. J. Pang, Y. Yang, and B. K. Cheon in 2026, tracked 61 participants who completed the month-long intervention. Investigators measured changes using dual-energy X-ray absorptiometry scans, tracking cardiometabolic biomarkers, and assessing physical function through the Asian Working Group for Sarcopenia 2019 criteria. Participants reduced their total body mass by an average of 2.2 kilograms, which included 1.3 kilograms of fat mass and 0.9 kilograms of lean mass.
Researchers Divide Participants into Leucine and Protein Groups
Researchers divided participants into three arms using a minimization procedure that balanced baseline body mass, body mass index, age, and physical activity. All provided supplements were calorie-matched per kilogram of body mass. The leucine group received 0.08 grams per kilogram of leucine per serving, while the protein group received 0.4 grams per kilogram of protein and 0.04 grams per kilogram of leucine per serving. Control participants consumed a calorie-matched chocolate supplement without added protein or leucine.
Participants consumed three supplement servings on feeding days and one serving on fasting days. On fasting days, individuals were permitted a self-selected meal of no more than 600 kilocalories between noon and 2:00 PM. Adherence to the protocol remained high, with about 93 percent reporting compliance with fasting-day meal timing and 63 percent adhering to the 600-kilocalorie limit. Participants completed 99 percent of the provided supplement regimens.
Cardiometabolic and Physical Function Shifts
Over the four-week intervention, systolic blood pressure and several body composition measures decreased significantly while bone mineral density remained unchanged. Lipid measures shifted favorably, showing reductions in low-density lipoprotein cholesterol, total cholesterol, the total-to-high-density-lipoprotein cholesterol ratio, and triglycerides. High-density lipoprotein cholesterol decreased slightly in the protein group, however, and fasting glucose, fasting insulin, glycated hemoglobin, and homeostatic model assessment of insulin resistance showed no significant changes.
Physical function measures yielded mixed results across the cohort. While appendicular lean mass index fell by 0.10 kilograms per square meter, participants demonstrated time-based improvements in functional sarcopenic outcomes. Five-times sit-to-stand test times improved by 0.92 seconds, handgrip strength increased by 1.44 kilograms, 6-meter gait speed rose by 0.05 meters per second, and short physical performance battery scores increased by 0.08. Eleven participants gained lean mass while losing weight, including five in the control group, four in the protein group, and two in the leucine group.
Resistance Training Requirements for Older Adults
The study highlights the physiological challenges of preserving lean mass during weight loss in older populations who face risks of sarcopenia. Because aging reduces the muscle response to amino acids and protein—a process known as anabolic resistance—older adults require higher thresholds to stimulate muscle protein synthesis. The authors note that the four-week alternate-day fasting intervention cannot establish causation on its own because the trial lacked a non-fasting control group.
The findings indicate that supplying body mass-scaled protein and leucine without mechanical loading does not prevent lean mass reduction. Consequently, researchers suggest that combining nutritional supplementation with structured resistance training is necessary to improve muscle-related outcomes during weight reduction in older demographics. The study cohort consisted of healthy, community-dwelling older men, meaning the results may not apply directly to older women, frail adults, or individuals with diagnosed sarcopenia.
Frequently Asked Questions About the Fasting Trial
Why did researchers study alternate-day fasting specifically in older men?
Investigators sought to examine how body mass-adjusted protein and leucine supplementation affects cardiometabolic biomarkers and body composition during weight loss in men aged 60 to 89. This demographic faces heightened risks of muscle loss and sarcopenia when reducing calorie intake.
What were the exact nutritional dosages given to the participants?
The leucine group received 0.08 grams per kilogram of body mass of leucine per serving, while the protein group received 0.4 grams of protein and 0.04 grams of leucine per kilogram. The control group received a calorie-matched chocolate supplement with no added protein or leucine.
Did the study observe any adverse side effects from the fasting and supplements?
Interventions were generally well tolerated, though two participants reported weakness and light-headedness on the first fasting day. One participant experienced bloating, constipation, and slight joint discomfort after consuming the supplements.
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