TL;DR
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A randomized trial found that older men lost an average of 0.9 kilograms of lean mass during four weeks of alternate-day fasting. Protein and leucine supplements did not prevent that loss compared with a calorie-matched control; the short study does not establish whether other diets, longer interventions or resistance training would change the result.
A randomized controlled trial in older men found that adding protein or leucine supplements did not prevent lean-mass loss during four weeks of alternate-day fasting. Participants lost an average of 0.9 kilograms of lean mass as they lost weight, and the supplement groups did not show additional benefits over a calorie-matched control, according to the study published in Nutrients.
The researchers recruited 80 men aged 60 to 89; 61 completed the intervention and were included in the analysis. Participants were assigned to one of three groups: a protein supplement containing leucine, a leucine supplement, or a chocolate control supplement matched for calories. The study assessed body composition, blood markers and measures of physical function.
On fasting days, participants could have a self-selected meal of up to 600 calories between noon and 2 p.m. They took one supplement serving that day and three servings on feeding days. Across the intervention, participants lost about 2.2 kilograms of body mass on average, including roughly 1.3 kilograms of fat mass and 0.9 kilograms of lean mass. Bone mineral density did not change significantly.
Some measures changed in a favorable direction over time, including systolic blood pressure and several lipid measures. The report also described modest average improvements in cognitive-screening scores and physical-function tests. Handgrip strength rose by 1.44 kilograms and five-times sit-to-stand time improved by 0.92 seconds. These changes did not differ significantly among the three supplement groups, so the results do not show that protein or leucine caused the improvements.
Muscle Loss Was Not Prevented
The findings address a practical concern for older adults trying to lose weight: weight reduction can include loss of lean tissue, and maintaining muscle is relevant to strength and everyday function. In this trial, adding the tested supplements did not prevent lean-mass loss during fasting. That means the study does not support relying on these specific supplements alone as protection against muscle loss in this setting.
The result is not proof that protein has no role in maintaining muscle. The researchers tested particular doses, schedules and participants over only four weeks; the trial compared supplements during one fasting approach rather than every form of weight loss. The report says the researchers suggest pairing nutritional supplementation with resistance training as a possible way to improve muscle-related outcomes, drawing on this result and earlier research. This trial itself did not establish that such a combination works.
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How the Fasting Trial Worked
Alternate-day fasting alternates fasting days with days when participants eat. The study focused on older men with a body mass index of at least 23 kg/m². Recruitment took place from November 2022 to May 2023, and data collection concluded by June 2023. The researchers used a three-arm, double-blind design and balanced groups on measures including age, body mass and activity level.
The protein group received 0.4 grams of protein and 0.04 grams of leucine per kilogram per serving. The leucine group received 0.08 grams of leucine per kilogram per serving; the control received a calorie-matched chocolate supplement without added protein or leucine. Researchers assessed body composition using DXA scans and included measures such as handgrip strength, gait speed and sit-to-stand performance. The study excluded people with certain conditions that could make fasting or physical testing unsuitable, including diabetes and kidney or liver conditions.
The report frames the question against age-related anabolic resistance—a reduced muscle response to protein and amino acids that can occur with aging. Leucine is involved in muscle protein synthesis, but whether the body-mass-scaled supplements would preserve muscle during this fasting schedule was uncertain before the trial.
“Body mass-scaled protein and leucine supplementation did not prevent lean mass loss or confer additional benefits as compared with a control supplement.”
— The study authors, as summarized by News-Medical
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Limits on the Trial Findings
The results apply to the 61 men who completed the study, not automatically to women, younger adults, people with health conditions excluded from the trial, or other fasting schedules. The intervention lasted four weeks, so it cannot establish longer-term effects on muscle, health or weight maintenance. The available report does not provide enough detail to determine why 19 recruited participants were not included in the final analysis.
The study also does not establish whether a different protein dose, timing, food source or exercise program would produce a different outcome. Although several physical-function measures improved on average, the supplement groups did not differ significantly, and the report does not establish what caused those changes. The findings should not be read as a personalized fasting or supplementation recommendation.
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Questions for Longer Studies
Further research would need to test whether longer interventions, different supplement schedules or the addition of resistance training can better preserve lean mass during weight loss in older adults. Studies including women and a wider range of health backgrounds would also show how far the current findings apply. The supplied report does not identify a follow-up trial or a planned next study, so the next research milestone is unclear.
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Key Questions
Did protein supplements prevent muscle loss during fasting in this study?
No. In the four-week trial, the protein and leucine groups did not lose less lean mass than the calorie-matched control group.
How much weight did participants lose?
The 61 men included in the analysis lost about 2.2 kilograms of body mass on average, including approximately 1.3 kilograms of fat mass and 0.9 kilograms of lean mass over four weeks.
Did the supplements improve strength or physical function?
Some physical-function measures improved across the study, including handgrip strength and sit-to-stand time. The changes were not significantly different among supplement groups, so the trial does not show that the supplements produced those improvements.
Does the result apply to everyone who fasts?
Not necessarily. The study involved older men and lasted four weeks; it excluded people with several health conditions. Its findings do not establish effects for women, younger adults, other fasting schedules or people with excluded conditions.
What did researchers suggest might help protect muscle?
The researchers suggested that resistance training could be combined with nutritional supplementation during weight loss. This trial did not test that combination, so its effectiveness remains a question for further research.
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