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In a randomized trial, 61 men aged 60 to 89 who completed four weeks of alternate-day fasting lost an average of about 0.9 kg of lean mass. Protein and leucine supplements did not prevent that loss compared with a calorie-matched control supplement. The short study does not establish whether resistance training or longer-term supplementation would change the outcome.
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. The 61 participants who completed the study lost an average of about 0.9 kilograms of lean mass, alongside weight and fat loss; the findings suggest supplements alone may not protect muscle during this type of weight-loss plan.
The study, published in Nutrients, recruited 80 men aged 60 to 89 with a body mass index of at least 23. A total of 61 completed the intervention and were included in the analysis. Participants were assigned to one of three calorie-matched supplement groups: leucine, protein plus leucine, or a control chocolate supplement without added protein or leucine.
On feeding days, participants took three servings of their assigned supplement; on fasting days, they took one serving and could eat a self-selected meal of up to 600 calories between noon and 2 p.m. 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. Researchers tracked body composition, blood markers, physical function and other measures.
Across the study, average body mass fell by about 2.2 kilograms, including roughly 1.3 kilograms of fat mass and 0.9 kilograms of lean mass. Appendicular lean mass index, a measure of limb muscle relative to height, also declined slightly. Researchers reported no significant advantage for either supplement group over control in preventing lean-mass loss or producing additional benefits. Eleven participants gained lean mass while losing weight, but that outcome did not alter the overall group finding.
Supplementation Did Not Spare Lean Mass
The result matters because older adults can be more vulnerable to muscle loss, and weight-loss strategies may reduce both fat and lean tissue. The trial challenges the idea that simply adding protein or leucine is enough to protect muscle during alternate-day fasting, at least under the conditions tested. It does not show that protein is generally ineffective for muscle health; it tests specific supplement doses and a short fasting program in a particular group.
Some physical-function measures improved during the four weeks: handgrip strength rose by an average of 1.44 kilograms, and the time to complete five chair rises improved by 0.92 seconds. Walking speed and the short physical performance battery score also increased modestly. These are changes over time across participants; the report says the supplement groups did not differ significantly on these changes. The mix of lean-mass loss and improved measured function means the findings should not be reduced to a claim that participants uniformly became weaker.
The researchers suggest resistance training alongside nutritional support as a possible approach to muscle-related outcomes during weight loss, drawing on the trial and prior research. That is a proposed implication, not a result tested here: the trial did not compare exercise programs.
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How the Four-Week Trial Worked
Alternate-day fasting alternates days with restricted intake and days with more usual eating. In this study, fasting days were not complete fasts: participants were allowed one meal of up to 600 calories, in addition to their assigned supplement. The researchers used a double-blind, three-arm design and matched supplements for calories, helping compare added protein and leucine with a control product under the same schedule.
Researchers measured body composition using dual-energy X-ray absorptiometry and assessed muscle-related function through grip strength, chair stands, walking speed and other tests. They also examined blood lipids, glucose-related markers, blood pressure and cognitive screening scores. Several measures changed during the intervention, including reductions in systolic blood pressure and some cholesterol and triglyceride measures; these short-term observations do not establish lasting health benefits.
The study excluded people with conditions that could make fasting or physical testing unsuitable, including diabetes, uncontrolled hypertension, and kidney or liver conditions. It also excluded smokers, vegetarians and vegans, and people with dairy allergies or intolerance. Its findings therefore apply most directly to the selected group of older men who met the eligibility criteria, not to all older adults.
“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 Applying the Findings
The intervention lasted only four weeks, and 19 of the 80 recruited men did not complete it or were not included in the final analysis. The supplied report does not give the reasons for each non-completion or detailed estimates of how much the results varied among participants. A short study cannot show whether the observed changes persist, reverse, or differ with a longer fasting period.
The findings do not establish what would happen in women, younger adults, people with different health conditions, or people following other fasting schedules. Nor did the trial test whether resistance training, different supplement doses, or protein from ordinary meals would change muscle outcomes. The report also describes changes in several blood and physical-function measures but does not establish that fasting caused durable clinical benefits. Longer and broader studies would be needed to address those questions.
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Longer Trials Could Test Exercise
The study reports a completed four-week intervention, with data collection ending in June 2023; it does not identify a next trial or a planned follow-up. The immediate research question is whether adding structured resistance training to fasting and adequate nutrition can better preserve lean mass in older adults, and whether any effect lasts beyond a few weeks.
Until those comparisons are available, the results support a limited conclusion: in this trial, the tested protein and leucine supplements did not prevent lean-mass loss during alternate-day fasting. The study is not a basis for recommending a fasting or supplement plan for every older adult, particularly because several health conditions were grounds for exclusion.
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Key Questions
Did protein supplements prevent muscle loss in the trial?
No. Among men who completed the four-week study, protein plus leucine did not prevent lean-mass loss compared with the calorie-matched control supplement.
How much weight and lean mass did participants lose?
Participants lost an average of about 2.2 kilograms of body mass, including approximately 1.3 kilograms of fat mass and 0.9 kilograms of lean mass over four weeks.
Did physical function change?
Several measures improved on average, including handgrip strength and five-times sit-to-stand performance. The reported changes were not significantly different between supplement groups, so they do not show a specific benefit from protein or leucine.
Does the study show resistance training protects muscle during fasting?
No. The researchers suggested resistance training may be worth combining with nutrition based on this study and earlier research, but this trial did not test an exercise program.
Can the results be applied to all older adults?
Not directly. The study involved selected men aged 60 to 89 who met its eligibility criteria, and it lasted four weeks. It does not establish effects in women, people with excluded health conditions, or people using other fasting plans.
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