TL;DR
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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 kilograms of lean mass. Protein and leucine supplements did not prevent that loss or produce additional benefits over a calorie-matched control supplement. The short study does not establish how the approach affects other groups or longer-term health.
A four-week randomized trial in older men found that body-mass-scaled protein or leucine supplements did not prevent lean-mass loss during alternate-day fasting, compared with a calorie-matched control supplement. The study, published in Nutrients, found that participants lost an average of about 0.9 kilograms of lean mass as their body weight fell; the findings do not show whether the same results would apply to women, younger adults or longer fasting programs.
Researchers recruited 80 men aged 60 to 89 with a body mass index of at least 23; 61 completed the intervention and were included in the analysis. Participants were assigned to one of three supplement groups in a double-blind trial. One group received protein with leucine, one received leucine, and the control group received a calorie-matched chocolate supplement without added protein or leucine.
All participants followed alternate-day fasting for four weeks. On fasting days, they could have a self-selected meal of no more than 600 calories during a set lunchtime window. They received three supplement servings on feeding days and one on fasting days. The researchers assessed body composition using DXA scans and also measured blood markers, physical function and cognitive screening scores.
Across the study, 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. The researchers reported no additional protection from either supplement. Several physical-function measures improved over time, including handgrip strength and performance on a timed sit-to-stand test, but those changes did not differ significantly between supplement groups. The reported average changes included a 1.44-kilogram increase in handgrip strength and a 0.92-second improvement in five-times sit-to-stand time.
Protein Alone Did Not Spare Lean Mass
The result addresses a practical concern for older adults trying to lose weight: weight reduction can include loss of muscle as well as fat, and age-related changes may make muscle less responsive to protein. In this trial, adding protein or leucine to the fasting regimen did not prevent the measured decline in lean mass. That finding cautions against treating supplementation as a proven stand-alone way to preserve muscle during fasting.
The study authors suggest that pairing nutrition with resistance training may be important for muscle-related outcomes during weight loss, drawing on their results and prior research. This trial did not test that combination, however, so it cannot establish whether exercise would have prevented the lean-mass loss observed here. The functional improvements and the absence of significant differences between groups also mean the results are more specific than a simple claim that fasting harmed every measure of physical performance.
For readers, the distinction matters: the trial examined a particular fasting schedule, supplement dosing and group of older men over only four weeks. It does not establish a general recommendation for fasting, supplementation or weight-loss programs, and the authors’ suggestion about resistance training is not a tested outcome of this experiment.
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How the Trial Was Set Up
The study focused on older men with overweight or higher body mass, recruited between November 2022 and May 2023. Participants with certain conditions that could make fasting or physical testing unsuitable were excluded, including diabetes, uncontrolled high blood pressure, and kidney or liver conditions. Smokers, vegetarians and vegans, and people with dairy allergies or intolerance were also excluded, limiting how broadly the findings can be applied.
Researchers used a three-arm, double-blind design and balanced assignments based on characteristics including age, body mass, body mass index and physical activity. The protein group received a supplement containing protein and leucine; the leucine group received leucine without the added protein dose; the control received a calorie-matched supplement. Participants recorded exercise and sleep, while investigators evaluated blood lipids and glycemic markers, blood pressure, body composition and measures used in sarcopenia assessments.
The reported changes included reductions in systolic blood pressure and several body-composition measures. LDL cholesterol, total cholesterol, the total-to-HDL cholesterol ratio and triglycerides changed favorably over time, while HDL cholesterol fell slightly in the protein group. These are observed changes during the study; the source report does not establish that the supplements caused the changes or that they would persist after the intervention.
“The researchers suggest combining nutritional supplementation with resistance training to improve muscle-related outcomes among older adults during weight loss.”
— The study authors, as summarized in the News-Medical report
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Limits on Applying the Findings
The study was small and short: 61 men completed four weeks, and the source report does not provide evidence about longer-term outcomes. It is not clear from the reported findings whether different fasting schedules, supplement amounts, or training programs would change the result. Nor can the results be assumed to apply to women, younger people, or older adults with health conditions that excluded them from participation.
The trial also found that some physical-function measures improved even as average lean mass declined. The report does not establish what explains that pattern or whether the changes would continue. Although 11 participants gained lean mass while losing weight, the study’s overall group findings did not show that protein or leucine supplementation preserved lean mass compared with control. The available information does not establish the clinical importance of the measured changes or their durability.
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Longer Trials Must Test Training
The published results establish what happened during this four-week intervention, but not whether the outcome holds over longer periods. Further research would need to test whether resistance training alongside fasting and nutritional support changes lean-mass or strength outcomes, and whether those results extend to women and older adults with different health profiles.
Until such evidence is available, the authors’ proposed role for resistance training should be understood as a direction for future study rather than a result demonstrated by this trial. The source report does not identify a scheduled follow-up study or a next publication date.
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Key Questions
Did protein supplements prevent muscle loss during fasting?
No. In this four-week trial, protein and leucine supplements did not prevent lean-mass loss compared with a calorie-matched control supplement.
How much weight and lean mass did participants lose?
Participants 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 physical function get worse?
Several measured function outcomes improved on average, including handgrip strength and timed sit-to-stand performance. The reported changes did not differ significantly between the supplement groups, and the study does not establish how lasting the improvements were.
Can these findings be applied to women or all older adults?
Not directly. The trial included older men only, and it excluded people with several health conditions. Its results cannot establish what would happen in women or other groups.
Did the trial prove resistance training protects muscle during fasting?
No. The authors suggested combining supplementation with resistance training, but resistance training was not tested as part of this trial.
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