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In a randomized trial of older men, four weeks of alternate-day fasting was associated with weight loss that included both fat and lean mass. Protein and leucine supplements did not prevent lean-mass loss compared with a calorie-matched control; the study was short and did not test resistance training.
A randomized controlled trial of older men found that adding protein or leucine to an alternate-day fasting plan did not prevent lean-mass loss over four weeks, according to research published in Nutrients. Participants lost weight in all three study groups, including a mean of about 0.9 kilograms of lean mass; the findings do not establish whether the supplements would have different effects over a longer period or alongside resistance training.
The trial enrolled 80 men aged 60 to 89, of whom 61 completed the four-week intervention and were included in the analysis. Participants were assigned to one of three groups: a protein-and-leucine supplement, a leucine supplement, or a calorie-matched chocolate control without added protein or leucine. The supplements were provided on both feeding and fasting days.
All participants followed alternate-day fasting. On fasting days, they could eat a self-selected meal of up to 600 calories between noon and 2 p.m. Over the study, average body mass fell by about 2.2 kilograms, including approximately 1.3 kilograms of fat mass and 0.9 kilograms of lean mass. Researchers found no additional lean-mass protection from either supplement compared with the control.
The report also described changes in some other measures. Systolic blood pressure and several body-composition measures declined, while some lipid markers, including LDL cholesterol, total cholesterol, the total-to-HDL cholesterol ratio and triglycerides, changed favorably over time. HDL cholesterol fell slightly in the protein group. Measures of strength and physical function improved on average, while appendicular lean-mass index declined slightly. These changes were observed across the intervention and do not show that a particular supplement caused them.
Muscle Loss Matters With Age
The results bear on a trade-off for older adults seeking weight loss: weight reduction can include loss of lean tissue, which may matter for strength and mobility. Age-related anabolic resistance can make muscle less responsive to protein and amino acids, while fasting limits the times available to eat. The trial suggests that simply adding the tested amounts of protein or leucine was not enough to spare lean mass under this particular plan.
The researchers point to resistance training alongside nutrition as a possible way to improve muscle-related outcomes, drawing on this study and earlier research. But this trial did not compare exercise programs, so it cannot establish that training would have prevented the observed losses. Its findings are a reason to study combined approaches, not a prescription for how an individual should fast or supplement.
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How the Fasting Trial Was Designed
The study was a three-arm, double-blind randomized trial involving men with a body mass index of at least 23. Researchers balanced group assignment using baseline body mass, BMI, age and physical activity. 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 supplement was calorie-matched.
Researchers assessed body composition with DXA scans and measured blood markers, blood pressure, grip strength, walking speed and other physical-function outcomes. Recruitment took place from November 2022 to May 2023, and data collection ended by June 2023. The report was published in Nutrients as a study of supplement effects during weight loss in older men.
“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 of the Four-Week Evidence
The study was small, lasted only four weeks, and included only older men who met the researchers’ eligibility criteria. It excluded, among others, people with some health conditions that could make fasting or physical testing unsuitable. The results therefore cannot be assumed to apply to women, younger adults, people with those excluded conditions, or longer fasting programs.
The report does not establish why some participants gained lean mass while losing weight, or whether the observed physical-function changes would persist. Although 11 participants gained lean mass, including people in each group, the overall analysis found no supplement advantage. It also remains unclear whether a different protein dose, meal timing, or a structured resistance-training program would change the outcome.
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Questions for Longer Combined Trials
The published study reports results from the completed four-week intervention; the source material does not identify a scheduled follow-up or a next trial. Further research would need to test longer interventions and compare fasting with and without resistance training to determine whether that combination helps preserve muscle while reducing weight.
For now, the evidence supports a narrow conclusion: the tested protein and leucine supplements did not prevent lean-mass loss in this group during alternate-day fasting. The findings do not determine whether fasting or supplementation is appropriate for any individual; people considering major dietary changes should discuss them with a qualified health professional.
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Key Questions
Did protein supplements protect muscle during fasting in this study?
No. In this four-week trial, the protein supplement did not prevent lean-mass loss compared with the calorie-matched control.
Did leucine perform better than protein?
The report found no additional lean-mass protection from the leucine supplement compared with the control. It did not establish a muscle-preserving advantage for leucine.
How much weight did participants lose?
Participants lost an average of about 2.2 kilograms over four weeks, including roughly 1.3 kilograms of fat mass and 0.9 kilograms of lean mass.
Does the study show that resistance training prevents muscle loss?
No. Resistance training was not tested. The authors suggested it may be worth combining with nutrition based on this study and previous research, but its effect was not established here.
Can these findings be applied to everyone who fasts?
The trial involved older men and lasted four weeks. Its results do not establish effects for women, younger people, longer fasting periods or people with health conditions excluded from the study.
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