Does Extra Protein Protect Aging Muscle During Fasting?

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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 compared with a calorie-matched control, though several physical-function measures improved over the study. The results do not establish how the approach affects older women, other age groups or longer-term outcomes.

A four-week randomized trial in older men found that added protein and leucine did not prevent lean-mass loss during alternate-day fasting, despite participants losing weight. The findings, reported in the journal Nutrients, suggest supplementation alone may not protect muscle during short-term weight loss; the researchers said resistance training may be worth combining with nutrition strategies, but this trial did not test that combination.

The study enrolled 80 men aged 60 to 89 with a body mass index of at least 23. Of those, 61 completed the intervention and were included in the analysis. Participants were assigned to one of three groups in a double-blind trial: a protein supplement with leucine, a leucine supplement, or a calorie-matched chocolate control without added protein or leucine.

On feeding days, participants consumed three servings of their assigned supplement. On fasting days, they had one serving and could eat a self-selected meal of up to 600 calories between noon and 2 p.m. The researchers measured body composition using DXA scans and assessed strength, physical function, blood markers and cognitive-screening scores.

Across the groups, body mass fell by about 2.2 kilograms on average over four weeks, including roughly 1.3 kilograms of fat mass and 0.9 kilograms of lean mass. The study reported no significant difference between supplement groups in lean-mass changes. Some functional measures improved over time: handgrip strength rose by an average of 1.44 kilograms, and five-times sit-to-stand time improved by 0.92 seconds. Those changes also did not differ significantly by supplement group.

Supplementation Alone Left Muscle Loss

The results address a practical concern for older adults trying to lose weight: weight reduction can include losses in lean mass as well as fat, and muscle preservation can become more difficult with age. In this trial, adding protein or leucine did not measurably prevent lean-mass loss compared with the control supplement, so the findings do not support treating either supplement as a stand-alone safeguard during this fasting plan.

The improvements in strength and timed physical-function measures occurred alongside a small decline in appendicular lean-mass index. That distinction matters: better performance on some tests does not mean lean tissue was preserved. The researchers proposed that resistance training could be paired with nutritional supplementation, drawing on prior research, but their trial did not compare exercise programs or show that training would prevent the observed loss.

The study also reported changes in some cardiometabolic measures, including lower systolic blood pressure and favorable changes in several lipid measurements over time. These results describe changes observed during the intervention; they do not establish that fasting or supplements caused lasting health benefits.

How the Fasting Trial Worked

The trial focused on alternate-day fasting (ADF), in which participants followed a restricted intake on fasting days and ate on feeding days. Researchers recruited participants between November 2022 and May 2023 through advertisements and word of mouth. Data collection ended by June 2023.

Leucine is an amino acid involved in muscle protein synthesis. The study rationale noted that older adults may have a reduced muscle response to protein and amino acids, sometimes described as anabolic resistance. The researchers tested whether body-mass-scaled supplementation would help during weight loss: the protein group received 0.4 grams of protein and 0.04 grams of leucine per kilogram per serving, while the leucine group received 0.08 grams of leucine per kilogram per serving.

The study excluded people whose health conditions could make fasting or physical testing unsuitable, including those with diabetes, uncontrolled hypertension, or kidney or liver conditions. It also excluded smokers, vegetarians and vegans, and people with dairy allergy or intolerance. Those criteria limit how readily the results can be applied to other groups.

“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 in the News-Medical report

Limits of a Short-Term Study

The intervention lasted only four weeks, and the analysis included 61 participants, all men aged 60 to 89. It is unclear whether the same results would apply to women, younger adults, people with different health conditions, or people following other fasting schedules. The trial also cannot show whether lean-mass changes would continue, reverse or affect health over a longer period.

Although 11 participants gained lean mass while losing weight—five in the control group, four in the protein group and two in the leucine group—the report does not establish why their results differed. The available findings also do not determine whether resistance training would change the outcome, because training was not assigned as part of the intervention. The study’s changes in blood markers and cognitive-screening scores likewise do not establish lasting effects.

Longer Trials Could Test Training

The reported study has completed its four-week intervention, and the source material does not identify a scheduled follow-up or a next trial. Further research would need to test longer interventions and include broader participant groups to establish whether the findings hold beyond older men in this study.

A trial that directly compares fasting with and without resistance training, alongside different protein strategies, could test the authors’ suggestion about combining exercise and nutrition. Until such evidence is available, the current results show that the tested supplements did not spare lean mass in this short-term trial; they do not establish which approach best protects muscle during weight loss.

Key Questions

Did protein supplements prevent muscle loss in the trial?

No. Among the 61 men included in the analysis, the protein and leucine groups did not have significantly different lean-mass changes from the calorie-matched control group.

How much weight did participants lose?

Participants lost about 2.2 kilograms on average over four weeks. The reported average included about 1.3 kilograms of fat mass and 0.9 kilograms of lean mass.

Did physical function get worse as lean mass fell?

Several measures improved over the study, including handgrip strength and five-times sit-to-stand time. Those changes occurred alongside a small decline in appendicular lean-mass index, and the trial does not show that one caused the other.

Did the researchers test resistance training?

No. The study compared supplement groups during alternate-day fasting. The authors suggested resistance training may be a useful partner to nutrition strategies, but this trial did not test that approach.

Can the findings be applied to all older adults?

Not from this study alone. It included men aged 60 to 89 who met specific eligibility criteria, lasted four weeks and analyzed 61 completers. The results do not establish effects in women or in people excluded from the trial.

Source: rss