Does Extra Protein Protect Aging Muscle During Fasting?

TL;DR

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In a randomized trial, 61 men aged 60 to 89 completed four weeks of alternate-day fasting and lost an average of 0.9 kilograms of lean mass. Added protein and leucine did not prevent that loss or produce additional benefits over a calorie-matched control supplement. The findings are limited to this short study and do not establish how fasting or supplementation affects older adults more broadly.

A four-week randomized trial found that protein and leucine supplements did not prevent lean-mass loss among older men following alternate-day fasting, despite an average body-weight reduction of about 2.2 kilograms. The study, published in Nutrients, compared the supplements with a calorie-matched control and found no additional body-composition or physical-function benefit from either supplement.

The trial 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 control chocolate supplement without added protein or leucine. The supplements were matched for calories per kilogram of body mass.

On feeding days, participants had three supplement servings. 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 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.

Across the study, participants lost an average of 2.2 kilograms of body mass, including about 1.3 kilograms of fat mass and 0.9 kilograms of lean mass. Appendicular lean mass index fell by 0.10 kilograms per square meter. The researchers reported no significant differences among the groups in changes to the measured muscle and physical-function outcomes. Some measures, including handgrip strength, gait speed and sit-to-stand performance, improved over time across participants, but the supplements did not produce an added benefit compared with control.

Why Lean-Mass Loss Matters With Age

The findings address a concern for older adults trying to lose weight: weight loss can include lean tissue as well as fat. Preserving muscle is relevant to strength and everyday function, and age-related anabolic resistance can make muscle less responsive to protein and amino acids. The trial tested whether extra protein or leucine could help counter that risk during an intermittent-fasting plan.

In this study, supplementation alone did not spare lean mass. The researchers suggested that resistance training alongside nutrition may be a useful approach to investigate for muscle-related outcomes during weight loss. That is an interpretation and proposed direction, not a result tested by this trial: participants were not assigned to a resistance-training program. The results also do not show that alternate-day fasting is unsuitable for all older adults, or that the supplements have no role in other settings.

How the Fasting Trial Was Designed

Alternate-day fasting alternates between eating days and days with sharply restricted intake. The researchers focused on older men because weight-loss strategies may pose a particular muscle-preservation challenge with age. Leucine is an amino acid involved in muscle protein synthesis, and the study tested whether adding leucine, alone or with protein, would change body composition or health measures during fasting.

The trial was double-blind and randomized, and groups were balanced for baseline characteristics including age, body mass and activity. Researchers assessed body composition using DXA scans and measured blood lipids, glycemic markers, blood pressure, cognition and physical-function indicators used in sarcopenia assessment. The study also recorded exercise and sleep patterns. Recruitment took place from November 2022 through May 2023, and data collection ended by June 2023.

Over the intervention, systolic blood pressure and several body-composition measures declined, while bone mineral density did not change. Some lipid measures, including LDL cholesterol, total cholesterol, the total-to-HDL cholesterol ratio and triglycerides, changed favorably over time. HDL cholesterol decreased slightly in the protein group. These findings were reported across the trial and do not establish that a particular supplement caused the changes.

“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

Limits of the Four-Week Evidence

The results come from 61 completers in a short trial, not from a long-term study of older adults in general. The participants were men who met the study’s eligibility criteria; people with several conditions that could make fasting or physical testing unsuitable were excluded. The findings cannot establish effects for women, people with those excluded health conditions, or older adults following different diets.

It is also unclear whether the results would differ with a longer intervention, another supplement dose, or a structured exercise program. Although some physical-function measures improved, the study found no significant differences in those changes between supplement groups. The trial does not establish whether the improvements were caused by fasting, practice with the tests, or other factors. The supplied report does not provide enough detail to determine the broader clinical importance of the observed changes.

What Future Studies Need to Test

The study authors point toward testing supplementation combined with resistance training during weight loss, since exercise was not part of the randomized comparison. Longer studies with more participants and a broader range of older adults could help determine whether effects vary by sex, health status, fasting pattern or training plan.

For now, the reported evidence is limited to the four-week trial: participants lost weight and lean mass, while added protein or leucine did not prevent the lean-mass loss relative to the control supplement. The source provides no scheduled follow-up study or further milestone. Whether a combined diet-and-exercise approach changes these outcomes remains to be tested.

Key Questions

Did protein supplements prevent muscle loss in the trial?

No. The researchers found that protein and leucine did not prevent lean-mass loss compared with the calorie-matched control supplement during the four-week intervention.

How much weight did participants lose?

The 61 men included in the analysis 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.

Did physical function get worse during fasting?

Some measured outcomes improved over time: handgrip strength, gait speed and sit-to-stand performance increased or improved. However, the study reported no significant differences in these changes among the supplement groups, and it cannot show what caused the improvements.

Does the study prove that resistance training protects muscle during fasting?

No. Resistance training was not tested in this trial. The authors suggested it as a possible partner to nutrition based on the findings and prior research, but this study cannot establish its effect.

Do the findings apply to all older adults?

Not necessarily. The analysis included older men who completed the study, and eligibility criteria excluded people with several health conditions. The results do not establish effects for women or for every older adult or fasting plan.

Source: rss