Alternate-Day Fasting in Older Men: Does Extra Protein Help?

A randomized controlled trial published on 2 October 2026 in Nutrients put 61 older men on alternate-day fasting for 28 days and gave them extra protein, extra leucine, or neither. Across all three groups, body composition measures went down, blood pressure and blood lipids improved, and the men did better on strength and function tests. The supplements added nothing the researchers could measure.

That is a null result for the supplements, and a useful one. It also comes with clear limits: four weeks, men only, and no group that did not fast.

Why the researchers asked the question

Alternate-day fasting means switching between fasting days and feeding days. It can produce weight loss, but the authors point out that weight loss diets carry a risk of losing muscle along with fat. For older adults that matters because of sarcopenia, the age-related loss of muscle mass and strength.

Protein is the usual answer to that worry. Leucine, one of the amino acids in protein, is described in the paper as the key driver of muscle protein synthesis, and it can be taken with far fewer calories than a full serving of protein. So the question was practical: if an older man fasts every other day, does adding protein or leucine, scaled to his body weight, protect his muscle or improve his blood markers?

What the study did

The trial was run by researchers at Nanyang Technological University in Singapore, with a co-author at the US National Institutes of Health.

Who	61 older men with a body mass index of 23.0 or higher
How long	28 days of alternate-day fasting
What was compared	Three calorie-matched supplement groups: control, leucine, protein
When the supplement was taken	Three doses on feeding days, one on fasting days
What was measured	Body composition by DXA scan, sarcopenia measures (strength and function), blood pressure, lipid panel, diabetes-related blood markers
Result	Everyone improved over time; no measurable added benefit from leucine or protein

All three groups fasted. The control supplement contained no protein and no leucine. The leucine supplement was scaled at 0.08 g of leucine per kg of body weight with no protein, and the protein supplement at 0.4 g of protein and 0.04 g of leucine per kg. Those are the amounts the trial tested, reported here so you can see what "body-mass-scaled" means. They are not a recommendation.

What it found

The analysis looked for two things: whether the men changed over the 28 days, and whether the change depended on which supplement they got.

On the first, the answer was yes. There were significant reductions over time in all body composition measures except bone mineral density, which did not change significantly (p = 0.274). Systolic blood pressure fell. So did total cholesterol, LDL cholesterol, the ratio of total to HDL cholesterol, and triglycerides. Every muscle strength and functional outcome improved.

On the second, the answer was almost entirely no. There were no significant differences between the supplement groups in body composition, sarcopenia measures or health outcomes, with two exceptions: diastolic blood pressure (p = 0.016) and HDL cholesterol (p = 0.013). The abstract does not say which group those two differences favored, so this post does not guess.

The authors' conclusion is restrained. Dietary interventions that increased leucine or protein intake "did not result in additional measurable improvements" on body composition, sarcopenic and cardiometabolic outcomes during this four-week intervention.

What it does not show

• It does not show that fasting caused the improvements. There was no non-fasting control group. Every man in the trial did alternate-day fasting, so the changes over time cannot be separated from other things that happen when people join a study, such as paying closer attention to food or getting used to the strength tests.
• It does not show that muscle was spared. The abstract says all body composition measures other than bone mineral density went down. It does not give separate figures for fat and lean mass, so you cannot tell from it how much of the loss was muscle. Strength and function improved anyway, which is reassuring but is not the same measurement.
• It does not show what happens after four weeks. Twenty-eight days is short for changes in muscle. A supplement that makes no difference in a month might or might not matter over a year. The authors limit their own conclusion to "this four-week intervention".
• It does not apply to women. Only men took part.
• It does not test exercise. The conclusion refers to dietary interventions alone. Whether protein matters more when combined with resistance training was not part of this trial.
• It does not show protein is unimportant. The trial tested one supplement design on top of the men's usual feeding-day meals. It says nothing about what happens if overall protein intake is low.

What this means if you fast

If you are an older man thinking about alternate-day fasting, this trial offers a modest piece of evidence that four weeks of it went along with weight loss and better scores on strength and function tests, and that adding a protein or leucine supplement did not change the picture. It is not a reason to buy leucine, and it is not proof that you can ignore protein.

It is also a reminder of who should be careful. Fasting every other day is a demanding pattern, and older adults are more likely to take regular medication and to have conditions that fasting affects. The men in this study were enrolled in a supervised trial with scans and blood tests. If you want a gentler starting point, the fasting schedules overview lists daily plans, and how to choose the right fasting schedule for your lifestyle walks through the trade-offs.

Fasting is not right for everyone, and this post is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting. That applies with extra force if you are older and worried about muscle or bone.

Tracking a fast with QoFast

QoFast is a fasting tracker app for iPhone and Android, for intermittent fasting and multi-day extended fasts, with a timer that works offline plus water and mood logging and a fasting history. It offers 16:8, 18:6, 20:4, OMAD and custom plans, so a longer custom fast can be timed too. QoFast gives general information, not medical advice, and is free to download with an optional Pro subscription on the App Store and Google Play.

FAQ

Does extra protein prevent muscle loss during alternate-day fasting? In this trial, a body-mass-scaled protein supplement produced no measurable added benefit over 28 days compared with a supplement containing no protein. That is one short study in older men, not a general rule.

Is leucine worth taking while fasting? The trial found no measurable advantage for the leucine group on body composition, strength, function or blood markers over four weeks.

Did the men lose muscle? The abstract reports reductions in all body composition measures except bone mineral density, without separate numbers for lean mass. Strength and functional outcomes improved over the same period.

Sources

• Pang BWJ, Yang Y, Cheon BK. "Effects of Body-Mass-Scaled Protein and Leucine Supplementation on Body Composition and Cardiometabolic Blood Markers During Weight Loss in Older Men: A Randomized Controlled Trial." Nutrients, 2 October 2026. DOI: 10.3390/nu18193255. PubMed: 42884706.