Fasting and muscle: the unanswered part of the weight-loss story

THE ARISING · 23 OCTOBER 2026 · FULL ARTICLE

A lower number on the scale can hide several different changes. Fat may fall, but so may water or lean tissue. For people in midlife and later life, preserving the ability to move and function matters alongside weight loss. A scoping review of intermittent fasting makes that unresolved question visible.

Santero and colleagues reviewed 20 randomized controlled trials involving 1,653 adults aged 45 and older. About 64% were women, and most participants had overweight or obesity. Published in Advances in Nutrition, the review mapped the evidence rather than pooling every study into a single effect estimate.

Trials had to study fasting for at least four weeks and measure body composition with validated methods. People with diseases causing muscle loss were excluded. The comparisons included a habitual diet and continuous calorie restriction—eating less overall without the same fasting schedule.

What the short trials suggest

Across the durations studied, commonly 12 weeks or less, differences between groups in total fat-free mass were generally small. Intermittent fasting produced weight, fat-mass and waist reductions comparable to continuous calorie restriction. Compared with habitual eating, reductions in weight and fat were reported more consistently with fasting.

The fasting approaches varied, including alternate-day regimens, modified fasting days and time-restricted eating. That variety is one reason the results should not become a blanket promise about a particular fasting window.

Lean mass is not the whole muscle story

Fat-free mass includes more than skeletal muscle. It also includes organs, bone and water. Most studies used DXA or bioelectrical impedance, so a finding about total lean mass cannot simply be translated into a finding about muscle strength or physical capability.

Strength, physical performance and quality of life were rarely assessed. Evidence on protein intake, insulin-related outcomes and the combination of fasting with exercise was limited and varied. Data on adults in advanced age were particularly sparse.

Those gaps matter because the question is not only what a body weighs after a few weeks. It is whether someone maintains strength, mobility and reserve over months and years. A short-term similarity between diets cannot settle that longer-term question.

What a stronger answer would require

The authors concluded that the available evidence does not permit definitive conclusions about preservation of fat-free mass, particularly over the long term in older adults. The review therefore offers context, not proof that fasting protects muscle or is safe for everyone.

Longer studies need standardized muscle and functional measurements, with clearer attention to nutrition and exercise. In practice, choosing a weight-loss approach requires attention to the whole person, including medications, nutritional needs and the capacity one hopes to preserve. The worthwhile goal is a change that supports life beyond the scale.