Protein2026-07

Intermittent Fasting and Fat-Free Mass Outcomes in Middle-Aged and Older Adults: A Scoping Review.

Santero Sara, Blagojević Bojana D, Merzah Mohammed, Celada Guerrero José Antonio et al. — Advances in nutrition (Bethesda, Md.)

Summary

Intermittent fasting is a popular weight-loss method, but there are concerns about its effect on muscle mass, especially in older adults who naturally lose muscle with age. A review of studies found that intermittent fasting generally helps reduce body fat and weight similar to continuous dieting, without significant differences in overall muscle mass compared to other diets over short periods. However, more long-term research is needed to fully understand its impact on muscle preservation.

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Key points

Intermittent fasting can help reduce body fat and weight in middle-aged and older adults.
Over short periods, intermittent fasting generally does not cause greater muscle loss compared to other dieting methods.
More research is needed to understand the long-term effects of intermittent fasting on muscle preservation in older adults.
The role of protein intake during intermittent fasting for muscle health requires further investigation.

What the study looked at

This review looked into how intermittent fasting affects muscle mass (fat-free mass) in middle-aged and older adults, comparing it to regular eating habits or continuous calorie restriction. It also considered other body measurements, physical function, and quality of life. Researchers systematically searched for randomized controlled trials published between 2000 and 2025. They included studies involving adults aged 45 or older who engaged in intermittent fasting for at least four weeks and reported muscle mass using reliable methods. The review ultimately included 20 such trials, mostly involving individuals who were overweight or obese. The review found that intermittent fasting was effective in reducing body fat, overall weight, and waist circumference, similar to continuous calorie restriction. Importantly, over the typical study durations (often 12 weeks or less), there were generally only small differences in total muscle mass between intermittent fasting groups and control groups. However, the evidence on long-term effects, specific muscle changes, and the role of protein intake was limited, highlighting a need for more comprehensive studies.

Dietary takeaway

If you're considering intermittent fasting for weight management, it appears that short-term use may help reduce body fat without significantly compromising overall muscle mass compared to other diets. To support muscle health during any weight loss strategy, including intermittent fasting, ensuring adequate protein intake from sources like lean meats, fish, eggs, dairy, and legumes is crucial. Remember, this review synthesizes existing studies, but individual results can vary, and more long-term research is needed to draw definitive conclusions.

Abstract

Intermittent fasting (IF) has gained popularity as a weight-loss strategy. Because muscle mass naturally declines with age, concerns arise about effects on fat-free mass (FFM) in aging populations. The objective of this review was to explore the current evidence on the effects of IF on: 1) FFM in middle-aged and older adults, compared with habitual diet (HD) or continuous energy restriction (CER); and 2) other anthropometric outcomes, functional performance, and quality of life. Three databases were searched, supplemented with snowballing and the clinicaltrials.gov registry. Eligible studies (2000-2025) were randomized controlled trial (RCTs) in adults ≥45 y, with IF lasting ≥4 wk, reporting FFM using validated methods, and excluding participants with disease-related muscle loss. Twenty RCTs (1653 participants; 64% females), primarily involving adults with overweight/obesity, were included. Over the intervention durations studied (commonly ≤12 wk), between-group differences in total FFM were generally small. Findings showed that IF produced reductions in fat mass (FM), body weight (BW), and waist circumference that were comparable with those observed with CER. Reductions in BW and FM were more consistently reported in IF compared with HD. Alternate-day fasting and alternate-day modified fasting were commonly studied and frequently associated with BW and FM reductions, particularly compared with HD. Evidence regarding insulin-related outcomes, dietary protein intake, and combined IF and physical activity interventions was limited and heterogeneous. Functional and quality of life outcomes were rarely assessed. Most studies measured total FFM or lean mass using dual-energy X-ray absorptiometry or bioelectrical impedance analysis; thus, it may not reflect skeletal muscle changes. Collectively, the current evidence does not permit definitive conclusions regarding the impact of IF on FFM preservation, especially in the long-term, in older populations. The findings highlight substantial gaps in study duration and methodological standardization. Longer-term, high-quality trials incorporating standardized muscle health and functional outcomes are needed to clarify the role of IF in strategies for healthy aging.

View on PubMed →DOI: 10.1016/j.advnut.2026.100663