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

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.