Researchers define intermittent fasting as a dietary intervention characterized by alternating periods of fasting and feeding. Time-restricted eating (TRE) represents a specific variant of this practice where individuals limit food consumption to a 4–12 hour daily window. While scientists recognize the potential for weight loss through TRE, inconsistencies in existing research led to a comprehensive systematic review and meta-analysis in the International Journal of Obesity on January 10, 2025. This investigation clarifies the effects of combining TRE with various exercise modalities including aerobic, resistance, and concurrent training on fat mass, body fat percentage, and fat-free mass in adults.
The authors identified 15 randomized controlled trials and crossover studies involving 338 healthy adult participants through a search of databases like MEDLINE and PubMed for literature published up to May 2023. Eligibility for inclusion was strictly limited to studies with a minimum duration of four weeks. The team performed data synthesis using a random-effects model to calculate weighted mean effect sizes (ES) and assessed heterogeneity among the studies via Cochran’s Q and I² statistics.
Groups following TRE with exercise showed a small but statistically significant reduction in fat mass (FM), evidenced by an effect size of −0.20 (p < 0.001). This effect size represented an additional loss of approximately 1.3 kg of FM relative to the control groups. Furthermore, participants significantly reduced their body fat percentage (BF%) by an estimated 1.34%. High heterogeneity was reported for these results; however, the application of the “trim and fill” technique indicated that potential publication bias did not alter the overall findings.
In contrast to the reductions in adiposity, the study detected no significant changes in fat-free mass (FFM) (p = 0.07). This suggests that athletes and active adults preserve muscle mass when they combine TRE with a structured exercise regimen. Subgroup and moderator analyses further determined that variables such as age, BMI, type of exercise, and energy intake levels did not significantly impact the observed outcomes. Additionally, meta-regression confirmed that age did not explain the variations in effect sizes across the analyzed studies.
The sources propose several biological mechanisms to explain these favorable changes in body composition. The authors identified unintentional energy restriction as a primary factor, as a spontaneous reduction in calorie intake often occurs due to the limited feeding window. On a molecular level, researchers noted the activation of the AMPK/ACC signaling pathway, which may promote fatty acid oxidation. Furthermore, the study suggests that aligning feeding windows with circadian rhythms improves insulin sensitivity and stimulates favorable hormonal changes, including increased levels of growth hormone and noradrenaline.
The authors acknowledged certain limitations, including the short duration of most trials—with many lasting only 4 to 8 weeks and a reliance on self-reported dietary data. Because the participants were largely healthy, active adults, the generalizability of these results to clinical populations remains limited. It was concluded that while TRE combined with exercise induces a significant decrease in fat mass, further long-term investigations are required.
Source:
Nature Publication: International Journal of Obesity
