Discover how syncing your eating habits with exercise can help shed fat, preserve muscle, and align with your body’s natural rhythms, according to groundbreaking research.

Systematic Review: Effects of time-restricted eating with exercise on body composition in adults: a systematic review and meta-analysis. Image Credit: goffkein.pro / Shutterstock

In a recent study published in the International Journal of Obesity, researchers assessed the effects of time-restricted eating (TRE) on body composition in exercising adults.

Intermittent fasting (IF) refers to dietary interventions wherein individuals complete periods of fasting alternating with feeding. Research suggests that IF may lead to weight loss and alleviate cardiometabolic risk. Various IF variations have been described in the literature, such as TRE, alternate-day fasting, 5:2 fasting, and Ramadan fasting. TRE involves fasting for 12–20 hours, with a feeding window of 4 to 12 hours.

TRE may also reduce systemic inflammation, fat mass (FM), and cardiometabolic risks. However, some studies have observed no changes in body composition or weight with TRE relative to controls, while others noted reductions in fat-free mass (FFM) with TRE. Combining exercise with diet is often believed to augment effects on body composition. The combination of TRE and exercise may particularly help enhance fat mass reduction while minimizing losses in FFM, though results across studies remain variable.

About the study

In the present study, researchers investigated the effects of TRE and exercise on body composition in adults. They performed a comprehensive literature search on five databases: PubMed, CINAHL, SCOPUS, SPORTDISCUS, and Medline. They considered randomized crossover studies and randomized controlled trials (RCTs) reporting the effects of TRE and exercise on body composition outcomes in healthy adults relative to exercise-matched controls on unrestricted eating.

Healthy adults were individuals without cardiometabolic or chronic conditions other than obesity/overweight. Search results were deduplicated, followed by title/abstract screening and full-text review. The researchers extracted data on study details and design, sample characteristics, and body composition outcomes: body mass index (BMI), FFM, FM, and body fat percentage. Separate meta-analyses were performed for each outcome variable.

The correlation coefficient was estimated between pre-and post-intervention measurements. A random effects model calculated the weighted mean effect size. The heterogeneity in the overall effect size was assessed using Cochran’s Q statistic. Further, moderator analyses were conducted for exercise type, BMI, energy intake, and intervention duration. A meta-regression analysis was performed to examine the effects of age on the overall effect size. The study adhered to PRISMA guidelines and included studies published up to May 2023.

Findings

The database search identified more than 1,000 articles; only 15 studies were selected for meta-analyses. These studies collectively included 338 participants with an average age of 28.7 years. Thirteen studies included healthy and active adults with a BMI of 25 or less, while two studies included healthy adults with obesity or overweight. Four studies were on aerobic training, six were on resistance training, and five were on concurrent training.

One of the aerobic training studies involved cyclists and three involved runners. Concurrent training studies involved both aerobic and resistance training or high-intensity interval training. Resistance training studies included lower and upper body exercise regimens generally performed under supervision. In five studies, participants were in a mild energy-restricted state throughout the intervention.

Most studies implemented a midday TRE protocol. Exercise was performed during the feeding window in all studies. Thirteen studies had a low risk of bias, while two were classified as unclear risk. Overall, there was a small but significant decrease in FM (approximately 1.3 kg) and body fat percentage (1.3%) with TRE and exercise relative to exercise alone. Nevertheless, there was a high level of heterogeneity in FM changes between studies. Besides, age did not explain the variations in effect sizes for FM.

Similarly, there was a small and significant reduction in body fat percentage following TRE plus exercise relative to controls. There was substantial heterogeneity between studies for body fat percentage. Moreover, age did not explain the changes in effect sizes for body fat percentage. Notably, no significant changes occurred in FFM with TRE and exercise. Likewise, there was significant heterogeneity in FFM effect sizes between studies.

Conclusions

The findings indicate a small and significant reduction in body fat percentage and FM with the combination of TRE and exercise for at least four weeks. When converted into raw scores, this reduction is estimated at approximately 1.3 kg of FM and 1.3% in body fat percentage.

The combined intervention had no impact on FFM. However, limitations such as short study durations (4–8 weeks in most cases), reliance on self-reported dietary intake, and high heterogeneity between study outcomes caution against overgeneralizing these results. Further studies are needed with longer-term interventions and different TRE protocols in diverse populations.