Discover how synchronizing your eating habits with exercise can help you lose fat, maintain muscle, and align with your body’s natural rhythms, according to groundbreaking research.
Systematic review: Effects of time-restricted dieting with exercise on body composition in adults: a systematic review and meta-analysis. Image credit: goffkein.pro / Shutterstock
In a recent study published in International Journal of Obesityresearchers evaluated the effects of time-restricted feeding (TRE) on body composition in exercising adults.
Intermittent fasting (IF) refers to dietary interventions where individuals complete periods of fasting alternating with feeding. Research shows that IF can lead to weight loss and reduce cardiometabolic risk. Various IF variants 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 have noted decreases in fat-free mass (FFM) with TRE. Combining exercise with diet is often thought to increase the effects on body composition. The combination of TRE and exercise may particularly help reduce fat mass while minimizing losses in FFM, although 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 in five databases: PubMed, CINAHL, SCOPUS, SPORTDISCUS and Medline. They reviewed 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 diet.
Healthy adults were individuals without cardiometabolic or chronic diseases other than obesity/overweight. Search results were abstracted, followed by title/abstract screening and full-text review. Researchers extracted data on study details and design, sample characteristics, and body composition outcomes: body mass index (BMI), FFM, FM, and percent body fat. Separate meta-analyses were performed for each outcome variable.
The correlation coefficient was calculated between pre- and post-intervention measurements. A random effects model calculated the weighted mean effect size. Heterogeneity in overall effect size was assessed using Cochran’s Q statistic. Further, moderator analyzes were performed for exercise type, BMI, energy intake, and intervention duration. Post-regression analysis was performed to examine the effects of age on overall effect size. The study complied with 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 a mean 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 involved aerobic training, six involved resistance training and five involved concurrent training.
One of the aerobic training studies involved cyclists and three involved runners. The concurrent training studies included both aerobic training and resistance training or high-intensity interval training. Resistance training studies included lower and upper body exercise regimens that were 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 reduction in FM (about 1.3 kg) and percent body fat (1.3%) with TRE and exercise relative to exercise alone. However, there was a high level of heterogeneity in FM changes between studies. Additionally, age did not explain variance in effect sizes for FM.
Similarly, there was a small and significant reduction in body fat percentage after TRE plus exercise relative to controls. There was considerable heterogeneity between studies for body fat percentage. Furthermore, age did not explain the changes in effect sizes for percent body fat. Notably, there were no significant changes in FFM with TRE and exercise. Similarly, there was considerable heterogeneity in FFM effect sizes across studies.
conclusions
The findings show 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 to raw scores, this reduction is calculated at approximately 1.3 kg FM and 1.3% body fat percentage.
The combined intervention had no impact on FFM. However, limitations such as short study duration (4–8 weeks in most cases), reliance on self-reported dietary intake, and high heterogeneity among study results caution against overgeneralizing these results. Further studies with long-term interventions and different TRE protocols in different populations are needed.
