TRE with exercise favourably alters body composition in adults

Time-restricted eating (TRE), when combined with several forms of exercise (aerobic, resistance training, or a combination of both), appears to induce reductions in fat mass and body fat percentage while conserving fat-free mass in adults who adhere to a structured exercise regimen as opposed to exercise-matched controls who do not follow a restricted eating regimen.

In this systematic review and meta-analysis of 15 studies (n=338; mean age 28.7 years), there were small but significant mean reductions in fat mass (effect size, -0.20; p<0.001) and body fat percentage (effect size, -0.23; p<0.001) following TRE with exercise as opposed to exercise only.

“When the effect sizes for fat mass and body fat percentage are converted into a clinically relevant measurement, the estimated additional loss of fat mass and body fat percentage may be close to 1.3 kg and 1.3 percent, respectively. Nevertheless, prediction intervals suggest inconsequential impacts on fat mass and body fat percentage in some populations, with potential variability in fat gain or loss,” the investigators explained.

Conversely, fat-free mass was not significantly altered when TRE was added to an exercise regimen (effect size, -0.04; p=0.07). [Int J Obes (Lond) 2025;doi:10.1038/s41366-024-01704-2]

Potential mechanisms

“It is reasonable to hypothesize that exercise and TRE activate tissue-specific and pathway-specific mechanisms,” the researchers said. “Both TRE and exercise may improve glucose metabolism by improving insulin sensitivity, with exercise promoting enhanced glucose uptake in muscles through GLUT-4 translocation and TRE reducing overall systemic insulin levels.”

The enhanced lipid oxidation by muscle via exercise and increased lipolysis via TRE may also lead to improved lipid metabolism. [Am J Physiol-Endocrinol Metab 2008;294:E726-E732; Nutrients 2020;12:3770]

TRE may have also triggered a spontaneous reduction in energy intake because of the restricted eating window, said the researchers. “Similar to calorie restriction, TRE stimulates the AMP-activated protein kinase/Acetyl-CoA-carboxylase signalling pathway. This pathway is activated under low cellular energy availability conditions, promoting ATP synthesis by enhancing fatty acid oxidation and upregulating glycolytic flux.”

Save for one, all of the studies included in the meta-analysis followed a TRE protocol that had a feeding window during daylight hours, the researchers noted. Hence, the feeding time for the intervention arm is in parallel with the light-dark cycle, as opposed to control participants who usually ate well into the evening. “Since growth hormones are highly regulated by circadian rhythms, alignment of feeding/fasting times with the light-dark cycle could be a mechanism for increased growth hormones observed in TRE.”

Limitations

However, the researchers reiterated that the findings should be interpreted with caution due to certain limitations.

For one, generalizability of the findings may be limited, as most studies included active, healthy adults with a BMI ≤25 kg/m². The mean BMI of participants across studies was in the normal to overweight BMI range (18.5–29.9 kg/m²). “This limitation prevents conclusions regarding any long-term body composition benefits of TRE against an unrestricted diet in an exercising population,” they said.

Moreover, self-reporting of dietary intake may have led to underreporting of energy intake. Also, the different body composition measuring devices used across studies do not always reflect similar results.

Another limitation that should be taken into context is the short durations in most studies, with only one study using a resistance training protocol and TRE for over a year. “Future investigations should focus on long-term studies utilizing various TRE protocols in diverse populations,” the investigators said.