Four weeks of 16/8 time restrictive feeding in endurance trained male runners decreases fat mass, without affecting exercise performance

Authors: TOVAR, ASHLEY - University Of California, Davis; RICHARDSON, CHRISTINE - University Of California, Davis; Keim, Nancy; Van Loan, Marta; DAVIS, BRUCE - Uc Davis Medical Center; CASAZZA, GRETCHEN - California State University

Submitted to: Nutrients
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2021
Publication Date: 8/25/2021
Citation: Tovar, A.P., Richardson, C.E., Keim, N.L., Van Loan, M.D., Davis, B., Casazza, G.A. 2021. Four weeks of 16/8 time restrictive feeding in endurance trained male runners decreases fat mass, without affecting exercise performance. Nutrients. 13(9). Article 2941. https://doi.org/10.3390/nu13092941.
DOI: https://doi.org/10.3390/nu13092941

Interpretive Summary: Competitive athletes are adopting many types of "fad" diets to maximize their performance, especially if the type of activity they do is affected by body weight. We conducted a study to test a time-restricted feeding study in competitive long-distance male runners to determine how this type of eating regimen affected body weight, body fat, and lean body mass. Our study demonstrated that when the eating window is limited to 8 hours a day, with the remaining 16 hours devoted to fasting, body fat decreased and lean body mass remained the same, compared to an eating regimen with a 12 hour eating window. These results suggest that endurance athletes who seek to lose body fat prior to competition could adopt this type of diet without losing muscle mass that is vital to endurance performance.

Technical Abstract: Background. The “16/8” diet (16 hours fasted and 8 hours fed) is a form of time restricted feeding (TRF) that is gaining popularity among endurance athletes as a means of losing body weight to maximize their performance. At present, there has only been one investigation of the 16/8 diet in endurance athletes, and while the authors reported promising results, the study did not control for calories, which lead to the consumption of far fewer calories in the TRF group. Objective. The purpose of the present study was to investigate the effects of TRF on body composition, resting energy expenditure (REE), and resting respiratory exchange ratio (RER) in competitive male endurance runners. Design. Fifteen competitive, endurance trained runners completed the randomized, cross-over intervention consisting of two 4-wk arms: a normal diet (ND) pattern arm (12 hours fasted and 12 hours fed) and the 16/8 TRF arm. Prior to the start and at the end of each intervention, subjects underwent a dual x-ray absorptiometry (DXA) scan for body composition, and indirect calorimetry was used to determine resting energy expenditure and respiratory exchange ratio. Caloric intake, macronutrients and running mileage were held constant for both interventions. Results. There was a significant 7% decrease in body fat percentage with TRF, compared to ND (TRF -1.0 ± 1.5% vs. ND +0.1 ± 1.3%, p<0.04). There was a tendency for fat mass to decline -0.8 ± 1.3 kg with TRF compared to the change of +0.1 ± 4.3 kg with ND p=0.05). There was no significant difference in fat-free mass, resting energy expenditure, and resting respiratory exchange ratio. Conclusion. TRF may be an appropriate strategy for endurance athletes who are seeking to lose body fat percentage, while maintaining fat free mass.