Six weeks of time-restricted eating improves basal fat oxidation and body composition but not fat oxidation during...

Time-restricted eating (TRE) is a type of intermittent fasting, requiring individuals to limit their eating timeframe to specific hours in the day, while maintaining a fasting period greater than 12 h. Fat oxidation (FOx) is a critical determinant in the pathophysiology of metabolic diseases, with impaired FOx contributing to conditions such as insulin resistance and obesity, whereas enhanced FOx is associated with improved metabolic health. However, the impact of the 16:8 TRE model on FOx remains largely unexplored. The aim of this study was to determine the effect of a 6-week TRE on resting and exercise substrate oxidation, body composition, and blood markers related to metabolic health.

Thirty-three healthy, young males (age: 27.5 ± 6 years, body mass: 76.5 ± 8.4 kg, maximal oxygen uptake [V˙O₂max]: 43.9 ± 6.6 mL·kg^-1·min^-1) were assigned to either TRE (n = 16) or control group (n = 17), with efforts to match baseline characteristics, including V˙O₂max and body composition. The TRE group followed a 16:8 program for 6 weeks, while controls maintained their existing dietary habits. Body composition, blood glucose, insulin, blood lipids, resting substrate oxidation, and FOx during cycling at 40 % V˙O₂max were assessed before and after the 6-week period. Data were analyzed using both intention-to-treat (ITT) and per-protocol approaches.

Thirty-three participants were included in the ITT analysis, while 31 participants were included in the per-protocol analysis. Compared to baseline, results showed a significant difference (p < 0.05) between TRE and control groups in body mass (TRE versus control) (Δ = -2.8 kg versus Δ = 0.7 kg), fat mass (Δ = -1.4 kg versus Δ = 0.4 kg), percent body fat (-1.7 % versus 0.4 %), lean mass (Δ = -1.4 kg versus Δ = 0.3 kg), and visceral adipose tissue mass (Δ = -39.7 g versus Δ = 46.4 g). There was a significant difference between TRE and control groups in resting respiratory exchange ratio (RER, Δ = -0.02 versus Δ = 0.02; p = 0.016), FOx (Δ = 0.33 mg·kg FFM^-1·min^-1versus Δ = -0.37 mg·kg FFM^-1·min^-1; p = 0.007), and carbohydrate oxidation (Δ = -0.39 mg·kg FFM^-1·min^-1versus Δ = 0.45 mg·kg FFM^-1·min^-1; p = 0.037) after the 6-week period. Exercise substrate oxidation and fasting blood glucose, insulin, triglycerides, total cholesterol, and high-density lipoprotein cholesterol did not significantly change over time in either group (p > 0.05).

In summary, a 6-week TRE significantly reduces body mass, fat mass, and resting RER as well as increases resting FOx in young, healthy males. However, it does not affect blood markers related to cardiometabolic health or exercise substrate oxidation. This trial was registered at https://clinicaltrials.gov/study/NCT06498102asNCT06498102.