The acute effect of time-restricted feeding (12 & 16 h) and varying exercise intensities on fat-oxidation rate in inactive young adults – a randomized control trial

Time-restricted feeding (TRF) is a dietary pattern that alternates between periods of fasting and feeding, which has gained significant attention in recent years. The 16/8 approach consists of fasting for 16 h and feeding for an 8-h window, while the 12/12 method consists of fasting for 12 h and a 12-h feeding window. Limited research exists comparing the effects of these methods coupled with physical activity (PA). The aim of this investigation was to examine the acute effects between conditions of varying TRF durations (12 and 16 h) and PA intensities on the fat oxidation rate (FOR). It was hypothesized that i) the TRF16 conditions would exhibit higher FORmax and that PA would enhance these effects, and ii) High Intensity Interval Training (HIIT) would result in greater effects on FORmax compared to Low-Moderate Intensity Steady State Continuous Training (MICT) PA. Eighteen young adults (age: 23 ± 2.0 yrs., body mass index: 23.5 ± 2.8 kg·m−2) were recruited and participated in the supervised intervention. The discrete component open circuit spirometry system was used to measure oxygen consumption (VO2), and Frayne's equation was used to determine the FOR plus FORmax. ANOVA was used to determine pre/post-intervention differences in FORmax. The FORmax for the TRF16 + HIIT intervention was significantly higher than the TRF12 (mean difference = 0.099 g·min-1, p = 0.011, 95% CI 0.017 to 0.180) and TRF16 fast alone (mean difference = 0.093 g·min-1, p = 0.002, 95% CI 0.027 to 0.159). The FORmax for TRF12 + HIIT intervention was significantly higher than the TRF12 fast alone (mean difference = 0.070 g·min−1, p = 0.023, 95% CI 0.007 to 0.134). The TRF16 + HIIT intervention was also significantly higher than the TRF12 fast alone (mean difference = 0.099 g·min−1, p = 0.011, 95% CI 0.017 to 0.180). This study contributes to the ever-growing body of literature on the acute effects of TRF and PA on young adult males and females. The findings suggest that the TRF16 + HIIT PA intervention results in the highest FORmax. Retrospective Registration ISRCTN # 10076373 (October 6, 2023).