Discussion
To the best of our knowledge, no prior systematic review with meta-analysis has been conducted to specifically compare the effects of TRF versus ND on physical performance and body composition in healthy adults with regular exercise habits. In total, 15 studies were identified, and all were available for meta-analysis. In the meta-analysis, we specifically investigated whether TRF could serve as a more efficacious nutritional approach compared with an ND when individuals in both groups undergo an identical exercise training regimen. Our findings suggest that the combination of TRF with regular exercise training does not result in significant changes or improvements in physical performance outcomes when compared with ND with the same exercise training programme. However, it successfully achieves a significant fat loss outcome.
In this meta-analysis, it is important to note that the indicator of aerobic capacity performance in all the included studies was assessed using the indicator of VO2max, which is considered a critical indicator for the assessment of aerobic performance.39–41 Our findings revealed that the combination of TRF with regular exercise training did not lead to a significant change in aerobic capacity performance compared with the ND group. This contrasts with the conclusions of a previous systematic review conducted by Correia et al, which reported a positive impact of TRF on VO2max.1 Additionally, our meta-analysis indicated that the combination of TRF and regular exercise training did not have a significant influence on power performance, a critical factor closely linked to athletic performance and essential for incremental tests.14 18 42 Subgroup analysis further showed that the combination of TRF with regular exercise training exhibited no significant influence on either aerobic capacity or power performance, regardless of the intervention duration (less than 8 weeks and 8 weeks or more). These findings emphasise that key performance indicators such as oxygen utilisation and power output are not reduced or physiologically affected following the implementation of such interventions. Discrepancies between our review and a previous systematic review24 can be attributed to differences in study characteristics. Our meta-analysis specifically focused on the 16/8 TRF strategy, consisting of a consistent 16-hour fasting period followed by an 8 hour eating window, maintained for 4 to 8 weeks. In contrast, the previous systematic review24 encompassed a broader range of IF strategies with varying durations and fasting periods, which may have led to diverse physiological adaptations among participants.10 Robust and comprehensive conclusions regarding the effectiveness of combining TRF with exercise on aerobic capacity and power performance in future clinical research necessitate well-designed large-scale studies.
Despite the existence of a narrative review43 and a systematic review1 on the topic, there is a lack of a meta-analysis in the literature examining the effects of the TRF programme in combination with daily exercise training on indicators related to muscle performance. Correia et al’s previous meta-analysis on IF and muscle strength reported non-significant effects. However, it is important to acknowledge the limitations of that study, including a limited number of included studies and the incorporation of diverse IF regimens (eg, Ramadan IF, TRF), which may not have fully taken into account the potential differences between the two fasting approaches.24 In our meta-analysis, we found no significant comparative effects of TRF versus non-TRF on muscle performance-related indicators, including strength performance, muscular endurance performance and jump performance in healthy adults with regular exercise habits. These findings indicate that the incorporation of TRF as a nutritional intervention alongside daily exercise training regimens may not have a significant impact on certain aspects of muscle performance when compared with the ND group. However, the existing RCTs on this topic have yielded mixed findings. For instance, the RCT conducted by Moro et al demonstrated that TRF plus resistance training resulted in no change in muscle cross-sectional area of the arm and thigh as well as maximal strength after an 8 week intervention by comparing to the ND with the same amount of exercise,8 while another study found a greater increase in lower body strength with TRF combined with resistance training.38 Furthermore, our subgroup analysis, which stratified the studies based on intervention duration, revealed a slight improvement in strength performance when TRF was combined with exercise. However, this increase was not statistically significant, suggesting that the effects of TRF on physical performance may diminish over time, potentially due to adaptive changes and the development of tolerance to TRF.43
In athletics, the optimisation of body composition and the adjustment of physical condition play vital roles. The meticulous management and equilibrium between lean muscle mass and fat are critical factors that substantially influence sports-related performance, particularly in sports such as boxing, weightlifting and others that rely heavily on strength and power.44 Our findings indicate that including TRF with regular exercise training is effective in achieving significant reductions in weight and FM with no significant difference in FFM compared with ND with regular exercise training. The potential benefits of TRF on body composition can be attributed to two main reasons. First, TRF may promote a reduction in daily calorie intake. When individuals are faced with time constraints, restricting food consumption becomes a practical strategy for reducing calorie intake, particularly when compared with the time-consuming tasks of meal preparation, cooking and calorie counting.3 In contrast, traditional dieting’s continuous calorie counting often leads to participant attrition. However, TRF offers an alternative approach by emphasising time rather than calorie monitoring. Previous studies have shown that differences in weight loss were not statistically significantly different between the TRF and daily calorie restriction.5 45 Moreover, TRF has been found to reduce overall calorie intake by approximately 25–38%,3 although individual responses to fasting may vary.1 Those characteristics of TRF enhance long-term adherence and enable sustainable weight control.9 45 46 Second, another reason may relate to metabolic benefits observed in the context of TRF, which can be attributed to the improved synchronisation of eating patterns with the individual’s biological circadian clock.47 Aligning eating patterns with the circadian clock has been associated with reduced fasting glucose concentration, improved insulin resistance and positive changes in lipid profiles.48 49 The slight improvements in glucose and lipid metabolism indicators observed with TRF could be partly attributed to the associated slight weight loss.47
While there is substantial evidence supporting the effectiveness of TRF with daily exercise training for fat loss, there are still divergent findings in the literature. For instance, the RCT conducted by Correia et al demonstrated that TRF in conjunction with regular training led to improvements in Wingate test performance but did not result in notable changes in body composition.14 The variability in fat loss across studies may be explained by the type of exercise performed in the intervention, duration of the experiment, total energy intake and participant characteristics. Specifically, in the subgroup analysis based on exercise type within our meta-analysis, encompassing four different types of exercise, the findings indicated that only the combination of TRF with endurance training and resistance training exhibited a statistically significant reduction in FM compared with the ND groups. Furthermore, intriguingly, shorter-term interventions demonstrated greater effects on FM reduction when comparing the TRF group with the ND group, potentially attributed to physiological adaptations.
Strengths and limitations
The current systematic review used a rigorous and comprehensive search strategy encompassing physical performance and body composition indicators. Distinguishing from previous studies, our review with meta-analysis adopted a more focused approach, specifically incorporating studies that only employed TRF as the intervention. However, this study still has several limitations that need to be addressed. First, although the form of TRF used in the included studies was predominantly the 16/8 method, there were variations in the daily fasting periods employed. These differences in fasting periods have the potential to influence biological rhythms and may impact the observed results. Therefore, future studies incorporating diverse fasting periods within TRF protocols would be valuable in further elucidating the effects of TRF on the outcomes of interest. Second, for some physical performance indicators (eg, endurance and power performance), the meta-analysis included a small number of studies, reducing statistical power. Additionally, our review did not specifically discuss blood test biomarkers and their implications for overall health and well-being. Further research in this area is needed to enhance our understanding of the physiological mechanisms involved and their potential impact on overall health and well-being. Finally, for the outcome of fat mass, our study is limited by potential publication and small study bias, which may affect the validity of the findings specifically for this outcome. Our methods to assess publication bias may not capture all forms of bias accurately. Additionally, we cannot differentiate between small study bias and publication bias for fat mass, introducing uncertainty in interpreting the overall effect size for this outcome.