Limitations and Strengths
This was an acute study evaluating the change in appetite hormones over the course of a single 36-h fast. While the results are informative, we are unable to describe how these results may change with repeated fasts over time. In addition, while each participant abstained from food for four hours prior to presenting at the lab and was fed a standardized meal at the beginning of each fast, the food intake prior to that was not controlled. The food and drink consumed prior to that point may have had a small residual impact on the participant’s metabolic state. However, participants were instructed to follow normal eating patterns and not overconsume at these meals in anticipation of the fast. There were no differences between appetite hormones at baseline, indicating that participants started each fast in a similar metabolic state. The fast was performed under free-living conditions, which lack the control of a lab environment. However, while this introduces some variability between fasts, the results are more generalizable since behaviors were not constrained. In addition, condition order was randomized, and any random differences in behavior between conditions should be controlled. Finally, blood draws were taken every 12 h, occurring at baseline, 12, 24, and 36 h. More frequent blood samples would better describe the time course of metabolic changes during the fast, especially within the first few hours after exercise. Additionally, we did not control the phases of the menstrual cycle of the women in the study. This could allow for more variability in appetite hormones since the hormone concentrations change with the phase of the menstrual cycle [Despite these limitations, this study adds to the current literature regarding the effects of combined fasting and exercise. While most research has evaluated the impact of fasting or exercise on appetite-regulating hormones separately, this study is one of the first to evaluate the behaviors in combination. In addition, most studies only evaluate a few hormones at a time or each hormone individually. The current study described the impact of exercise and fasting on seven hormones, which allows a broader perspective on how fasting and exercise influence the biological drive to consume food. While much of the existing literature captures the acute impact (30 min to 3 h) of exercise or fasting on appetite hormones, this study evaluated the impact up to 36 h. This enabled us to describe the prolonged effects of exercise on these hormones and to observe the non-linear response of both GLP-1 and ghrelin. Finally, the practice of intermittent fasting and time-restricted eating is very popular because of their potential to improve body composition and metabolic health. This study adds to the current body of literature by describing how exercise could be used in conjunction with fasting to potentially enhance the benefits of this behavior while mitigating the negative effects of hunger.