Fasted vs. Fed State Exercise

This study will be the first to evaluate a long-term (16-week) aerobic exercise intervention performed in the fasted state (compared to fed state) at a guidelines-based dose. Primary outcomes for Aim 1 will include changes in fat \& fat-free mass (DXA), energy compensation, blood pressure \& lipids, aerobic fitness, and substrate oxidation (respiratory quotient, RQ) at rest and during activity. It is also important to understand sources of response variability to an exercise program. Individuals tend to compensate for the energy they expend during exercise, primarily by increasing energy intake (EI). Such compensatory eating behaviors likely stem from a series of evolutionarily conserved responses, ensuring energy is available for vital organ function and reproduction when faced with an energy deficit. This proposal will be the first to evaluate how fasted exercise training influences (Aim 2 outcomes): physiological hunger (hormonal responses to a standardized meal) and behavioral constructs shown to influence EI and weight status (food reinforcement and attentional bias towards food cues). The overall hypothesis is that fasted state exercise will result in greater energy compensation, attenuating weight loss compared to an identical dose of exercise performed in the post-prandial state. This study will further hypothesize an attenuated weight loss with fasted exercise training will negate beneficial cardiovascular adaptations stemming from improvements in oxidative metabolism commonly associated with fasted exercise. Adults (aged 18-59 years, BMI: 25-45 kg/m2) will be randomized to a 16-week supervised aerobic exercise intervention (progressing to a guidelines-based dose of 1600 kcal/wk). Exercise sessions will be performed between 0500h and 1100h, 4 days per week in either the fasted state (FAST, 8-12 hour fast) or post-prandially (FED, within 3 hours of eating at least 300 kcal).