Time-of-Day Effects of Fasted Exercise on Energy Intake

In a five-way crossover design, participants will complete three fasted and one non-fasted exercise sessions on a treadmill in randomized order on weekdays with one week in between sessions: once in the morning (~1h after participants' habitual wake-up time) following a 12h overnight fast (12h AMEx), once in the evening (exactly 12h after the 12h AMEx condition) after a 6h fast (6h PMEx), and once in the evening (identical to the 6h PMEx condition) following a 12h fast (12h PMEx). As control conditions, a subsample of participants (n=17) will also complete a fed morning exercise condition (AMEx Fed; identical timing to 12h AMEx) and a fed evening exercise condition (PMEx Fed; identical timing to 12h PMEx), following a small pre-exercise snack (~10% of individual energy requirements; 30 min before exercise). The standardized meals provided before the fasting period contained 30% of the individual's daily energy requirements and consisted of ~50% carbohydrates, ~30% fat, and ~20% protein.

Inclusion criteria

• BMI of 18.5.0-30.0 kg/m2
• weight stability (≤2.5 kg weight change during the past 3 months)
• ability to exercise at a vigorous intensity for 30 minutes

Exclusion criteria

• pregnancy or breastfeeding
• smoking
• history of or current eating disorders
• medical condition or use of medication that could affect appetite or pose any contraindications to exercise

As the menstrual cycle can cause fluctuations in food cravings and appetite, women of childbearing age were only eligible if they used hormonal contraceptives. To ensure constant exposure to exogenous estrogen and progestin throughout the study phase, participating women were further asked to use active hormone pills throughout the study period in both studies, a practice that is considered both safe and efficacious.

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