Exercise Timing and Postprandial Glucose Responses in Healthy Adults

This study is conducted as a repeated measures, randomized crossover trial. Each participant completes five intervention conditions within a 14 day study period while wearing a continuous glucose monitoring device. The order of conditions is randomized, and all participants complete all conditions.

The five conditions include endurance exercise performed before breakfast, after breakfast, before dinner, after dinner, and a recovery day without exercise. On exercise days, participants complete a 45 minute outdoor run at a vigorous intensity at predefined times relative to standardized meals. Breakfast, lunch, and dinner are consumed at fixed clock times (08:00, 13:00, and 18:00, respectively).

Postprandial glucose outcomes are assessed for the three consecutive meals following each exercise session. In conditions involving exercise performed before or after breakfast, outcomes are assessed following breakfast, lunch, and dinner on the same day. In conditions involving exercise performed before or after dinner, outcomes are assessed following dinner on the same day and following breakfast and lunch on the subsequent day.

Dietary intake is fully standardized across all intervention days. Meal composition is identical across intervention days within participants, with macronutrient composition held constant. Meals are designed to have a high glycemic load, calculated using published glycemic index tables and established food composition databases. Meals must be consumed within a predefined consumption window, with the start of meal intake serving as the reference time point for postprandial glucose analyses. Adherence to meal timing and consumption is verified using time-stamped photographs of meals and packaging, along with a standardized checklist documenting consumption times.

Habitual waking times compatible with the fixed breakfast time are required for study participation. During the intervention period, participants are instructed to standardize bedtime as closely as possible across study days and to record wake time and sleep duration daily by self-report. Caffeine consumption is restricted on intervention days and on the day preceding each intervention day. On the day before each intervention day, participants are also instructed to refrain from structured exercise and alcohol. Participants are further instructed to replicate, as closely as possible, the type and timing of meals consumed on the day preceding each intervention condition in order to standardize pre-intervention nutritional status. This applies particularly to the day before conditions involving morning exercise and to earlier meals consumed on days with evening exercise.

All exercise sessions are completed outdoors on the same route. Ambient temperature and precipitation are recorded for each exercise session using smartphone-based weather data. Exercise sessions may be rescheduled in cases of extreme weather conditions or acute illness to ensure participant safety and protocol consistency.