Exercise Timing and Postprandial Glucose Responses

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.

Inclusion criteria

• Age 18 to 35 years
• Body mass index 18.5 to 24.9 kg per square meter
• Able to complete 45 minutes of continuous running at an intensity corresponding to 75 to 85 percent of maximal heart rate without stopping
• Engages in regular endurance-type physical activity
• Habitual waking times are compatible with the fixed breakfast time of 08:00
• Willing and able to comply with all study procedures, including standardized meals, exercise sessions, behavioral restrictions, and continuous glucose monitoring
• For female participants: non-pregnant and using stable hormonal contraception for at least 3 months

Exclusion criteria

• Acute or chronic illness
• Metabolic disorders or conditions associated with altered glucose or lipid metabolism
• Use of medications known to interfere with glucose metabolism or study outcomes
• Intolerance or allergy to components of the standardized study diets
• Pregnancy or breastfeeding
• Body mass index greater than or equal to 25 kg per square meter
• Current or past eating disorder, or any medical condition or medication that could affect appetite regulation, glucose regulation, or make vigorous endurance exercise unsafe
• Current smoking or use of nicotine-containing products