The timing of exercise can influence glucose and substrate metabolism, particularly in individuals at risk of type 2 diabetes. The European Association for the Study of Diabetes (EASD) 2025 study examined whether acute high-intensity exercise performed in the morning or afternoon alters 24-hour substrate rhythmicity in older adults with elevated body mass index.

Fourteen participants (4 females, 10 males; 40–75 years, BMI >25 kg/m²) underwent three separate 24-hour stays in a respiration chamber in a randomized crossover design: control (no exercise), morning exercise (09:00), and afternoon exercise (17:00). Exercise sessions consisted of repeated 2-minute intervals at 80% and 50% of maximal workload. Continuous glucose monitoring and blood sampling tracked metabolic changes, while meals were standardized.

Both morning and afternoon exercise reduced post-exercise respiratory exchange ratio and increased fat oxidation compared to control. Notably, only afternoon exercise lowered nocturnal RER and increased fat oxidation during the night. Energy expenditure immediately post-exercise was higher for morning exercise, but metabolic benefits during nocturnal periods were superior with afternoon exercise.

These findings suggest that scheduling exercise in the afternoon may enhance nighttime fat utilization and improve 24-hour substrate metabolism in adults at risk of developing type 2 diabetes.