High-intensity interval training (HIIT) improves metabolic health in adults with overweight or obesity, but the time of day may influence its benefits. Findings presented at the European Association for the Study of Diabetes 2025 suggest that afternoon and morning HIIT programs confer distinct advantages.

Twenty-five adults participated in a 12-week HIIT program on a stationary bike, exercising either in the morning (08:00–11:00) or afternoon (14:30–18:00). Peripheral insulin sensitivity was measured via hyperinsulinemic-euglycemic clamps, muscle mitochondrial function was assessed using muscle biopsies and 31P-MRS, and intrahepatic lipid content and composition were evaluated by 1H-MRS.

After training, afternoon HIIT produced greater improvements in peripheral insulin sensitivity, while morning HIIT led to larger increases in muscle oxidative proteins and citrate synthase activity. VO2max increased similarly in both groups. Additionally, afternoon training more effectively reduced saturated fatty acids in liver lipids, highlighting timing-specific effects on metabolic outcomes.

These results indicate that the timing of HIIT can differentially enhance insulin action, muscle mitochondrial health, and liver lipid metabolism, offering practical guidance for personalized exercise interventions in metabolically compromised adults.