Diurnal differences in postprandial glucose and triglyceride metabolism reveal metabolic flexibility and resilience.

Objective The study investigated the diurnal variance in metabolic resilience (i.e., the robustness, the recovery and re-orientation of metabolism) and metabolic flexibility in glucose and fat oxidation rates to three identical test meals. Methods Eight young, healthy subjects consumed identical liquid mixed meals three times a day (33 % of energy requirement each), followed by a defined bout of physical activity conducted in a whole-room indirect calorimeter to continuously assess energy expenditure and postprandial changes in substrate oxidation rates, as a measure of metabolic flexibility. A mathematical metabolic resilience model was used to analyze the postprandial blood parameters. Results Throughout the day, postprandial glucose area under the curve (AUC) increased (breakfast mean ±SD 17.3 ±2.4 vs. dinner 20.8 ±2.0 g/180min; p<0.001) while triglyceride AUC decreased (breakfast 434 ±158 vs. dinner 365 ±104 mg/180min; p=0.039) at identical insulin AUC and energy balance. Fat oxidation increased from breakfast 24.8 ±8.7 to dinner 28.0 ±8.7 g/180min (p=0.029), while respiratory exchange ratio declined from 0.035 ±0.026 to 0.012 ±0.029 (p=0.005). Metabolic resilience model reveals a diurnal increased rate of lipolysis of circulating triglycerides at a concomitant decrease in the rate of exogenous and endogenous triglyceride appearance. Conclusion Meal-to-meal changes in glucose AUC indicate rising insulin resistance during the day. However, this reflects a resilient metabolism that shifts to triglyceride metabolism in the evening while maintaining insulin AUC and energy balance.