Ultramarathon running alters glucose dynamics in three distinct phasesGlucose spikes during ultramarathons may not match standard diet or exercise rules

Ultramarathon running is characterized by extreme duration and substantial physiological stress, making individualized nutritional strategies essential for performance and completion. This mini-review examines glucose dynamics during ultramarathon running using continuous glucose monitors (CGMs) and proposes a conceptual multi-phase framework. Glucose dynamics during ultramarathon running can be conceptualized into three phases. The first phase is characterized by transient elevations in blood glucose. In the second phase, a gradual decline in blood glucose is observed. The third phase is characterized by an increase in glucose variability, leading to transient elevations in blood glucose. Importantly, late-stage increases in blood glucose levels cannot be fully explained by conventional determinants such as carbohydrate intake or exercise intensity. Potential mechanisms underlying these observations include changes in substrate utilization and cumulative inflammatory stress. These changes may impair glucose uptake in skeletal muscle and contribute to a mismatch between glucose availability and utilization. Although direct evidence remains limited, CGMs may provide a useful tool for identifying overall trends in glucose dynamics and supporting individualized nutritional strategies during ultramarathon running.