The present study was conducted to obtain basic information about blood glucose fluctuation and relation with race performance during 100 km marathon. Subcutaneous glucose of one well-trained runner was measured by continuous glucose monitoring system (CGMS) at 5 min interval and blood samples for biochemical analysis were drawn at pre, middle and post of the race. Energy balance during one week prior to the 100 km race was recorded, and the whole energy and fluid intake during the race was analyzed. Blood glucose fluctuated reflecting duration of exercise and energy supply during the race. During the latter part of the race (65–70 km), abrupt declines in blood glucose level, which reflected insufficient carbohydrate intake before the race (119 g), were accompanied by decrease in running speed. The present report suggests that continuous glucose monitoring supplemented with standard nutritional and physiological measurement provides precise and valuable information on runner’s energy state during the ultra-endurance race, and that athletes need to reassess their preparation for the race and planning of energy intake during the race.
