Preexercise Carbohydrate Ingestion and Transient Hypoglycemia: Fasting versus Feeding

Carbohydrate (CHO) ingestion 30 to 45 min before exercise results in transient hypoglycemia after starting the exercise in some, but not all, subjects. However, whether transient hypoglycemia is more likely to occur under fed or fasted condition remains unknown. This study aimed to directly compare the effects of fasting versus feeding on plasma glucose responses after preexercise CHO intake and to examine the relationship between insulin responses and onset of transient hypoglycemia.

Sixteen subjects performed 60-min cycle ergometer exercises at 75% maximal oxygen uptake (V˙O2max) under overnight fasted and fed (4 h after breakfast) conditions. In both conditions, they consumed 500 mL of beverage (150 g of glucose) 30 min before beginning exercise.

The mean plasma glucose concentrations 15 min after starting the exercise did not fall below 4.0 mmol·L (criteria for hypoglycemia) in both states; however, individual differences in the occurrence of transient hypoglycemia were noted. In the fasted state, plasma glucose levels transiently dropped below 4.0 mmol·L in five subjects, who had substantially higher serum insulin levels at the start of exercise, compared with those who did not develop hypoglycemia. Although seven subjects developed transient hypoglycemia in the fed state, no relationship was observed between insulin responses and hypoglycemia. Three subjects developed hypoglycemia in both fasted and fed states.

These results suggest that transient hypoglycemia after preexercise CHO ingestion occurs in some, but not all, subjects, under both conditions. Furthermore, subjects with enhanced insulin responses seem to be more prone to transient hypoglycemia in the fasted condition.