EFFECT OF PRE-EXERCISE CARBOHYDRATE FEEDING ON GLYCEMIC AND INSULINEMIC RESPONSES DURING EXERCISE AT DIFFERENT INTENSITIES

Abstract

Rebound hypoglycemia has been reported after pre-exercise carbohydrate (CHO) ingestion in some studies but not in others. Differences in the experimental design and factors such as the exercise intensity are likely to be responsible for the discrepancies between these studies. Exercise intensity might be a crucial factor since it affects both insulinemia, liver glucose production and muscle glucose uptake. Purpose: Therefore the aim of the present study was to compare the glycemic and insulinemic responses to exercise at different intensities after ingestion of a standardized pre-exercise CHO load. Methods: Eight trained subjects (VO2max 58.4 ± 1.9 ml/kg/min) consumed 75g of glucose 45min prior to 20min of exercise at 55 ± 1, 77 ± 1 and 90 ± 1%VO2max. Blood samples were collected before glucose ingestion, at 15min intervals at rest and 5min intervals during exercise. Results: There were no significant differences in glucose or insulin concentrations between the three trials during exercise. Immediately pre-exercise, plasma glucose concentration had returned to pre-ingestion levels, while the insulin concentration was more than 3 times higher than pre-ingestion (45 ± 10, 62 ± 14, 62 ± 15 vs 16 ± 2, 16 ± 2, 16 ± 1 μU/mL for the 55%, 77% and 90% trials resp.). During exercise, plasma glucose concentration decreased from 4.7 ± 0.3, 5.5 ± 0.3, 4.8 ± 0.3 mmol/L before the start of exercise to 4.1 ± 0.2, 4.0 ± 0.2, 4.1 ± 0.2 mmol/L during the first 5min in the 55%, 77% and 90% trials respectively and then stabilized in all trials. During the 55% and 77% trials, 3 subjects developed hypoglycemia (glucose concentration < 3.5mmol/L), while hypoglycemia occurred in 4 subjects during the 90% trial. Insulin concentrations decreased during the first 10min of exercise and then leveled off at pre-ingestion concentrations. Conclusions: These data suggest that the glycemic response to 75g of CHO 45min pre-exercise is similar during exercise of different intensities. The discrepancies found in studies regarding hypoglycemia after pre-exercise CHO feedings cannot therefore be explained by differences in exercise intensity employed in those studies.