Effects of high and low-glycemic index isoenergetic breakfast on glucose homeostasis and substrate oxidation during high intensity intermittent exercise

Most studies have used moderate or low intensity endurance exercise to examine the effect of food intake with high and low glycemic index on metabolic responses and oxidation of substrates. However, the effect of glycemic index on high intensity intermittent exercise, which is a major characteristic of many sports, has not been studied. The purpose of the present study was to investigate the effect of high-glycemic (HGI) and low-glycemic (LGI)index isoenergetic breakfast on glucose homeostasis and substrate oxidation during high intensity intermittent exercise (HIIE). Eight male students with age 23.4 ± 0.9 years, weight 76/21± 4/38 kg, maximal oxygen uptake 53.7±1.0ml/kg/min, participated in two expermintal trials separated by 7 days. At each trial, 60 minutes after consumption isoenergitic breakfast (819 kcal), high intensity intermittent exercise (HIIE) was performed. Blood samples were evaluated for plasma glucose, insulin and glucagon, total fat and carbohydrate oxidation, evaluated by through respiratory equations. The results showed that in the postprandial period, increase in plasma glucose concentrations in HGI (from 86 to 127mg/dl) (P <0.05). In HGI, glucose concentration decreases rapidly in the early period of HIIE (to 79 mg/dl), but remains almost stable in LGI (P <0.05). AUG of glucose and insulin in HGI were higher (1.08%) than LGI (1.37%), respectively. The oxidation of fat during activity in LGI (2.5 g) was higher than that of HGI (3 g) (p <0.05). HGI causes hyperglycemia and hyperinsulinemia in the postprandial period, and higher levels of insulin before exercise can lead to a sudden drop in blood glucose over the course of the activity, but LGI, due to lower insulinemia, helps to maintain better blood glucose and glucose homeostasis during HIIE.