Does prior exercise affect glycemic response to a glucose load?

Abstract

When oral glucose tolerance tests (OGTT) are repeated in individua1s, relatively large variations often occur in the magnitude of the blood glucose response from one occasion to another. Little is known about what causes this within-subject variability. One potential contributor to this variability may be the subject's prior extent of physical activity and/or the amount of stored glycogen present at the time of the OGTT. This research tested the effect of a bout of exercise (of the type known to significantly deplete muscle glycogen) performed within 24 hours prior to an OGTT on blood glucose and insuIin responses. Ten male endurance athletes underwent an OGTT without prior exercise on one occasion and after a glycogen-depleting bout of exercise the day prior to testing on another occasion. Venous blood was sampled at standard intervals (0, 15,30, 45,60,90, and 120 minutes) for 2 hours following consumption of an OGTT beverage containing 50 grams of glucose. Capillary blood samples also were taken at the same intervals by finger stick. Venous and capillary blood samples were analyzed for glucose. Venous samples also were analyzed for insuIin concentration. Glucose and insulin areas under the curve (AUC) were calculated. There was no significant difference between the glucose AUC measured by capillary and venous blood samples and no significant difference between the exercise and non-exercise conditions. Insulin AUC values were significantly lower on the days following exercise (P=0.03). There was a significant difference between exercise and non-exercise glucose levels when insulin levels were held constant (P=0.03). Thus, when comparing people at the same insulin levels glucose AUC levels were significantly greater the day after exercise. There was no predictable difference in insulin sensitivity between exercise and non-exercise conditions based on Insulin Sensitivity Index (ISI) values. Based on this evaluation often male endurance athletes, it does not appear that glycogen-depleting exercise has a predictable effect on the OGTT.