The effect of the glycaemic index of a pre-exercise meal on the glycaemic and insulin responses during acute exercise

Abstract

Introduction : The objectives of this study were to examine the role of the glycaemic index of a pre-exercise meal, ingested 1 hour before an acute exercise test (15 -20 min), on the metabolic profile of elite athletes compared to control subjects in order to advise sportsmen on the ideal carbohydrate composition of the pre-exercise meal. Subjects and methods: Fifteen elite long distance male athletes (mean age: 22 yr.) and 11 control subjects (mean age: 21 yr.) participated. Each subject underwent three interventions. 1) Intervention I (baseline): Fasting subjects exercised until exhaustion. 2) Intervention II: Each subject consumed a high glycaemic index meal consisting of hot maize porridge and 10 g glucose one hour prior to exercise. 3) Intervention III: Each subject consumed a low glycaemic index meal consisting of cold samp and 10 g sucrose one hour prior to the exercise session. Interventions II and III were done in random order, with both meals providing 50 g carbohydrate. The exercise session was an acute graded exercise test (15 - 20 min) until exhaustion on a Monark cycle ergometer. Exercise started at 70 rpm against a 50 watt resistance which was increased with 50 watt every four minutes until exhaustion was reached . Exhaustion was determined by using the maximal age adapted heart rate (220 - age) or until the subjects reported exhaustion according to a 10-point Borg scale. Blood samples were taken during all interventions at the following time intervals: fasting, prior to exercise, at exhaustion, and 30 min after rest. Parameters measured included serum glucose, insulin, triglycerides, total protein, albumin and haematocrit (to correct for plasma volume changes that might occur during exercise). A DAX profile was also included. Results: Results showed the following: a) The athletes were on average significantly more insulin sensitive than the control subjects and they also had significantly lower fasting serum glucose and insulin values than the controls (effects of long-term exercise).

b) Glucose values at the end of the exercise session were significantly higher when the subjects exercised in a fasted state compared to when a pre-exercise meal was ingested. c) The low glycaemic meal resulted in a better/more stable blood glucose and insulin control than the high glycaemic meal. Conclusion: It is concluded that for acute bouts of exercise a low glycaemic index pre-exercise meal might be a better option than the high glycaemic meal as far as blood glucose and insulin values are concerned. Partaking in this type of exercise in the fasted state, resulted in higher serum glucose values at the end of exercise, but a sharp decline during the 30 min after exercise (hypoglycaemia did, however, not occur).