The effect of the glycemic index on endurance performance

Abstract

There exist a wide variety of metabolic responses to different types of carbohydrates and their influence on metabolism during endurance training. Recent studies revealed that the physiological responses to food are far more complex than was previously appreciated. The rapid release of insulin and the decline in blood sugar levels during the first stages of endurance training are linked to the Glycemic Index of foods. Researchers cannot still make use of the old distinction between starchy and sugary food or simple and complex carbohydrates. These distinctions are based on the chemical analysis of the food, which does not totally reflect the effects of these foods on the body. The Glycemic Index is a more reliable guideline to apply in nutritional management for endurance athletes. The major object of the study was to indicate the importance of utilizing the Glycemic Index (GI) as part of the nutritional preparation for endurance events. The study investigated the advantages of ingesting a Low Glycemic Index meal prior to exercise and compared it with the ingestion of a High Glycemic Index meal. A pretest-posttest design was used. Twelve healthy, male and female cyclists participated in the study. Subjects were selected according to their level of training. The total test period consisted of 14 days, which included two different dietary interventions of 7 days each. Diet -and training analysis were done on the subjects prior to the commencement of the study. Each subject completed three exercise trials. The first exercise trial consisted of a V02max test until exhaustion. Two submaximal trials (65 - 70 % of V02max) followed and were preceded by two dietary interventions. The dietary interventions (7 days each) had the same amount of CHO, fat and protein but differed in the Glycemic Index of the pre-exercise meals. The first pre-exercise meal was a High Glycemic Index (HGI) meal. The second pre-exercise meal was a Low Glycemic Index (LGI) meal. The results of the study indicated the advantages of ingesting a Low Glycemic Index meal prior to endurance exercise. The drop in blood glucose levels significantly differed (p<0.05) with an average of 0.68 mmol/L between the two tests after 10 minutes of cycling. It took 20 minutes for the blood sugar level of the first testto reach the same level of the blood sugar level of the second test. After the ingestion of the High Glycemic Index meal in Test 1, the blood lactate levels were significantly higher (p<0.05) during the first 15 minutes. The total distance covered by the subjects was 22.86 km after the first dietary intervention (High glycemic Index food) and 27.43 km after the second dietary intervention (Low glycemic Index food) although it is not statistically significant due to the small sample size. The difference in the distance covered of the two tests is 4.57 km in a period of 50 minutes. Subjects indicated that they experienced more physical strain (higher RPE values) in Test 1 (High Glycemic Index food) than in Test 2 (Low Glycemic Index food) (p<0.05). The study results support the fact that Low glycemic index food may confer an advantage when eaten prior to prolonged strenuous exercise by providing a slow¬releasing source of glucose to the blood without causing extensive hypoglycemia. Proper preparation and the correct choice of the pre-exercise meal can exclude the occurrence of sudden drops in the blood sugar levels. The Glycemic Index can also be successfully applied during and after events to improve performance.