The glycaemic index is a method for the classification of carbohydrate foods based on the rise in blood sugar they cause upon ingestion. Athletes understand the importance, prior to exercise, of the correct carbohydrate selection based on the glycaemic index. The glycaemic index of the meal prior to training is known to influence substrate utilisation in any subsequent exercise. Generally, selection of foods with a low glycaemic index rating before exercise will result in a higher fat oxidation relative to carbohydrate. Alternatively selection of high glycaemic index rating foods will increase carbohydrate oxidation relative to fat. High intensity short duration exercise may therefore benefit from pre-exercise high glycaemic index foods, and low intensity long duration exercise may benefit from low glycaemic index foods. Females consuming a low glycaemic index breakfast increased fat oxidation by ≈55% in a 60 minute run compared to consuming a high glycaemic index breakfast.
Because increased fat oxidation during exercise might be beneficial to non-athletes with weight loss as a goal, researchers1 have tested the effects of the glycaemic index of foods on fat oxidation during exercise. In a cross over design study, low and high glycaemic index breakfasts were fed to sedentary eight women 3 hours before walking for 60 min. Both high and low glycaemic index meals contained 1 gram of carbohydrate per pound of body weight and the values of the high and low glycaemic index meals were 44 and 78, respectively. Following the meal, plasma glucose and insulin area under the curve were higher following the high glycaemic index meal. Fat oxidation was suppressed after both meals, but remained higher following the low glycaemic index foods. During exercise, total fat oxidation was significantly higher following the low glycaemic index foods when compared to the high glycaemic index foods.
These results support he contention that low glycaemic index foods are beneficial to weight loss because they allow increased oxidation of fatty acids during exercise. Interestingly, fat oxidation was suppressed less following the meal when consuming the low glycaemic index foods compared with the high glycaemic foods, suggesting that benefits in weight loss may occur even without exercise. Although carbohydrate foods are rated on the glycaemic index, addition of protein foods to a carbohydrate meal reduces the glycaemic index significantly. This is because protein foods slow the release of chyme from the stomach, thus increasing digestive time. This may explain some of the weight loss effects of higher protein diets seen in numerous studies. Carbohydrates foods with high amounts of soluble fibre also tend to have low glycaemic index because of increased digestive tract transit time.
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