The glycaemic index (GI) is a relative measure of the rise in blood glucose levels following ingestion of carbohydrate foods, compared to the same amount of a standard food. The standard food is usually white bread that is designated a value of 100. While the glycaemic index tables may be useful as a guide to the relative glycaemic responses of single foods, the complexity of mixed meals diminishes the practicality of the glycaemic index considerably. For example, the glycaemic index of mixed meals containing rice, spaghetti or lentils is not different despite differences in the individual glycaemic indexes of the foods. While the GI of individual foods are available in tables, combinations of foods can have their glycaemic index predicted using a formula. Differences in the measured and predicted GI of food may be explained by differences in ripeness, processing, cooking and storage.
Researchers have tried to assess the reliability of the GI tables by measuring the difference between the measured and the predicted values of food contained within meals. In one study1, 30 subjects were fed meals containing potato mash, white rice, dried spaghetti pasta, tomato sauce, plain frozen peas and frozen diced carrots to test the glycaemic index of the foods when combined. Each meal contained 50g of carbohydrate to make a potato meal a white rice meal and a spaghetti meal, along with 50g of pan fried chicken. The predicted and measured GI for the combined foods were 63 and 53 for potato mash, 51 and 38 for the white rice meal and 54 and 38 for the spaghetti meal, showing that the predicted GI was higher than the measured glycaemic indexes by 22, 40 and 50%, respectively.
The formula used to calculate the GI for mixed meals is therefore not a reliable measure of the actual GI experienced. The formula used to predict the GI of mixed meals came from a study that included beef protein, and the authors acknowledged that the results may have been confounded by the inclusion of protein. It could be assumed that the addition of chicken (containing 10g of protein and 7.6g fat) to the meal also altered the GI. However, the addition of chicken to individual foods only altered the GIof the food by around 13%. The magnitude of this reduction is therefore small compared to the figures seen with the three test meals, suggesting that other factors are involved. These findings suggest that the GI of foods is lost when combined with other foods in a mixed meal, although the exact reason for the disparity is not known.
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