The glycaemic index is a measure of the blood glucose elevation following ingestion of a single carbohydrate food containing 100 grams of carbohydrate, when compared to a reference food, usually glucose. However, while many foods have high glycaemic indices, this can be an artifact of the large amount of carbohydrate that is used in the test. Parsnips for example have a high glycaemic index, but we rarely eat enough of them to provide 100 grams of carbohydrate and so in reality they do not elicit high blood sugar responses. The glycaemic load is therefore a better measure of the effects of carbohydrate foods on the blood sugar because the glycaemic load takes account of the amount of carbohydrate consumed. The glycaemic index can be further modified by protein and fat that is consumed concomitantly, as both have a downward effect on the blood sugar response. The glycaemic load of a meal in its entirety is therefore a more useful tool when assessing the effects of diets on blood sugar.
In this regard, researchers have tested the effects of varying the glycaemic load of diets on the fat loss experienced by overweight male and female subjects1. For example in one study researchers fed a group of overweight individuals one of four diets that varied in their carbohydrate content or glycaemic load. Diet 1 comprised of 55 % carbohydrate with a glycaemic load of 127, diet 2 comprised of 55 % carbohydrate with a glycaemic load of 75, diet 2 comprised of a carbohydrate content of 45 % and a glycaemic load of 57 and diet 4 comprised of a carbohydrate content of 45 % and a glycaemic load of 54. The fat content of each diet was similar and protein substituted for carbohydrate variation. The results of the study showed that the proportion of subjects who lost 5 % or more of their body weight was highest in groups 3 and 4 (56 and 66 %, respectively), in comparison to groups 1 and 4 (31 and 33 %, respectively). The women consuming diets 3 and 4 also lost significantly more (80 %) body fat than the women consuming diets 1 and 4.
These results therefore suggest that the glycaemic load play some part in fat loss, but that other modifying factors are clearly present as the lowest glycaemic index diet did not cause the most fat loss. This likely relates to the fact that the subjects were all provided with diets that possessed whole grains, and so all the diets were of relatively high quality compared to the typical Western diet. Carbohydrates in the diet are not the problem and fat loss is perfectly possible on a high carbohydrate diet. The presence of carbohydrates in their refined state is the driver of insulin resistance and weight gain, and by allocating each diet with whole grain sources of cereals, the researchers lessened the beneficial effects of the lower carbohydrate and lower glycaemic load diets. This relates to the fact that it is not high glycaemic index foods that are damaging per se. High glycaemic index foods are generally damaging because they tend to be refined and processed, but some high glycaemic index foods are perfectly healthy, particularly vegetables.
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