The glycaemic index is a measure of the speed of digestion of carbohydrate and the subsequent absorption of glucose from foods. Obviously for this reason the glycaemic index is only applied to carbohydrate containing foods. Foods with a high glycaemic index are foods that are rapidly digested and absorbed and thus produce rapid increases in blood glucose postprandially. Regular consumption of high glycaemic index foods are thought to contribute significantly to the development of Western lifestyle diseases because the rapid rise in blood glucose following their consumption leads to nutrient overload in the cells, particularly the liver, and this may be a cause of oxidative stress which ultimately leads to, or at least contributes to, the development of insulin resistance. A number of factor including the fat, protein and the fibre content of the food, makes determining the glycaemic index of mixed meals problematic because these factors modulate downwards the blood glucose response.
Mixed meals therefore tend to neutralise any high glycaemic index foods contained within them. High glycaemic index foods are therefore at their most damaging when taken in isolation of other food components. Sugar sweetened soft drinks are particularly damaging to the health, but the sugar and high fructose corn syrup within such drinks has a low glycaemic index. This point illustrates that while a high glycaemic index may be considered detrimental, it is not the only factor to consider when assessing a food for its health properties. However, as a general indicator of the healthiness of a food, the glycaemic index can give some useful information. Population based studies have investigated the association between high glycaemic index diets and low glycaemic index diets and found for example, that low fibre high glycaemic diets increase the risk of developing type 2 diabetes by 40 %. In the same study, possession of a high body mass index magnified the risk 10-fold.
The glycaemic index is a useful measure of the blood glucose response to a single food and avoiding high glycaemic index foods is generally sound advice. However, some foods that are healthy have high glycaemic indices and this can sometime leads to anomalies on food tables. For example, parsnips have a high glycaemic index, but are considered a healthy food as they are a vegetable. The discrepancy lies in the fact that although the carbohydrate in the parsnip is quickly digested and absorbed, there is only a small amount of starch present in the parsnip because the fibre and water content of the vegetable is high. Therefore following consumption, the total rise in blood glucose, although rapid, is only small. In this respect the glycaemic load is a better measure of the disease causing capacity of carbohydrate foods as it is a measure of the blood glucose rise per fixed quantity of that food. Measured in this way, parsnips have a low glycaemic load and therefore are considered acceptable foods in a healthy diet.
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