The GI is defined as the incremental area under the curve (AUC) for the test food expressed as a percent of the response after 50g anhydrous glucose taken by the same subject. In practice individual GI values are calculated for each test taken by each subject. The resulting values for each test food are averaged to give the food GI. If white bread is used as the reference food, the resulting GI values are multiplied by 0.71 (or divided by 1.4) to convert to the glucose scale, because the GI of white bread, relative to glucose, is 71.
GI is measure of the extent to which the available carbohydrate in foods raises blood glucose on a gram-for-gram basis. The GI is independent of the amount of food consumed, and is independent of the glucose tolerance status of the subject tested (eg. it is the same in people with or without diabetes). GI can be thought of as a measure of the quality of the available carbohydrate in the food. The GI cannot be applied to foods containing no available carbohydrate (ie. it is meaningless to say, for example, that bacon has a GI of 0). Fat and protein influence glycaemic responses independently from the quality of the carbohydrate; this has 3 implications for the GI: 1) the GI is determined by feeding carbohydrate foods alone; 2) the GI is best applied to high carbohydrate foods, foods containing large amounts of fat and protein may have low glycaemic responses due to the fat or protein; 3) the GI of mixed meals is not measured, but calculated from the GI values of the individual foods in the meal.
GR is the extent to which any test meal raises blood glucose. GR is affected by the glucose tolerance status of the subject. GR is also affected by the amount of carbohydrate in the meal, the digestibility of the carbohydrate (eg. undigestible carbohydrates such as fructo-oligosaccharides do not raise blood glucose), the GI of the available carbohydrate and the amount and type of fat and protein. The effects of fat and protein on glycemic responses probably differ in insulin sensitive and insulin resistant people.
GL is defined as GI _ g, where GI is the food GI and g is the grams of available carbohydrate. Hence, GL is thought to be a measure of the overall glycaemic impact of the food consumed. However, this has not been verified in practice. GL is useful as a way of thinking about the acute glycaemic effects of foods; for example, even though carrots may have a high GI, they contain very little carbohydrate, so do not affect blood glucose very much. However, we believe that GL is not useful for dietary management because it can be changed either by changing diet GI or by changing the amount of carbohydrate in the diet. GI and amount of carbohydrate are completely different things. Reducing diet GI with no change in carbohydrate intake has different effects on beta-cell function, blood pressure, and plasma insulin, triglyceride and free fatty acid levels than reducing carbohydrate intake.
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