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The definition of the glycemic index is often given as the area under curve (AUC) of their two-hour blood sugar response. However, it's essentially meant to be a measure of whether food causes a fast or slow rise in blood sugar level. Why is the two-hour AUC a good measure of the speed of the rise?

When ingesting the "same" amount of sugar, no matter the form, the AUC should be the same, right? The only explanation I can find is that the two-hour cutoff means some carbohydrates are metabolized beyond that cutoff, so their AUC and hence their GI is lower, is that what's happening?

The reason I'm a bit confused is that all of the images of blood sugar curves illustrating low & high GI seem to show two curves with the same AUC, both going to 0 before the two-hour cutoff.

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The AUC of identical dosages of sugar depends on digestion speed because of the rate at which sugar is removed from the blood stream. If sugar exponentially decayed in a first order rate, then the AOC would be the same, but it's save to assume some 0 order mechanisms contributing. E.g. alcohol is mostly metabolized in a 0 order reaction rate: A fixed amount per hour.

Edit: If you take up sugar slowly, it might never reach high concentrations in the blood stream and lead to lower AUC then if taken up fast. Each molecule has a longer lifetime if taken up faster.

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