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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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First of all, yes, the cut-off at 2 hours will make most of the difference. But I think your question asks about what would happen without that 2h cut-off, e.g. measuring 10 hours instead.

In theory, the AUC of identical dosages of sugar can vary, dependent on the kinetics of how sugar is removed from the blood stream.

Mathematically, if sugar exponentially decayed in a first order rate, which seems true during the initial insulin spike, then the long-time AUC would be identical, independent of the digestion speed.

If we assumed that some aspects don't fully scale-up (less than first-order), e.g. alcohol is mostly metabolized in a 0 order reaction rate [2], then even the long-term the AUC would be lower if digestion was slower.

However, reality is complicated. Looking at an example insulin and glucose day-curve [3], you can see, that lower insulin levels drastically reduce the glucose elimination (via uptake by cells). From this, it's not clear how long-term AUC would look like and one could actually speculate, that actually slowly digested food could have a higher long term AUC, given its glucose acts during smaller concentrations of insulin.

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