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The AUC Glucose/Blood Level curve Integral (area under curve) is used to calculate the Glycemic Index of certain foods. I have a question on why two sample foods with the same mass of carbohydrates can have different AUC Integrals or Glycemic Indices. I have some possible theories and would appreciate guidance.

Are these numbers different between the same sample size of 50 g of carbohydrates due to differing blood glucose levels, yet the same energy delivered to the body (perhaps in another form other then rising glucose)? Are the levels different due to differences in the body's digestion of the food? Or, is part of the glucose/blood integral of some lower Glycemic Index foods passing beyond the 2-hour window of measurement, which would cause a smaller integral and lower Glycemic Index?

BACKGROUND:

To be clear on definitions, the Glycemic Index is calculated using the relative rise of a blood glucose level two hours after consuming a food and it is the integral of (area under) a 2 hours glucose/blood response curve, following a 12 hour fast and ingestion of a sample food with 50 g of carbohydrates. The Index is calculated specifically using the ratio of a blood glucose AUC integral, ignoring the area below the fasting baseline (start of curve rise), for some carbohydrate source and a reference source. The reference source is usually either glucose or white bread. The trapezoidal rule is typically used to calculate the integral, although any method can be used. The Glycemic scale places pure Glucose at 100. The index is calculated using a ratio of two integrals using this formula: (SAMPLE_FOOD mmol/L * time)/(REFERENCE_FOOD mmol/L * time) * 100.

Where I'm confused is that two sample foods with 50 g of carbohydrates should have the same energy in calories, typically 200 kcal (4 kcal/g carb). I see the integral of the AUC curves for different foods can be different, indicating different glucose/blood level changes for each food over the two hour window. This makes me think, the foods have the same energy content, but are changing the body's blood glucose levels differently, resulting in different curves and Glycemic Indices. I first questioned whether the reason for this was that part of a food's curve lied outside the 2-hour window, raising the Glycemic Index slower than another food. I also read that the glucose levels will change differently depending on the type of carbohydrate, and some form of carbohydrates are even impossible for the body to digest and are either digested by bacteria in the gut or simply excreted in the feces, which would resulting in a lower glucose/blood level rise.

For example here is a 2-hour AUC curve of some fruits against glucose reference: a

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Your question points to the entire point of glycemic index: two foods with the same carbohydrate content (by mass) do not necessarily enter the bloodstream at equivalent rates.

Picture a gram of glucose; we know it's energy content (about 5 kcal) but how quickly will it get into the bloodstream? Whatever that answer is, now picture the same gram of glucose packaged as a time-release capsule. The time-release glucose will get absorbed into the bloodstream much slower so the peak glucose concentration will be much lower.

This fact -- that the peak blood glucose is lower for the time-release 'food' -- is what's quantified by the glycemic index. Since foods vary in the rapidness in which sugars are absorbed (and metabolized into insulin-eliciting sugars) the total carbohydrate content isn't a direct predictor for the peak blood glucose (or insulin); that's why glycemic index is a (theoretically) useful concept to assess the endocrinologist effects of different foods.

And yes, the slower peak means a) more of the curve lies outside a 2h window and b) more of the sugar is metabolized as its absorbed. Both factors decrease the total area of the integral. But it's the peak concentration that's thought to have the largest effect as the endocrine signal to release insulin.

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  • $\begingroup$ Asparagus has a Glycemic Index of 15, does that imply that metabolism of glucose into the bloodstream occurs over about 6 hours, vs 2 hours for pure glucose (100 GI)? $\endgroup$
    – Nick
    Dec 30, 2022 at 3:56
  • $\begingroup$ I don't think I follow your math...the GI is only based on the 2h post-ingestion part of the blood glucose. The whole treatment doesn't address how long it takes for all th glucose to be absorbed into the bloodstream. 50g of asparagus only has about 0.94g of glucose in it! So the peak blood sugar could be as much as (50/0.94) 53 times higher for pure glucose compared to asparagus. $\endgroup$
    – Ryan
    Feb 13, 2023 at 18:34

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