Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Glycemic load : I don't get it

  1. Dec 21, 2012 #1

    From what I have read, the glycemic index is a limited tool because it doesn't take the amount of ingested carbs into account, hence why the glycemic load was created.

    But then I read an example that threw me: a serving of 10g with an GI of 100 is equivalent to a serving of 100 g with a GI of 10, since they have the same GL of 10.

    So my question is: how do we know that blood glucose levels will rise constantly? Is glucose absorption really constant and unlimited? I don't think it is the case with other nutrients, is it?
    I mean, what if they rise constantly with the first 90g of GI=10 food, and then the rise slows down with the final 10 grams (because of some kind of saturation for example, although I haven't studied glucose metabolism yet), which would cancel the equivalence suggested by GL calculation? Or the other way round? What if it increases 'normally' at first, and then more and more as the intake gets bigger?
    Is it an established fact that blood glucose increase is directly proportional to quantity x GI ?

    I hope my question is clear...

    Last edited: Dec 21, 2012
  2. jcsd
  3. Dec 21, 2012 #2

    jim mcnamara

    User Avatar

    Staff: Mentor

    Simple answer - Type I diabetics do not make insulin. They were fed a carefully measured amount of some food and the rise in blood sugar was measured.

    That is simplified model for glycemic index calculation. Pure glucose was assigned a de facto 100 value as a standard. If administration of whole wheat bread caused a rise of 80% of glucose it got a number: GI 80.

    Multiply the glycemic index by the amount of carbohydrate in grams provided by a food and then divide the total by 100. This is glycemic load

    Glycemic load was developed by scientists to simultaneously describe the quality (via glycemic index) and the quantity of carbohydrate in a meal or diet.


    In other words glycemic load is meant to be used for real food - a mixture of carbs, protein, fat, water, etc.
  4. Dec 22, 2012 #3
    Many thanks for taking the time to post!
    However this doesn't answer my question, probably because I wasn't clear enough.

    I know what GI and GL are and how they are calculated.

    What I wanted to know is whether it has been established that glucose absorption is constant and unlimited, as opposed to other nutrients such as Ca and Fe whose absorption decreases as the amount of Ca and Fe in the intestine increases.

    And I'd also like someone to confirm (or not) my understanding that constant and limitless absorption is a condition for GL to be an accurate tool (eg for the common statement that '10 g of GI 100 food = 100 g of GI 10 food' to be true).

    Last edited: Dec 22, 2012
  5. Dec 22, 2012 #4
    The short answer is no. This is a field of active research. There are at least two classes of systems that are involved in glucose transport across cell membranes including the gut. One is the sodium dependent SGLT system and the other is the non N+ dependent GLUT system. There are a variety of variables that seem to affect the activity of these systems (medications, dietary factors, gender). You can search the literature for some of these.

    The GTT "normals" are based on population averages for non-diabetics and should be considered as approximate.


    Last edited: Dec 22, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook