Saturday, November 11, 2006

Glycemic Index, Glycemic Load

Great question from Melinda in the comments section that I thought I'd resurrect and discuss here ... because I get off on numbers:
Have been reading up on glycemic load. Understand the basic principle, but am confused by radically differing measurements in different tables of value from different sources. F'rinstance, the International table of GI & GL values linked to Mendosa's site lists the GL of an apple at an average of 6. I borrowed a book from library (hoping not to have to crank up the computer every time I want to check a value) titled Glycemic Load Diet by Rob Thompson, MD (2006). He lists the GL of an apple at 78 (and recommends keeping total daily GL below 500). An Australian website with a large table of values seems to be using yet another scale: an apple is listed w/ a GL of 684 (!) and you're recommended to keep your daily GL total at 3000 or less ( . Online Harvard Health Publications lists an apple as having a GL of 6.

Needless to say, this is confusing. I know you recommend Mendosa's site, but if you use his GL listings, approx. what amount of GL total for a day would you be aiming at in order to lose a little weight & lower cholesterol and glucose blood levels?


I think that Mendosa's and the Australian site are in sync, except the latter does not divide by 100.

Basically, GL = GI x CHO(g)

That is, the glycemic load of a serving of food is equal to the glycemic index of that food multiplied by available carbohydrate. If you consider the GI as a percentage then the above result would be divided by 100.

To answer your question of how much GL in a day...
Say you planned on eating a 2000 calorie/day diet. Say you wanted about 55% of those calories to come from carbohydrate. At 4 calories per gram of CHO, you would end up eating 275g CHO. If you could possibly get all of your CHO to fall into a midrange GI of 60, you would end up eating a GL of 60x275/100, or 165. Since many people eat more than 275g carbohydrate in a day, and since those carbs are often of the processed variety (and thus have a higher GI) I can understand setting 300 as a daily GL limit.

Curiously, whenever I initiate a discussion of glycemic index and glycemic load, my conversation partner begins checking their watch. I've learned that statements like, "Try to eat fewer foods made from flour, especially wheat flour." or "Dilute your juices." condenses the message and gives the concept a working chance :)

Keeping a cap on glycemic load is one method of maintaining healthful blood sugars. This is another method.


Re: Thompson. He seems to be out of sync with the others but that doesn't necessarily make him wrong.

I wasn't familiar with Dr. Thompson or his GL calculation. I went to his site and read his explanation.

I then played with the numbers (boy did I) and it looks like his system is somewhat comparable (although not identical) to the more established system, in that an apple leads to a lower blood sugar response than a slice of bread, a bagel a higher response than bread (because of its larger size), etc. Without a better explanation of his methodology that's all I can say.

I will say - None of these numbers are set in stone. Apples (and other foods) vary in the amount and types of sugars they contain, which would affect their GI. The best we can do is measure lots of apples (and other foods) and use a mean/average. When I consider the wiggle room in calculation of GI, serving size, calculation of CHO in serving size, lab error, etc., I can appreciate how tables of values might differ.

As if you don't have enough :) here's another good GI/GL table with explanation (scroll down a bit and select "View this table"):
International table of glycemic index and glycemic load values: 2002

1 comment:

williwm said...

A calorie ratio of 40 percent carbohydrates, 30 percent proteins, and 30 percent fat minimizes your glycemic load (insulin) and thus the fat you retain. This balance also provides the three key macro nutrients needed to keep a body in hormonal balance.