How to Use GI and GL for Better Control

Published on October 30, 2012   ·   No Comments

A little bit of knowledge and a pencil and paper are all you need to make use of these great tools.

By Chris Sparling

Learning how to use the glycemic index and glyemic load to help better manage your blood sugar is easier than you may think — and it may well be worth your while. Plenty of research has indicated that low-glycemic-load eating can contribute to reducing health risks, and not only for people with diabetes. The first step is to understand what these two terms mean.

The Glycemic Index (GI) is a measure of the effects of carbohydrates on blood glucose levels. Carbohydrates that break down quickly during digestion, releasing glucose rapidly into the bloodstream (like those found in white bread), have a “high GI” (70 or higher); carbohydrates that break down slowly, releasing glucose gradually into the bloodstream (like those in whole grains and legumes), have a “low GI” (55 or lower).

The Glycemic Load (GL) is a ranking system for the glycemic impact of foods, based on their carbohydrate content, portion size, and Glycemic Index. Low = 1 to 10; Medium = 11-19; High = 20 or higher.*

As explained in the book The New Glucose Revolution for Diabetes (Marlowe, 2007), the GL was developed by Harvard researchers, who posited that eating a small amount of a high-GI food would have the same effect on blood sugar as would eating large amounts of a low-GI food. Another issue with looking only at the GI of a food is that it’s tied to the number of grams of carbohydrates in that food and, obviously, that number varies by large amounts. Watermelon is a good illustration of this problem. Watermelon’s GI is high, 72. The GI, however, is based not on a normal portion, but on 50g of carbohydrates — whatever the food . To get 50g of watermelon carbs, you’d have to eat almost 5 cups. GL combines both the quality and the quantity of the actual carbohydrates consumed — and provides one “number.” The GL of one cup of watermelon is about 9, which is low.

This leads us to our next step: understanding the math.

Not to worry, we’re not talking about advanced algebraic equations; rather, the formula for calculating GL requires only some basic multiplication and division.

GL = (GI x the amount of carbohydrate) divided by 100.

Using an apple as an example, let’s input the fruit’s GI score of 38 and its 13 grams of carbohydrates into the equation and see what the resulting GL turns out to be.

GL = (38 x 13) / 100

GL of an apple = 5

Easy enough, right? Well then, how about we try something a bit starchier, like a baked potato (GI = 85; Total Carbs = 14)?

GL = (85 x 14) / 100

GL of a baked potato = 12

Based on these simple calculations, we see that the potato will have twice the glycemic effect on blood sugar as the apple will. Knowing this can help you make better decisions toward keeping blood sugar steady — and reducing the chances of unexpected spikes.

So, GI or GL? The Answer is Both

To keep your blood sugar in a healthy range, the best strategy is to consume a lot of low-GI carbohydrates but watch your portion sizes and use the GL calculation as a reality check on your choices. And remember, these tools are for carbohydrate foods — grains, starches, legumes, fruits, and veggies. Other rules apply when making choices about proteins and fats, of course. And even if you master the GI and GL approach, remember that diabetes is very individual, and the only way to know how foods will truly affect your blood sugar levels is to test yourself.

For more information on GI and GL, check out these resources:

The Official Site of the Glycemic Index

Oregon State University’s Linus Pauling Institute: Micronutrient Research for Optimum Health, “Glycemic Index and Glycemic Load”

NutritionData: Glycemic Index

* Glycemic load ranges, according to David Mendosa

Reviewed by Susan Weiner, RD, MS, CDE, CDN. 4/11

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