What Is The Glycemic Index?
The glycemic index is a way to rank different types of carbohydrate (such as pasta, rice, or cereal) according to their effect on blood sugar levels. It's very popular with people who want to lose weight, increase their energy levels, or protect their health.
If you're not familiar with the glycemic index, here's a brief history.
Traditionally, carbohydrates are known as either simple or complex. Foods high in sugar, such as chocolate, fruit or cakes are classed as simple carbohydrates. Scientists used to think these foods were quickly digested, leading to a rapid rise in blood sugar.
Complex carbohydrates, such as potatoes, rice and pasta are supposed to break down more slowly, producing a gradual rise in blood sugar.
This is where the glycemic index comes in.
Researchers from Europe first brought attention to the effects of various foods on blood sugar levels in the early 1970's. However, the glycemic index was conceived in the early 1980's by Dr. David Jenkins.
Dr. Jenkins — a professor of nutrition at the University of Toronto — set out to establish the type of foods that were best for people suffering from diabetes.
Jenkins found that foods such as potatoes — traditionally defined as a complex carbohydrate — actually led to a rapid rise in blood sugar. Some foods high in simple carbohydrates appeared to digest more slowly, leading to a gradual elevation in blood sugar.
This led researchers to classify foods according to their glycemic index. The glycemic index refers to the change in blood sugar that occurs after you eat a food high in carbohydrate.
Foods with a high glycemic index lead to a bigger change in blood sugar levels over a 2-3 hour period than foods with a low glycemic index.
A glycemic index list assigns a numerical value to a food. This value indicates how much and how rapidly 50 grams of its carbohydrate content will raise blood sugar levels, compared to 50 grams of a reference food (glucose or white bread). The reference food is given an arbitrary value of 100.
Glycemic load
Many people leap to the conclusion that all foods with a high glycemic index should be avoided. Using the glycemic index is better than the old method of identifying carbohydrates as either simple or complex. Unfortunately, the glycemic index doesn't cope well with low-energy density carbohydrates, such as vegetables.
To establish the glycemic index of a food, researchers use a portion size containing 50 grams of carbohydrate. The problem is that a Snickers® bar contains roughly 37 grams of carbohydrate. A serving of cooked carrots has only eight grams of carbohydrate.
Yet the glycemic index of both foods is roughly the same. This leads some people to conclude that a serving of carrots provides the same nutritional value as a Snickers® bar.
To help clear up some of the confusion, Harvard researchers have coined the term glycemic load. The glycemic load tells you how much sugar is in the food, as well as the degree to which it raises blood sugar levels.
While the glycemic index tells you how rapidly a particular carbohydrate raises blood sugar, it doesn't tell you how much of that carbohydrate is in a serving of a particular food. To understand how that food will affect blood sugar levels, you need to know both. That's where the glycemic load helps.
The carbohydrate in a carrot, for example, has a moderate glycemic index. But there isn't a lot of it, so the glycemic load of a carrot is relatively low.
The glycemic load takes the glycemic index into account, but gives a more complete picture than the glycemic index alone. To calculate the glycemic load of a given food, simply multiply the grams of carbohydrate in a serving of food by that food's glycemic index.
Cooked carrots, for example, have a glycemic index (using white bread as a reference value) of 56. A typical serving of cooked carrots contains roughly 8 grams of carbohydrate.
To calculate the glycemic load, multiply 8 by 0.56. This gives a glycemic load of 5. The Snickers® bar has a glycemic index of 57 (again, using white bread as a reference value). One bar contains 37 grams of carbohydrate. Thirty-seven multiplied by 0.57 gives a glycemic load of 21.
Despite the fact that the glycemic index of both foods is virtually identical, the glycemic load of the Snickers® bar is four times higher than the cooked carrots.
When you take into account both the glycemic index AND the total carbohydrate content of a food, it gives a much clearer picture of how that food affects blood sugar levels.
Remember — to calculate the glycemic load of a given food, simply multiply the grams of carbohydrate in a serving of food by that food's glycemic index.
You can see an international table of the glycemic index and glycemic load values here.
Use of the glycemic load as a way to improve the quality of your diet is certainly not universally accepted. Some critics contend that it takes an already imprecise measure (the glycemic index) and magnifies it further through the calculation.
But Dr. Jennie Brand-Miller, an Australian researcher involved in the original studies of the glycemic index, thinks that the glycemic load is a valuable concept.
"A diet with a very high glycemic load should be avoided," says Brand-Miller. "This means that the higher the carbohydrate content of your diet, the more important it is that the carbohydrate comes from low glycemic index sources."
About The Author
 Christian Finn holds a master's degree in exercise science, is a certified personal trainer and has been featured on BBC TV and radio, as well as in Men's Health, Men's Fitness, Muscle & Fitness, Fit Pro, Zest and other popular fitness magazines.
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References
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M., Marchie, A., Jenkins, A.L., & Axelsen, M. (2002). Glycemic
index: overview of implications in health and disease. American
Journal of Clinical Nutrition, 76, 266S-273S
2. Schenk, S., Davidson, C.J., Zderic, T.W., Byerley, L.O., & Coyle,
E.F. (2003). Different glycemic indexes of breakfast cereals are
not due to glucose entry into blood but to glucose removal by tissue.
American Journal of Clinical Nutrition, 78, 742-748
3. Pi-Sunyer, F.X. (2002). Glycemic index and disease. American Journal
of Clinical Nutrition, 76, 290S-298S
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