The glycemic index: What you need to know...

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.

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.

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 number 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.

One fundamental assumption about foods with a low glycemic index is they release glucose into your blood more slowly, mainly because of a slower rate of digestion. This, in turn, is supposed to minimize the release of insulin.

Although it might sound like an elegant idea, the reality is far more complex.

The profit of a business, for example, depends not only on how much money a company is making, but also on how much it's spending. In much the same way, blood sugar levels depend on the rate at which glucose appears in the blood, as well as the rate at which it's cleared.

The speed at which glucose is removed from the blood depends mainly on the hormone insulin. In other words, a food can have a low glycemic index because the glucose is released into your bloodstream more slowly, or because it's cleared more quickly.

A good example of this comes from a study published in the October 2003 issue of the American Journal of Clinical Nutrition. Researchers from the University of Texas at Austin compared two breakfast cereals — Kellogg's All-Bran Original and Kellogg's Corn Flakes. Both cereals contained 50 grams of available carbohydrate (available carbohydrate is the total amount of carbohydrate minus fiber).

The glycemic index (measured over a three-hour period) of the Corn Flakes was more than twice that of the All-Bran (131.5 compared with 54.5).

However, there were no significant differences in the rate at which sugar appeared in the blood. This might come as a surprise to some people, especially when you consider the large amount of fiber (38.5 grams) in the All-Bran.

However, All-Bran is composed mainly of insoluble wheat bran fiber. This type of fiber has little effect at slowing the rate at which glucose enters the blood stream when combined with carbohydrate.

Instead, the lower glycemic index of the bran flakes was due to an earlier rise in insulin levels. Insulin levels after 20 minutes were roughly 75% higher following the consumption of the All-Bran compared with the Corn Flakes. This led to an earlier increase in the rate of disappearance of glucose.

All-Bran contains about 3.5 times more protein than Corn Flakes. When protein is eaten at the same time as carbohydrate, there is a much greater insulin response compared to eating carbohydrate alone.

However, the difference in insulin levels was only temporary. When they looked at insulin levels over a three-hour period, the researchers found only a small difference (about five percent) between the All-Bran and Corn Flakes.

This doesn't mean that all foods have a low glycemic index because they raise insulin levels more quickly. But it's a mistake to assume that a low glycemic index automatically means that glucose will enter the blood at a slower rate.

So, what does all this mean for you?

The glycemic index was originally developed to establish the type of foods that were best for people suffering from diabetes. Many popular diet books now recommend it as a way to make weight loss faster and easier.

However, some of the assumptions and experimental methods used to develop the glycemic index means that it has a limited practical value. The index is an average. True numbers vary considerably from one person to another, from one time to another, and from one study to another.

It's almost impossible to work out how any given food at any given time is going to behave in your body, unless you take blood sugar readings before and after eating.

Of course, there's nothing wrong with following a low glycemic index diet. Foods with a low glycemic index tend to have a low energy density. They're also rich in fiber, and contain phytochemicals that have important health benefits.

However, the most important part of any diet designed for weight loss is to consume fewer calories than you burn. The glycemic index can make planning your diet a lot more complicated and time-consuming than it needs to be.

References
Jenkins, D.J., Kendall, C.W., Augustin, L.S., Franceschi, S., Hamidi, 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
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
Pi-Sunyer, F.X. (2002). Glycemic index and disease. American Journal of Clinical Nutrition, 76, 290S-298S