Introducing the Glycemic Index and Glycemic Load

To figure out which carbs are best and worst for blood sugar, scientists had to do some serious detective work.

To figure out which carbs are best and worst for blood sugar, scientists had to do some serious detective work. First, they needed to come up with a way to measure a food’s effect on blood sugar. Nutrition scientist David Jenkins, M.D., Ph.D., developed a system called the glycemic index (GI) back in 1981 (the prefix glyc– means “sugar”). He had volunteers eat different foods, all containing 50 grams of carbohydrate. Then he measured the volunteers’ blood sugar over the following 2 hours to see how high it went.

Glycemic Index and Glycemic Load
Glycemic indexing has turned out to be a powerful way to think about not just individual foods but also whole meals and even entire diets.
As a control he used pure glucose, the form of sugar that’s identical to blood sugar — your body converts glucose very quickly to blood sugar — and assigned it the number 100 on his new index. The glycemic index opened a lot of eyes. Almost everyone had assumed that table sugar would be the worst offender, much worse than the “complex carbohydrates” found in starchy staples such as rice and bread. But this didn’t always prove true. Some starchy foods, like potatoes and cornflakes, ranked very high on the index, raising blood sugar nearly as much as pure glucose.

Where the Glycemic Index Fell Short
Something was wrong, however. Some of the results pointed fingers at healthy foods, such as carrots and strawberries. Watermelon was just about off the top of the GI chart. But no one ever gained weight from eating carrots, nor do carrots, in the real world, raise blood sugar. What was the GI missing?

The GI measured the effects of a standard amount of carbohydrate: 50 grams, or about 1 1/2 ounces. But you’d be awfully hard-pressed to eat enough carrots — seven or eight large ones — to get 50 grams of carbohydrate. The same holds true for most other vegetables and fruits. They’re full of water, so there’s not much room in them for carbohydrate. Bread, on the other hand, is crammed with carbohydrate. You get 50 grams by eating just one slice.

To solve the problem, scientists came up with a different measurement: the glycemic load (GL). It takes into account not only the type of carbohydrate in the food but also the amount of carbohydrate you would eat in a standard serving. (To get a bit technical, a food’s GL is the GI multiplied by the amount of carbohydrate in one serving.)

This made more sense. By this criterion, carrots, strawberries, and other low-calorie foods are clearly good to eat — they all have low GL values, since the amount of carbohydrate they contain is low.

The GL has turned out to be a powerful way to think about not just individual foods but also whole meals and even entire diets. When scientists looked at the GL of typical diets in different populations, they found that the higher the GL, the greater the incidence of obesity, diabetes, heart disease, and cancer. A study showed that men who ate the most sugar-boosting foods were 40 percent more likely to get diabetes. That’s GL we were talking about. The Nurses’ Health Study found that women were twice as likely to develop heart disease over 10 years if they ate more sugar-boosting foods. Again, the GL. The converse is also true: The lower the GL of your diet, the more likely you are to keep your weight under control and stay free of chronic disease.

When it comes to eating right, controlling weight, and preventing disease, the GL is a heavy hitter. It’s a more powerful factor in keeping you healthy than the amount of carbohydrate — or fat — you eat.

Originally Published in Reader's Digest

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