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The Glycemic Index debate: Does the type of carbohydrate really matter?

by Janine Freeman, RD, CDE

As the low-carbohydrate-diet fad slowly loses steam, another may be moving in to take its place: the glycemic index fad.

The glycemic index (GI) ranks carbohydrate foods based on how they affect the body's blood glucose levels. "High GI" foods such as corn flakes, instant potatoes, and white bread greatly affect blood glucose levels. "Low GI" foods such as oatmeal, most fruits and vegetables, legumes, and nuts produce less of an effect. Recently, some weight loss diets have popularized the concept of the glycemic index, linking low GI foods to weight loss and high GI foods to weight gain.

The usefulness of the glycemic index in the treatment of diabetes has been controversial since its creation some 25 years ago. It's easy to understand why. The idea of classifying foods into groups -- those that greatly affect blood glucose and those that do not -- sounds very appealing to people with diabetes. It seems like it could make meal planning easier and improve after-meal blood glucose levels.

Unfortunately, it's not that simple. Here are a few reasons why.

  • The GI of a food varies substantially depending on the kind of food, its ripeness, the length of time it was stored, how it was cooked, its variety (potatoes from Australia, for example, have a much higher GI than potatoes from the United States), and how it was processed.

  • The GI of a food varies from person to person and even in a single individual from day to day, depending on blood glucose levels, insulin resistance, and other factors.

  • The GI of a food might be one value when it is eaten alone and another when it is eaten with other foods as part of a complete meal.

  • The GI value is based on a portion that contains 50 grams of carbohydrate, which is rarely the amount typically eaten.

  • Most GI values reflect the blood glucose response to food for only 2 hours, whereas glucose levels after eating some foods remain elevated for up to 4 hours or longer in people with diabetes.

Figuring out which foods are "high GI" and "low GI" can be complicated, too, because it depends on which base reference is used to determine the GI -- white bread or glucose. And believe it or not, many high-fat foods, such as candy bars and pizza, have a low GI. If food manufacturers begin lowering the GI of processed foods by adding high-fat ingredients or high-fructose corn syrup (which has a low GI), we'll have the same dilemma we had when low-carb products began inundating the market: junk foods that have been altered and labeled as healthy.

Some studies show small improvements in A1Cs among people who are attentive to the glycemic index. But reducing calories, weight loss, and basic carbohydrate counting have been shown to be more effective in improving A1Cs among people with type 2 diabetes than basing diet decisions on the GI.

I don't suggest eliminating "high GI" foods in favor of "low GI" foods to gain better blood glucose levels for two reasons. First, there is not enough evidence yet to show that such an action actually will improve your blood glucose levels; and second, choosing foods based solely on GI will compromise healthy eating.

I suggest basing your food choices on a nutritionally balanced diet, while controlling total carbohydrates, as a first measure. Then, if you find that your after-meal blood glucose is much higher after eating certain foods, you can either choose to eat smaller portions of those foods or adjust your mealtime diabetes medication.

This guest editorial was taken from the September 2005 issue of Diabetes Forecast.

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