Carbohydrates are not created equal. Although made of the same building blocks—sugar molecules—carbohydrates differ in how the body handles these molecules. Some, like white bread, are digested quickly and dump their sugar into the bloodstream all at once, causing rapid fluctuations in blood sugar levels. Others, like oat bran bread, are digested at a more leisurely pace and dole out their sugars slowly, causing a gradual rise in blood sugar. A food’s impact on blood sugar—called its glycemic effect —is measured by glycemic index (GI) and glycemic load (GL).

The glycemic index indicates how quickly 50 grams of a food’s carbohydrates turn into blood sugar. Compared with 50 grams of glucose (GI=100), for example, white bread has a GI of 73 and oat bran bread has a GI of 31. The glycemic load considers both the GI and the amount of carbohydrate in one serving. One slice of white bread has a GL of 10; oat bran bread, 3 (GL of glucose=10). For both GI and GL, a lower number is believed desirable for good health. A diet rich in high-GL foods, on the other hand, is linked to a greater risk of obesity, type 2 diabetes , and heart disease.

To determine if changing the GI and GL of a diet would affect weight and heart disease risk, Australian researchers put 129 overweight people on four different diets. Their findings, in the July 24, 2006 Archives of Internal Medicine , show that volunteers eating a moderately reduced GL diet—one rich in low-GI carbohydrates—lost the most weight and decreased their risk for heart disease.

About the Study

Australian researchers recruited 129 overweight people, ages 18-40, to follow a low-calorie, low-fat, modest-fiber diet for 12 weeks. The volunteers were randomly assigned to one of four diets (Table 1). The GL was highest in diets 1 and 3 and lowest in diets 2 and 4.

Table 1. Composition of Study Diets

CarbohydrateProteinGlycemic indexGlycemic load
Diet 1High, 55%Average, 15%High127
Diet 2High, 55%Average, 15%Low75
Diet 3Reduced, 45%High, 25%High87
Diet 4Reduced, 45%High, 25%Low54

Each week, volunteers were weighed and given pre-measured foods. Body fat was measured at the start and conclusion of the study. Blood was drawn at three visits to monitor glucose, cholesterol, and related factors. Researchers compared changes in weight, body composition, and blood levels across the four groups.

Diets 2 and 3 were most effective for weight loss; after 12 weeks, significantly more people on diets 2 or 3 dropped at least 5% of body weight than those on diets 1 or 4. Among the 98 women in the study, those on diets 2 and 3 also lost significantly more body fat than women on the other two diets. For heart health, diet 2 appeared to stand out as protective. Both total and LDL cholesterol (the “bad” cholesterol) were lowered in participants on diet 2, while these two factors increased in the diet 3 group.

Although diet 4 had a low GL and low GI, the researchers determined that it may have been too "extreme" and required greater discipline. Not enough volunteers, therefore, stuck to the diet for it to distinguish itself from the other diets.

This study lasted for 12 weeks; the long-term effects of these diets are unclear.

How Does This Affect You?

In this study, moderately reducing the GL of the diet, by choosing foods rich in low-GI carbohydrates, was the most effective means for achieving weight loss and lowering heart disease risk. This finding fits with previous studies that have suggested benefits from low-GI foods.

While the GI and GL lingo is fairly new, the underlying message is not. Foods with a low GI and GL are those that are high in fiber, less refined, and less processed. In short, these are the types of foods that nutrition experts have long been recommending to improve health and decrease the risk of chronic disease.

To choose a healthful diet with a low GL, follow these steps:

  • Choose low-GI grains: whole oat products, brown rice, barley, rye, and whole wheat breads and pastas
  • Choose nuts, legumes, fruits, and nonstarchy vegetables
  • Limit intake of starchy high-GI foods like potatoes (especially in the form of French fries), white rice, and white bread
  • Limit intake of sweets, such as cookies, cakes, candy, and soft drinks