Over the past several years, books such as Sugar Busters, Dr. Atkins’ New Diet Revolution, and The Zone have presented the idea that eating carbohydrates leads to weight gain and obesity. Essentially, these books promote the idea that some carbohydrates are “high-glycemic-index” foods that raise blood sugar levels, cause excess insulin to be secreted, and lead to the storage of fat. The authors encourage people to eat foods with low glycemic indices or no glycemic index such as proteins (meat, fish, poultry, eggs) and fats (oils, margarines, nuts, full-fat salad dressings), and to eat only small amounts of carbohydrates, to promote weight loss and good health.

This sounds very plausible and scientific, except for the fact that the glycemic index is very difficult to apply to foods consumed in the real-world environment, where the glycemic index can vary widely depending on the ripeness of a food, the degree to which a food is cooked, and other factors. That is why there are questions about its use as a measure on which to base dietary recommendations for the general population.

Glycemic index is a measure of how carbohydrate-containing foods affect blood glucose levels. All foods that contain carbohydrates, such as starchy vegetables (potatoes, corn), desserts, fruits, bread, pasta, and rice, can be tested for how they affect blood sugar levels after being eaten. Glycemic index is assessed by having one or more people eat a specific amount of a single food [usually 50 grams of digestible carbohydrate (total carbohydrate minus fiber)] and then measuring the change in blood sugar levels compared with the levels achieved after they have eaten a control food containing the same amount of digestible carbohydrate, such as white bread or glucose. The average change in blood sugar levels over a set period of time relative to the levels after consumption of the control food, usually white bread or glucose, is the food’s glycemic index.

Foods can be ranked according to how much blood sugar levels rise. Given sufficient quantities, all carbohydrate-containing foods eventually raise or maintain blood sugar levels to some extent. High-fat and high-fiber foods typically have lower glycemic indices. Both the fat and the fiber in these foods can slow the rate of gastric emptying and therefore reduce the rate of delivery of carbohydrate to the small intestine, where it is digested and glucose is absorbed. Fat actually inhibits or delays gastric emptying, and many (but not all) fibers simply increase the viscosity or increase the mass to slow the emptying rate. Fibers have also been shown to decrease blood glucose levels.

You might expect that foods with more sugar, such as candy, soft drinks, and sweetened cereals, would have higher glycemic indices than other less sugary foods such as baked potatoes or corn flakes. It turns out, however, that baked potatoes and corn flakes have higher glycemic indices than jellybeans and soft drinks.

Ranking and eating foods according to their glycemic indices has a number of problems as a prescription for healthful eating. First, a person’s blood sugar response to eating a food can vary from day to day and also according to how the food was prepared. Even something as simple as ripeness, e.g., in the case of bananas, can affect the glycemic index (the riper the banana, the higher its glycemic index). Second, once a food is combined with other foods (such as cereal and milk or peanut butter and bread), the glycemic index of the meal will be very different from that of either food alone. Finally, the amount of food eaten to measure the glycemic index is often different from the amount of food eaten in a typical serving. The bottom line is that creating a diet based on the glycemic index alone is impractical, can result in an eating plan that may exclude some nutrient-rich foods, and render the diet less palatable overall.

Advice about dietary carbohydrates has not changed much over the past 30 years: experts generally encourage people to build their diets around nutrient-dense carbohydrates, for the most part, and then to include a moderate amount of protein and some fat. Also, to help reduce risk for many of the lifestyle-related diseases such as heart disease, diabetes, and high blood pressure, keeping your weight in check and exercising regularly will help a great deal. You can include all types of food in your eating plan. Be sure to plan for the calories of the foods you eat and not just their glycemic indices alone.

Sources of information for this article:

Pi-Sunyer X. Glycemic index and disease. Am J Clin Nutr 2002; 76(Suppl): 290S-298S.

Wolever T, Jenkins D, Jenkins AL, and Josse RG. The glycemic index: methodology and clinical implications. Am J Clin Nutr 1991; 54:846-854.

Foster-Powell K, Miller JB. International tables of glycemic index. Am J Clin Nutr 1995; 62:871S-890S.