Mainstream groups, such as the American Heart Association and the American Dietetic Association, endorse a unified set of guidelines for the optimum diet. According to these organizations, the majority of calories in the daily diet should come from carbohydrates (55% to 60%); fat should provide no more than 30% of total calories; and protein should be kept to 10% to 15%.

However, many popular diet books turn the standard diet on its head. As described in the entry on low-carbohydrate diets, the Atkins diet, the Zone diet, Protein Power, and other “alternative” dietary approaches turn thumbs down on carbohydrates and advocate increased consumption of fat and/or protein. According to theory, the low-carb approach aids in weight loss (and provides a variety of other health benefits) by reducing the body’s production of insulin.

The low glycemic index (low-GI) diet splits the difference between the low-carb and low-fat approaches. It maintains the low-carb diet’s focus on insulin, but it suggests choosing certain carbohydrates over others rather than restricting carbohydrate intake.

Evidence suggests that carbohydrates are not created equal. Some carbohydrates, such as pure glucose, are absorbed quickly and create a rapid, strong rise in both blood sugar and insulin. However, others (such as brown rice) are absorbed much more slowly and produce only a modest blood sugar and insulin response. According to proponents of the low-GI diet, eating foods in the latter category will enhance weight loss and improve health. However, as we shall see, despite some promising theory there is as yet no solid evidence that low-GI diets enhance weight loss.

Besides weight loss, preliminary evidence suggests that the low-GI approach (or, even better, a related method called low glycemic load) may help prevent heart disease. The low-GI approach has also shown promise for treating and possibly preventing diabetes.

The precise measurement of the glucose-stimulating effect of a food is called its glycemic index. The lower a food’s glycemic index, the less potent its effects on blood sugar (and therefore insulin).

The glycemic index of glucose is arbitrarily set at 100. The ratings of other foods are determined as follows. First, researchers calculate a portion size for the food to supply 50 g of carbohydrates. Next, they give that amount of the food to at least 8-10 people and measure the blood sugar response. (By using a group of people rather than one person, researchers can ensure that the idiosyncrasies of one individual don’t skew the results.) On another occasion, researchers also give each participant an equivalent amount of glucose and perform the same measurements. The glycemic index of a food is then determined by comparing the two outcomes. For example, if a food causes half of the blood sugar rise of glucose, it is assigned a GI of 50; if it causes one-quarter of the rise, it is assigned a GI of 25. The lower the glycemic index, the better.

When scientists first began to determine the glycemic index of foods, some of the results caused eyebrows to rise. It didn’t surprise anyone when jellybeans turned out to have a high glycemic index of 80—after all, jellybeans are mostly sugar. Nor was it unexpected that kidney beans have a low glycemic index of 27 because they are notoriously hard to digest. But when baked potatoes came back with a reading of 93, researchers were taken aback. This rating is higher than that of almost all other foods, including ice cream (61), sweet potatoes (54), and white bread (70). Based on this finding, low-GI diets recommend that you stay largely away from potatoes. (However, the concern regarding potatoes is probably unnecessary. See the discussion of glycemic load below.)

There are other surprises hidden in the glycemic index tables. For example, fructose (the sweetener in honey) has an extraordinarily low glycemic index of 23—lower than brown rice and almost three times lower than white sugar. Candy bars also tend to have a relatively good (low) glycemic index, presumably because their fat content makes them digest slowly.

It’s difficult to predict the glycemic index of a food without specifically testing it, but there are some general factors that can be recognized. Fiber content tends to reduce the glycemic index of a food, presumably by slowing down digestion. For this reason, whole grains usually have a lower GI score than refined, processed grains. Fat content also reduces GI score. Simple carbohydrates (such as sugar) often have a higher GI score than complex carbohydrates (such as brown rice).

However, there are numerous exceptions to these rules. Factors such as the acid content of food; the size of the food particles; and the precise mixture of fats, proteins, and carbohydrates can substantially change the GI measurement. For a measurement like the glycemic index to be meaningful, it has to be generally reproducible between people. In other words, if a potato has a glycemic index of 93 in one person, it should have pretty much the same glycemic index when given to another person. Science suggests that the GI passes this test. The glycemic index of individual foods is fairly constant between people, and even mixed meals have a fairly predictable effect according to most (but not all) studies.1-6

Thus, the GI of a food really does indicate its propensity to raise insulin levels. Whether a diet based on the index will aid in weight loss, however, is another story.