By Donna Feldman, MS, RD

If you are looking for a diet that promises weight loss by numbers, the glycemic index (GI) diet might be for you. The promise is this: By eating only foods with low GI, you will avoid the dreaded blood sugar rush that is blamed for weight gain. Before you buy into this promise, you should learn more about exactly what it means and whether it really does work as claimed.

What is the Glycemic Index?

GI was originally supposed to help diabetics with food choices. Diabetics need diets that keep blood sugar from swinging up and down too much. Researchers devised a system to measure how a food affects blood glucose. To measure the glycemic index of carrots, for example, test subjects eat an amount of that food that supplies 50 grams of carbohydrate. That's over a pound of carrots. Then the researchers measure the subjects' blood glucose level at set intervals, for 2 hours. The glycemic index of the food is computed using those measurements. It's a number between 0 and 100. Pure glucose has a GI of 100.¹

Main problem with the Glycemic Index

For carrots, the number is, well...take your pick. There are numerous websites that will give you a number. One says the GI for carrots is 65.² Another says 31,³ and another gives a range of 16-92.⁴ That's a very wide range for a simple food. There are plenty more websites and books that will give you GI numbers for a variety of foods. It is never clear where the numbers come from.

Most lists categorize foods into three groups: Low (less than 55), Medium (55-69), and High (over 70) GI foods. Low GI foods will stimulate the least blood glucose and are presumably better to eat. Vegetables generally are low GI. Foods with lots of sugar have higher values. But what should we make of the fact that fructose, a sugar, has a GI of 20 while bananas have a GI of 56? Does this mean it is better to eat fructose-sweetened foods like candy than to eat a banana? This is one of the main problems with the glycemic index. Some numbers can be interpreted to say that foods such as potatoes (98) and cornflakes (80) are poor choices, compared to Peanut M&Ms (32), ice cream (31) and even Pop Tarts (70). Nutritionally speaking, this is nuts.

Other problems with the Glycemic Index

Another criticism of the GI diet is that the measurement does not reflect the actual way foods are eaten. People do not eat individual foods one at a time, especially in the unusually large amounts used in research. The effect of the whole meal is more important than the GI of each individual food in the meal. Protein and fat in a meal slow digestion. The glycemic index does not take that into account at all, nor does it account for differences in each person's digestion or different food preparation methods.

Will the Glycemic Index Diet help with weight Loss?

For all the effort a GI diet requires, there is little apparent pay-off in terms of improved weight control. The Glycemic Index diet has not been shown to affect weight loss, despite the belief that controlling blood sugar will control appetite and insulin, and therefore control food intake and fat storage. The International Food Information Council recently summed up current knowledge with a quote from the Dietary Guidelines Advisory Committee: "Current evidence suggests that the glycemic index ... is of little utility for providing dietary guidance for Americans."⁵

Stick to the diet principles of the GI Diet

While you may not adopt the number-crunching glycemic index system to improve your diet, the general principles of GI foods are fairly easy to incorporate: Stick to a low-sugar, high-fiber, plant-based diet with adequate protein and healthy fats like olive oil. No one can argue with that.