Harvard Health Letter

Choosing good carbs with the glycemic index

Low glycemic foods help you feel full longer; help keep blood sugar even.

Bread, rice, pasta, breakfast cereal, dairy foods, fruits, and vegetables are staples in many diets. All deliver carbohydrates. For providing calories, one carbohydrate is as good as another. When it comes to health, though, some are better than others. Picking good carbs can help you control your weight and ward off a host of chronic conditions, from diabetes and heart disease to various cancers. One way to identify good carbs is with the glycemic index (GI). This tool measures how much a food boosts blood sugar.

"Glycemic index categories can be very helpful for people trying to choose a healthy diet," says Dr. Frank Hu, professor of nutrition at the Harvard School of Public Health.

The rise of blood sugar levels

The rise of blood sugar levels

High glycemic foods result in a quick spike in insulin and blood sugar (also known as blood glucose). Low glycemic foods have a slower, smaller effect.

What's a carb?

Carbohydrates are a family of molecules. They occur in three main forms. Sugars are the simplest. They include glucose (the type of sugar that travels in the blood stream), fructose (fruit sugar), lactose (milk sugar), and others. Starches are more complex carbs. A starch is a long chain of sugar molecules. Some starches, like those in the average baked potato, the body digests in a flash, quickly elevating blood sugar. Other starches, like those in whole grains and beans, are digested more slowly, and so don't boost blood sugar as high. Fiber, another complex carbohydrate, can't be broken down by the human digestive tract. Fiber tends to move though the stomach and gut slowly, making you feel full without adding calories.

Understanding the glycemic index

Blood sugar and insulin levels rise every time you eat something containing carbohydrates. How high they rise, and how fast, depend on the food. A serving of white rice has almost the same effect as eating pure glucose—a quick, high spike in blood sugar and insulin. A serving of lentils has a slower, smaller effect. The glycemic index captures these changes by rating the effect of a specific amount of a food on blood sugar compared with the same amount of pure glucose. A food with a glycemic index of 28 boosts blood sugar only 28% as much as pure glucose; one with a GI of 95 acts almost like pure glucose.

Over the past three decades, researchers have measured the glycemic index of several thousand foods. You can see the values for 100 commons foods and get links to more at health.harvard.edu/glycemic.

The glycemic index of a diet can affect health in various ways. Some of the latest studies suggest that:

  • a low glycemic index diet can help maintain weight loss

  • a high glycemic index increases the risk of breast, prostate, colorectal, and pancreatic cancers

  • a high glycemic index diet increases the risk of developing type 2 diabetes and cardiovascular disease

Using the glycemic index

Using the glycemic index is easy: choose foods in the low GI category instead of those in the high GI category, and go easy on those in between. Using the glycemic index would be even easier if U.S. food makers put a symbol on low GI foods, as they do in Australia. (It's "under discussion in the U.S.," says Dr. Jennie Brand-Miller, professor of human nutrition at the University of Sydney and a long-time glycemic index researcher and advocate.)

Low glycemic index (GI of 55 or less): Most fruits and vegetables, beans (Brand-Miller calls beans "star performers"), minimally processed grains, pasta, low-fat dairy foods, and nuts.

Moderate glycemic index (GI 56 to 69): White and sweet potatoes, corn, white rice, couscous, breakfast cereals such as Cream of Wheat and Mini Wheats.

High glycemic index (GI of 70 or higher): White bread, rice cakes, most crackers, bagels, cakes, doughnuts, croissants, most packaged breakfast cereals.

Everything in moderation

The glycemic index is a useful guide for choosing healthy foods. But it shouldn't be the only one. The amount of carbohydrate you take in matters too. Spaghetti, for example, has a low glycemic index (42). But eat a huge plate of it and your blood sugar will head into the stratosphere. And "not all foods with a low glycemic index are health foods," cautions Dr. Hu. A Snickers bar has a GI of 43; Coca Cola one of 63. Both deliver little more than sugar.

Swaps for lowering glycemic index

Instead of this high-glycemic index food

Eat this lower-glycemic index food

White rice

Brown rice or converted rice

Instant oatmeal

Steel-cut oats


Bran flakes

Baked potato

Pasta, bulgur

White bread

Whole-grain bread


Peas or leafy greens