Keep your weight down and your energy up with the glycemic index

Harvard Women's Health Watch

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A low-glycemic diet is based on
whole, natural foods.

Using this simple concept as a guide can help you stay sated longer without gaining weight.

In 1984 a Time magazine cover used a scowling plate of bacon and eggs to symbolize the dietary culprit of the day—high-fat, cholesterol-laden food. Today, a similar graphic might feature a baked potato. Over the last 30 years researchers have discovered the secret to weight control may lie not in reducing dietary fat but in lowering the amount of refined carbohydrates you eat, or in more precise terms, choosing foods with a lower glycemic index (GI). Moreover, rapidly accumulating evidence is linking a low-glycemic diet with a reduced risk of diabetes, heart disease, and certain cancers.

Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Harvard-affiliated Boston Children's Hospital, and his colleagues have discovered some clues to why people on a low-glycemic diet find it easier to lose weight and keep it off. In 2012 they determined that, after a weight loss of 10% to 15%, people on a low-glycemic diet burned more calories than those on a high-glycemic diet. In 2013 they looked at the effects of high- and low-glycemic meals with the same number of calories four hours after people ate them. They found that people were hungrier, had lower blood sugar, and had more activity in the area of the brain that is associated with craving and reward after they consumed a high-glycemic meal.

How high-glycemic foods keep you hungry

"The glycemic index is the best measure we have today of how rapidly carbohydrate gets digested into glucose," Dr. Ludwig explains.

Here's how it works. When you eat a high-glycemic food, the sugar in that food becomes readily available as soon as it passes through the stomach to the intestines. You may feel a sudden surge of energy as sugar (in the form of glucose) pours into your blood. Your body will react to the glucose elevation by producing more insulin to metabolize it. However, the insulin rush will deplete that blood glucose within the next couple of hours. You may even feel exhausted, shaky, and woozy if your glucose level drops too low too quickly, a state called hypoglycemia. And you'll probably crave a high-glycemic snack, which certainly won't help with weight loss.

In contrast, low-glycemic foods require more processing time in the digestive system as enzymes work to separate the sugar from other components. Glucose flows slowly into the bloodstream, and insulin is released gradually, too. As a result, you remain sated longer and are less likely to overeat.

Another measure, the glycemic load, takes into account both the GI of a food and the carbohydrate content in a serving. Although some foods, like watermelon, have a high GI, they have a moderate glycemic load because a serving has relatively few carbohydrates. Foods like white potatoes that are both high-GI and high-carb pack a greater glycemic load.

Finding low-glycemic foods

As a rule, high-GI foods are those with lots of concentrated sugars and refined starches—for example, white flour, packaged cereals, and instant oatmeal. Foods with a low GI are likely to contain few sugars. And when they do contain sugars, the sugars are part of the natural food structure and aren't as readily available, so they enter the blood slowly.

The following pointers can start you on the path to low-glycemic eating :

  • Avoid processed starches and sugars. In other words, approach the center aisles of the grocery store with caution.

  • Pass up "quick-cooking" or "instant" rice or grains and most cold cereals. Steel-cut oats, wheat berries, and brown rice are good alternatives.

  • Buy fresh fruit instead of juice. Not only are several fruits required to make a glass of juice, but when fruit is squeezed, the cells are broken open, releasing the sugars. As a result, a 6-ounce glass of apple juice has a much higher glycemic load than a 6-ounce apple does.

You can find a chart listing the glycemic index and glycemic load of 100 foods at health.harvard.edu/glycemic.