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On Nutrition

Can a low-glycemic diet help you improve your health and reduce your waistline?

That’s a question scientists have been trying to answer since the glycemic index was developed more than 30 years ago. Current research suggests — and many health experts argue — that choosing lower-glycemic foods may help prevent some chronic diseases.

The glycemic index (GI) ranks foods on a scale of 0 to 100 based on how much they raise blood sugar (glucose) levels in the few hours after a meal. Pure glucose has a GI of 100. Foods with a high GI (above 70) are digested faster, so they cause sharper spikes in blood sugar. Most of these foods are high in carbohydrates. High-fat and high-protein foods have minimal effect on blood sugar.

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A food’s GI tells you how quickly a set amount (50 or 100 grams) of the type of carbohydrate in that food is converted to blood sugar. It’s about the quality of the carbohydrate, but not the quantity.

The amount of carbohydrate in one serving of a food, together with the GI value of that carbohydrate, is used to come up with that food’s glycemic load (GL). For example, watermelon has a high GI of 72, but because there is so little digestible carbohydrate per serving, it has a low glycemic load of 4 (on a scale of 0 to 60). Pasta (cooked al dente) has a low GI of 22 but a high GL of 46 (any GL above 20 is high).

Foods that are both low GI and low GL include whole fruit, lentils, most beans and most vegetables (including carrots). Many traditional diets, including diets of the Mediterranean, Middle East, Latin America, Africa and South and East Asia have a low GI.

A food’s GI is not easily identified from how it looks or the list of ingredients on its label. Take whole grains. Not all whole grains are low-glycemic. Most actual whole grains (brown rice, quinoa, wheat berries, bulgur, steel-cut or thick rolled oats) are low-glycemic. But when those grains are milled into flour or rolled thin and baked into bread and other products, they may be digested almost as rapidly as foods made from white flour. This is one reason why some health experts say that GI ratings should be included on food labels here in the U.S., as they are in Australia and a few other countries.

How our blood-sugar levels change after we eat carbohydrates depends on what other foods we eat them with. Mixed meals and snacks that include high-glycemic foods along with whole grains, vegetables, whole fruits, protein and healthful fat are digested more slowly, promoting gentle fluctuations in blood sugar.

The research investigating whether low-glycemic diets are associated with lower rates of chronic disease (and whether high-glycemic diets are associated with higher rates of disease) is not all in agreement. The strongest evidence suggests that choosing lower-glycemic carbohydrates may be beneficial for heart health, especially if they replace some of the saturated fat in the diet. (Replacing saturated fat with high-glycemic carbohydrates is not heart-healthy.)

The relationship to various cancers is inconclusive, but lower-glycemic diets may help prevent or delay type 2 diabetes in people who are at risk because of being sedentary, overweight and having a family history of the disease.

Lower-glycemic diets have not been shown to help with weight loss, although they may help prevent weight gain, and some studies have linked a high-glycemic diet to excess abdominal fat.

Carrie Dennett: Dennett is a graduate student in the Nutritional Sciences Program at the University of Washington; her blog is time: Seeds — not just for birds

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