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Decoding the glycemic index

Learn how to interpret the glycemic index and find out which carbs are the smartest dietary choices (hint: they're not all evil!).

If you’ve heard of the glycemic index (GI), you probably associate it with diabetes: To help keep blood sugar levels even, people with diabetes should eat foods that score low on the GI and avoid high-glycemic foods, which tend to spike levels. In fact, research shows eating too many high-GI foods raises risks not only for developing type 2 diabetes, but also inflammation, some cancers, and cardiovascular disease. High blood sugar also prompts the pancreas to pump out lots of insulin, upping blood triglyceride levels and promoting fat storage in the belly area.

“Including more low-glycemic foods in your diet can be a healthy way to lose weight, improve energy, and alleviate mood swings, especially if you have insulin resistance,” says David Edelson, MD, an obesity expert and assistant clinical professor of medicine at Albert Einstein College of Medicine in New York City. But a food’s GI value often doesn’t tell the whole story. For instance, a high-GI snack or drink can boost energy and improve performance during an intense workout. Here’s our guide to more exceptions to the GI rule, plus tips on how best to use GI values to assess foods.

Not all high-GI foods are evil

Consider the plain baked potato. Or watermelon. Or parsnips. Low-calorie? Check. Nutritious? Check. Low-glycemic? Not by a long shot.On the other hand, lower-glycemic pizza, chocolate cake, and potato chips are loaded with calories and saturated fat and are scarce on nutrients.

Choose whole-food carbs over more processed carbs. “Think whole fruit instead of fruit juice, whole-grain bread instead of white bread, carrots instead of carrot cake,” says Joan Clark-Warner, RD, coauthor of The Complete Idiot’s Guide to Glycemic Index Weight Loss (Alpha, 2005).

It’s all about portions

To determine a food’s GI value, researchers feed a person an amount that provides 50 grams of digestible carbohydrates (starch plus sugar, minus fiber). But for many foods, this isn’t a real-world portion. Take watermelon: You’d need to eat 4 cups to get the required 50 grams. Enter the glycemic load (GL), devised by Harvard scientists to take into account both GI value and normal portion size. To calculate the GL, divide a food’s GI by 100, and then multiply by grams of digestible carbs in a typical serving. Thus the high-GI parsnip (GI score: 97) becomes the low-GL parsnip (GL score: 12).

Practice portion control for both good and bad foods to keep total calorie intake appropriate, says Clark-Warner. “Use half of your plate for nonstarchy vegetables such as leafy greens, a quarter of the plate for starches like potato or brown rice, and the remaining room for proteins.”

Balance is crucial

Although a plain baked russet potato’s GI value is about 85, few people eat spuds that way. Fat, protein, and fiber—whether added or naturally present in a food—all slow blood glucose’s release into the bloodstream. Hence the low GI score for a doughy slice of pizza loaded with fat-laden cheese. “To lower the overall GI of your diet, it’s important that your meals and snacks contain a balance of healthy carbohydrates, fats, and protein,” says Clark-Warner.

Add sautéed vegetables, chicken, and a drizzle of olive oil to pasta rather than dumping straight tomato sauce on noodles. And don’t forget the soluble fiber, found in foods like beans, lentils, oats, flaxseed, and chia seeds. “Soluble fiber forms a gel in your stomach that slows the release of sugar into the bloodstream,” Clark-Warner says.

Do label math

To assess how a packaged food will affect your blood sugar, find the carbohydrate numbers on the nutrition panel. From this number, subtract the grams of fiber and sugar alcohols (added to some foods to boost sweetness without a significant impact on blood sugar). Your result will be grams of digestible carbohydrates. “The higher the number, the bigger the likely impact on blood glucose,” says Edelson.

The most important thing to remember about the glycemic index, experts say: Don’t be a slave to the numbers. Use them as a guide to make better carbohydrate choices, eat them in moderation, and keep moving.

How does the glycemic index work?

Developed nearly 30 years ago, the GI measures carbohydrates’ effect on blood sugar levels. Faster carbs (which break down quickly during digestion to release glucose into the bloodstream) have a high GI; slower carbs have a low GI. Glucose, the reference food, has a value of 100; in comparison, a white baguette scores 95, and chickpeas score around 31.

How do 5 common foods rate?

(Glycemic index, Serving size, Glycemic load per serving)

Apple, Braeburn                          (32, 120 g, 4)

Ice cream, low-fat, vanilla            (54, 50 g, 3)

Kidney beans, canned                 (52, 150 g, 9)

Microwave popcorn, plain            (72, 20 g, 8)

Wheat bread with peanut butter   (59, 100 g, 26)

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