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The glycemic index (GI) is a ranking of carbohydrate-containing foods on a scale of 0 to 100. The higher the number, the more quickly you'll digest the food and trigger extreme fluctuations in blood sugar. Low scores (55 and lower) mean the food is digested slowly and produces only gradual changes in blood sugar. For almost 30 years, the GI has been a useful tool for medical science, allowing researchers to gauge the relationship between specific carb-rich foods and their effect on physiology. Though dieters and diabetics are starting to use the index for their own purposes, Ludwig advises that you avoid "eating by the numbers" and instead follow these simple guidelines:

1. Eat plenty of fiber-rich vegetables (dark leafy greens—good; corn—not so good), beans (all of them), and fruit (apples, pears, peaches, and berries have a lower GI than tropical fruits, like papaya and mangoes).

2. Limit potatoes to small side dishes.

3. Choose grains in their least processed states. For example, replace refined and white breads with stone-ground whole wheat, sourdough, or pumpernickel. Swap jasmine and arborio rice for basmati, brown, or long grain. Instead of processed cereals like cornflakes and instant oatmeal, stick with old-fashioned oats or cold cereals that have at least four grams of fiber per serving. Ration white-flour sweets like doughnuts and cookies for the occasional treat—there are no healthy substitutes for these!

4. Avoid sugar-sweetened beverages, and drink no more than one cup of 100 percent fruit juice daily.

5. Consume protein and fat at most meals and snacks. Eating a balance of nutrients will help keep your blood sugar steady and your hunger in check. Vegetable and lean animal sources (including dairy) are your best options for protein. Olive oil, nuts, avocados, seeds, and nut butters are healthy fats. Cut back on saturated fats, and banish trans fats completely.