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If calories don't make you fat, what does? Carbohydrates, especially simple ones like sugar, honey, and refined white flour. When you eat carbohydrates, your body breaks them down into sugar, which enters your blood. Your pancreas responds to blood sugar by producing the hormone insulin, which tells your fat cells to hold on to fat and store it – but not necessarily as permanent fat. When everything's working right, fat acts "more like a wallet than a savings or retirement account," says Gary Taubes, author of Why We Get Fat: And What to Do About It. You're constantly putting money in and taking it out again. In the case of the body, fat gets stored in cells and rereleased after digestion to ensure steady energy.

Simple carbs, like those found in cake, candy, and fruit, contain lots of sugar and will eventually cause a large insulin response. If you eat simple carbs with any regularity – or worse, drink them – your fat cells hold on to accumulated fat because there's always more sugar, and consequently more insulin, in the body. Over time, your fat cells continue to acquire more fat than your body can use as energy. To lose weight, then, you have to cut back on your carbohydrate consumption, to push your insulin level low enough that it forces your body to release the stored fat that your muscles and organs burn as fuel.

If you're looking to maintain weight, remember that not all carbs are created equal. Complex carbs, such as legumes, whole grains, and leafy vegetables, produce only a moderate effect on blood sugar. All legumes, except baked beans, are particularly healthy because they're high in fiber and protein; aim to eat them on a daily basis. You shouldn't eat pasta, cereal, and baked goods as frequently, but when you do, choose only those made from whole grains. Look for products that specify whole flours, but read labels carefully: Without the word whole, "100 percent wheat flour" doesn't count. Opt for sprouted-grain, oat-bran, or 100 percent whole-wheat bread; Alvarado Street Bakery, Food for Life, and Rudi's Organic Bakery are all reliable brands. Choose less processed, high-fiber cereals, such as Nature's Path Organic SmartBran and Cascadian Farm Hearty Morning, and whole-grain pastas, such as Eden Organic Kamut & Buckwheat Rigatoni. Brown rice is preferable to white, but whole-kernel substitutes like barley, farro, and quinoa are even better because they contain more protein and fiber.

Eat Right with the glycemic index.

The glycemic index ranks foods that contain carbohydrates from 0 to 100, based on how they affect blood sugar. High-GI carbs (>70) cause blood sugar to spike; low-GI foods (<55) have a more moderate effect on insulin levels. For a database of more foods, visit

Low GI

Meat pizza (30)

Oat-bran bread (32)

Most vegetables (33)

Peanuts (13)

Chocolate cake (37)

Black beans (20)

High GI

White bread (70)

Watermelon (72)

Winter squash (75)

Cornflakes (81)

Pretzels (83)

Fruit roll-ups (98)

*Values based on data from the National Cancer Institute