Surprise! Ten myths that can trip you up

"We have a new study under way to see if we can reduce or eliminate the bad effects of carbs by switching to low-glycemic carbs," says Sacks. That would mean replacing some breads, cereals, and sweets with foods like beans, pasta, or oats, which cause less of a rise in blood sugar levels.

"I don't want to bash all carbs, because some carb-rich foods are very healthy, and some low-carb diets are not," he cautions. "But the scientific community is grappling with the glycemic index. We don't know how much it matters."

4 Butter is better for you than margarine.

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Butter may rule in the taste department, but it's still high in saturated fat (2 1/2 grams per pat and 7 grams per tablespoon).

What's more, nearly all margarines have now ditched their damaging trans fat.

Only a handful of brands--nearly all sticks--are still made with partially hydrogenated oils. The rest have switched to a mix of unsaturated oils (like soy or canola) and smaller amounts of saturated fats (like palm oil). And you can even find sticks--Promise and Earth Balance Buttery, for example--that have zero grams of trans.

If you still prefer butter, consider one of the better butters. Land O' Lakes makes Whipped Butter (3 1/2 grams of sat fat per tablespoon), Whipped Light Butter (3 grams), and Light Butter with Canola Oil (2 grams). That's not as low as the best margarines (1 gram), but it's not bad.

Since "whipped" has added air and "light" has added water, neither works for baking or sauteing. But they should spread and melt just fine.

5 You needn't worry about getting enough protein.

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In recent years, as attention shifted from low-fat diets (like Ornish) to low-carb diets (like Atkins), protein more or less dropped off the radar screen.

That's title if, like most Americans, you're eating enough protein. But some of US aren't.

"As people get older, calorie needs go down," explains protein expert Wayne Campbell of Purdue University in West Lafayette, Indiana. "And since most people are creatures of habit, they tend to eat the same types of food, just less of it."

But protein needs don't drop as you age. "That puts people at risk for not getting adequate amounts of protein," says Campbell.

The Recommended Dietary Allowance for protein is 46 grams a day for women and 56 grams for men. "USDA survey data suggest that up to 15 percent of people in their 60s are not eating at least the RDA," says Campbell.

"And by age 75, the number goes up to 40 percent." Even though surveys underestimate what people eat, those percentages are worrisome.

What happens if you eat too little protein? "Your body may have to draw on muscle for the amino acids it needs for vital organ functions," Camphell explains. (Proteins are made of amino acids.)

Over time, that lost muscle leads to weakness. At first, it's imperceptible. Eventually, it means you have trouble carrying heavy grocery bags or climbing stairs.