Beware the "who knew?" effect — that scary moment when you learn the whopping calorie count of a favorite food. Take this quiz from FITNESS to find out whether you're making healthy choices or sabotaging your diet.

1. HOW LONG WOULD YOU HAVE TO WALK TO BURN OFF ONE SODA?

A. 40 minutes

B. 50 minutes

C. 1 hour 18 minutes

D. 1 hour 30 minutes

Answer: C. One 20-ounce cola has 250 calories. Soda is the biggest single source of calories in the U.S. diet. We drink more than 49 gallons of the stuff per person per year, research shows.

2. WHICH BREAKFAST HAS THE MOST CALORIES?

A. Ham, egg and cheddar on an English muffin

B. An oat-bran muffin

C. A bowl of yogurt, berries and granola

D. Oatmeal with maple syrup and brown sugar

Answer: B. One oat-bran muffin has 454 calories and 12 grams of fat. Even worse, some store-bought muffins are high in trans fats, which are as bad for your waistline as they are for your heart. In a study of overweight women, for every 1 percent of calories they ate from trans fats they gained 2.3 pounds. Even if a food is labeled "trans-fat free," by law it can contain up to 0.5 grams of trans fat per serving, and those grams add up quickly. If the label says "partially hydrogenated," stay away.

3. WHICH OF THESE FOODS HAS THE MOST CALORIES PER SERVING?

A. 1/4 cup of tropical trail mix

B. A 1-ounce bag of vegetable chips

C. 30 chocolate-covered raisins

D. 1/2 cup of Edy's Slow Churned Rich and Creamy Chocolate Fudge Chunk Ice Cream

Answer: B. Just 14 veggie chips have 150 calories and 9 grams of fat - and no, they don't count as a serving of vegetables. "They're potato chips in disguise," says Madelyn Fernstrom, Ph.D., a FITNESS advisory board member and the director of the UPMC Weight Management Center in Pittsburgh. "Because people think they're healthy, they're more likely to polish off the whole bag."

4. HOW BIG IS A SINGLE PORTION OF CHEESE?

A. The size of two dice

B. The size of a matchbook

C. The size of your index finger

D. The size of a deck of cards

Answer: C, or 1 ounce. Cheese is a great source of calcium, which researchers at Rutgers Univer-sity found can help women maintain bone mass while dieting. "As long as you keep portion sizes in check, you don't have to choose low-fat or fat-free. Eat the type you enjoy most," Koff says.

5. HOW MANY CALORIES SHOULD AN ACTIVE WOMAN EAT IN A DAY?

A. About 1,500

B. About 1,800

C. About 2,000

D. About 2,500

Answer: C. If you work out three or more times a week for at least 30 minutes, you should consume about 17 calories per pound to maintain your weight. "If you're looking to shed pounds, you need to cut about 500 calories a day, and you can't do it with exercise alone," Fernstrom says. Those who are most successful at keeping weight off do so by eating less, according to researchers at the University of Alabama. Your best bet: Take in 250 fewer calories daily and burn 250 more with exercise.

6. A BAGEL IS EQUAL TO HOW MANY SLICES OF BREAD?

A. 3

B. 5

C. 6

D. 8

Answer: B, or 5 servings of grains. That's more than two-thirds of your daily intake, and you've only had breakfast! Since most bagels are made from refined carbs - yep, usually even the wheat ones - you'll be hungry again soon after eating one. Spread a tablespoon of peanut butter (about the size of your thumb) on your bagel, Fernstrom says. "The protein will increase the time it takes for your body to process the bagel, so you'll be energized and satisfied for longer."

7. HOW MANY TIMES A WEEK IS IT OK TO EAT DESSERT?

A. 2

B. 4

C. 7

D. 10

Answer: C. You can eat dessert every day (even if you're on a diet!), as long as you keep the portion sizes in check. "Depriving yourself will only make you crave it more," Fernstrom says. That doesn't mean you should dig into a piece of cake every night, however, or the calories will pile on fast. A few smart choices: fruits and berries with toasted oats, a small piece of dark chocolate or a scoop of low-fat ice cream (about the size of a tennis ball).

8. WHAT'S BETTER:

A. A turkey burger?

B. A hamburger?

Answer: About equal. Surprise! "Some turkey burgers contain ground meat from all parts of the bird, which includes the high-calorie, high-fat dark meat and skin," Fernstrom says. When making a turkey burger at home, choose lean, all-white meat, which is similar in fat and calorie content to a 95 percent lean hamburger (140 calories and 5 grams of fat), says Lisa Young, Ph.D., R.D., a FITNESS advisory board member and author of "The Portion Teller."