The Secret to Weight Loss? Calories.

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What you need to know about counting them, cutting them—and why you should stop obsessing about carbs and fat.

by Suzanne Schlosberg

The truth is, all diets boil down to a simple formula-eating fewer Calories than you burn. Break that rule and all the carb-cutting, fat-banning, low-glycemic-index-eating in the world won't make a bit of difference. This is why weight-loss experts are now advocating a back to-the-basics approach: Calorie counting.

Weren't paying attention in high school biology? No worries. This crash course on the science of Calories will catch you up fast. You'll learn to make smarter choices about the foods (and portion sizes) you put on your plate and how to maximize your daily Calorie budget. The savvier you are, the quicker you'll drop pounds.

Q: Exactly what is a Calorie anyway?

A: In science equations, a calorie is a measure of potential energy in food. Technically, one calorie is the amount of energy it takes to raise the temperature of 1 gram of water by 1 degree Celsius. The Calorie we refer to generally in nutrition, which is found on food labels, is the kcal, or kilocalorie (which equals 1,000 calories). "Calories are like gasoline. In the same way that gas makes your car go, calories fuel your body," says D. Milton Stokes, R.D., president of a nutrition counseling firm in Norwalk, Connecticut.

Q: How does my body convert Calories?

A: Enzymes in your digestive system break down the chemical bonds that hold food molecules together, explains Gary Miller, Ph.D., an associate professor of nutrition at Wake Forest University. Your digestive system then releases the energy contained in those bonds, making it available for use.

Q: What does my body do with that energy?

A: It fuels everything from basic activities like breathing, thinking, and growing hair to bigger tasks, like carrying a pregnancy or running a marathon. However, when you don't use the Calories you've consumed (maybe you decide to skip the gym today), those Calories get shuttled to your liver to refill your glycogen stores. Glycogen is your body's quick, easy-access energy reserve. Your ability to store it means you don't have to eat continuously to keep your body revved up. Still, it gets depleted every three to four hours. When the liver is holding as much glycogen as it can, some of it is passed on to muscles for short-term storage (to be used as needed to move your body and get you through a workout).

Between your liver and muscles, you have a ready supply of Calories (roughly 300 to 400, depending on your weight and metabolism) that you can access as necessary throughout the day. When you eat more than you can save in these temporary "accounts," the Calories get converted to fat and distributed throughout your body.

Q: How many Calories do you have to burn to lose a pound?

A: About 3,500. That means you can eat 100 fewer Calories a day for 35 days, or 500 fewer calories for seven days, or walk an hour a day for 22 days, or do a combination of the two by eating less and moving more. Remember, even if you're exercising more than usual, the Calories-in, Calories-out rule still applies: If you take in more than you burn, you'll gain weight.