Women Need Fewer Calories

"A woman and man of the exact weight and percentage of fat would burn the same amount of calories for the same amount of exercise," says Sharon B. Spalding, MEd, CSCS, professor of physical education and health at Mary Baldwin College in Staunton, Va. "However men are usually larger with a higher lean weight and will burn more calories."

Body composition comes into the picture, she says, because we know that muscle takes more calories to maintain -- even when you're not exercising -- than fat.

So women need fewer calories than men in part because they tend to be smaller and have higher fat percentages than men. That means women have to be choosier about what they eat. If you need fewer calories, the calories you take in need to pack a lot of nutritional punch.

In general, women need around 1,200 calories every day and men need a few hundred calories more. If you exercise you'll need much more depending on how active you are.

"Remember that to determine caloric expenditure one must take into consideration the intensity and duration of the activity, as well as the body weight of the person exercising," said Spalding.