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When age goes up, calories need to go down.

So you've hit 40, and you notice that you've put on a few pounds. But you're still exercising and eating just as you did in your 30s. What's going on?

For the answer, WebMD turned to Liz Hill, RD, a nutrition specialist with the Food and Nutrition Center at the U.S. Department of Agriculture.

As you age, you need fewer calories. Why? Once you reach adulthood, your muscle mass gradually decreases while the proportion of fat increases. Since muscle tissue burns more calories than fat, you'll need fewer calories to maintain your usual weight. If you keep eating the same amount as you get older, those extra calories will turn into extra pounds.

For example, a woman who's 5'4", 130 pounds, and who engages in light exercise for an hour each day might need 1,980 calories at age 40. At age 50, weighing and exercising the same amount, she might need only 1,880.

Those are only rough estimates, and they don't take into account the effect of your particular metabolism and genetic make-up.

The Answer is Simple

What are your options? They're pretty straightforward: either eat less or exercise more. If that hypothetical woman were to make her hour of exercise a little more vigorous, she could keep eating the same number of calories at 50 that she did at 40.

However, remember that calories aren't everything when you're trying to stay healthy as you age, says Hill. Seniors tend to eat fewer calories anyway, so they need to make sure they get enough nutrients and vitamins in their foods. "When you're looking at people who are 80 or 90 years old," says Hill, "the real problem is keeping weight on."

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