Worried about your metabolism idling with age? Here's how to keep it revved for the long haul.

By Jill Weisenberger, M.S., R.D..

YOU'RE STILL FUELING YOUR DAY AND YOUR WORKOUT THE SAME WAY you always have, balancing calories in with calories burned, and indulging in Ben & Jerry's only every so often.

     Problem is, you're just not getting the same results. There was a time when your body blazed through calories like a raging fire, converting them into the power you needed to keep moving on overdrive. Now, it's more like a slow burn—and eating what you want whenever you want is no longer the option it was.

    The bad news: For many reasons, your metabolism slows as you age.

     The good news: With an active, healthy lifestyle, there's a lot you can do to keep your internal fires burning. Here's a look at what metabolic rate is and how you can influence your calorie-burning potential.

What is Metabolism?

    You burn calories all day and night—even if you do nothing but stare at the wall or count sheep. The speed at which you burn them is your metabolic rate. Most of the calories spent each day are for breathing, circulation of blood, maintenance of body temperature and other things your body does to keep you alive. This is your resting metabolic rate (RMR), and it accounts for 60 to 75 percent of the total calories you burn. Physical activity and the digestion, absorption and storage of food, called "diet-induced thermogenesis," make up the rest.

    So just what is it that revs your RMR? The greatest factor is your fat-free mass, made up in large part by your musculoskeletal system. In fact, your skeletal muscles at rest burn up about 18 percent of your RMR. This explains why men generally have greater metabolic rates than women. Men have more muscle. Even at the same height and weight, men burn about 10 percent more calories than women because of the gender differences in body composition.

    The metabolic slump with aging is largely due to a decrease in muscle mass. And we lose muscle mass mostly because we decrease activity, either through less planned exercise or because people are generally less active as they age.

Keep Pumping Iron

    Fat burns much less energy at rest than muscle does. So if a woman becomes less active as she ages, she'll likely lose muscle mass and gain body fat, causing her metabolic rate to lose steam. Whether muscle loss is solely the result of less physical activity isn't clear, but what is clear is that you can maintain your muscle mass as you grow older.

    When researchers challenged 15 men and women, ages 61 to 77, to 26 weeks of resistance training, the participants increased strength by 36 percent and lean body mass by 4.5 pounds. Their reward was a near 7 percent increase in RMR, which translated into 88 extra calories burned per day.

    You can keep your own metabolic fire stoked by including strength training at least twice a week in your exercise routine to build strength and maintain muscle mass.

Keep Sweating

    "Aerobic exercise is the only way to increase energy expenditure at will," says Roger Fielding, Ph.D., of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. To burn more calories, you must increase your cardiovascular exercise. For most people, daily physical activity accounts for 15 to 30 percent of the total calories burned. So at any age, run, bike, kayak, swim, and just move, move, move to use up more calories.

     Does the extra calorie-burn from aerobic exercise come from changes to the RMR or from the exercise itself? Researchers continue to debate this theory, but a study in the Journal of Clinical Endocrinology and Metabolism reports that the RMR of active women did not show the same age-related sluggishness as the RMR of sedentary women. The researchers found that muscle in aerobically trained postmenopausal women adapted to burn more calories than in untrained postmenopausal women of the same age.

Keep Watching Calories

One of the worst ways to burn calories is to restrict them too much. According to Fielding, over-restrictive dieting will eventually rob your body of muscle mass, wreaking havoc on both metabolic rate and sports performance.

     To estimate the number of calories you should be eating, see "How Many Calories Do I Need?" on page 31.

     Taking in adequate amounts of protein can also help. Eating lean meats, legumes or other protein helps you stay trim in two ways. Protein increases satiety—that feeling of being satisfactorily full. It also affects the total calories you burn each day. Protein uses more calories in its digestion, absorption and storage than either carbohydrate or fat. But don't overdo it. Too much protein can do more harm than good. Most women should be getting no more than 10 to 35 percent of their calories from protein, with 25 percent being a safe recommendation for many active women (for a more accurate recommendation consult a sports nutritionist).

     The overall effect of diet-induced thermogenesis from all food accounts for 5 to 15 percent of the total calories you burn each day depending on your diet and activity.

     As always, it's about balancing calories taken in with calories burned. So keep watching your calorie needs as you age and your activity level changes.

Jill Weisenberger, M.S., R.D., is a registered dietitian and diabetes educator for Hampton Roads Center for Clinical Research in Norfolk, Va.