Feb. 28, 2000 (Denver) -- From middle age to at least age 70, this is true for most people. There are a number of reasons for this, one being lifestyle. In the United States, we tend to become more sedentary as we get older. Jobs tend to require less physical activity as people move up the corporate ladder, so that fewer calories are expended during the workday. Also, once kids grow up and move out of the house, the demands of day-to-day activity are fewer and eating out becomes more common. It's easy to slip into a less-active lifestyle, in which fewer calories are burned and which, without cutting back on caloric intake, leads to weight gain.

Around age 50, we begin to lose muscle mass, causing a decline in the resting metabolic rate -- yet another setup for weight gain. To counteract this decline in energy expenditure and not gain weight, you will either have to eat less or be more physically active. Also, as you get older, you have to exercise longer to burn the same number of calories as when you were younger. This is because as you age, there is a decline in your exercise capacity, even if you are in fairly good shape. At 25, for example, you might have been able to burn 14 calories a minute while running; but at age 65, you might be able to burn only eight calories a minute

Even though the rate at which you burn calories decreases with age, the number of calories you have to burn to lose a pound of fat -- about 4,000 -- stays the same. So, as you get older, you have to exercise longer to lose weight.

You might not be able to avoid weight gain completely, but you can minimize it. Make it a point to fit in more activity during your day, whether you are still working or retired. Take the stairs instead of the elevator. Start exercising, being sure to check first with your physician, especially if you have not exercised in many years. You can prevent the loss of muscle mass by doing weight training if your doctor approves it. Your goal should be to maintain, as closely as possible, the recommended weight for your height. Even if health problems prevent you from doing certain types of exercise, remember: Any exercise is better than none at all.

Wendy Kohrt, PhD, is an exercise physiologist and professor of medicine in the division of geriatric medicine at the University of Colorado Health Sciences Center, Denver.

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