DEAR DR. DONOHUE: I am a swimmer, and I swim every day. I do it to keep fit, but I also happen to love swimming. I have read that swimming is not a good exercise for prevention of osteoporosis. Why? What exercises are? – G.R.

ANSWER: Swimming is an excellent exercise. I have never seen an overweight swimmer. Not only does it keep a person from gaining weight, but it strengthens the heart and lowers blood pressure.

It is not the greatest exercise for osteoporosis. The reason is that water buoyancy neutralizes the pull of gravity on bones. Land exercises, where the body must fight gravity, are better osteoporosis exercises.

Astronauts face a big problem with osteoporosis when they are on lengthy missions. Theirs is a gravity-free environment, and they must take measures to reduce their loss of bone strength while they orbit the Earth.

The best exercise for osteoporosis prevention is weightlifting. I know it sounds preposterous to tell an 80-year-old to pump iron, but no matter what the age, weightlifting strengthens bones (and muscles).

A sacred tenet of weightlifting is overload. Overload is a constant increase in the amount of weight lifted so bones and muscles are constantly challenged. The response to such challenges is stronger bones and muscles.

It’s perfectly fine to start a weightlifting program without weights. Take advantage of body weight alone. For example, when sitting in a chair, raise the legs so they are parallel to the ground and hold them in the raised position for 10 or more seconds. Eight consecutive lifts constitute a set. Take a short break after completion of a set, then repeat two more sets of the exercise.

You can overload by wearing ankle weights when performing the above exercise. Start with only 1 pound. The goal is to reach three sets of 12 lifts. When you reach that goal, add another pound of weight.

I could detail more exercises, but this gives you the idea of how to go about an osteoporosis prevention program.

DEAR DR. DONOHUE: My sister-in-law and I have motorized treadmills in our homes. She tells me I should try backward running on it. She says it’s a better exercise than forward running. Is it? – K.K.

ANSWER: Backward running burns more calories than forward running. It takes about one-third more effort to run backward. Your sister-in-law is right.

Backward running exercises different leg muscles, so it strengthens muscles not used in forward running — another point in its favor.

Furthermore, it hones the body’s sense of balance.

When starting this sort of exercise, do so at slow speeds. It takes a while to get the hang of it.

I must issue a cautionary note. There is a risk of falling when people begin running backward. For that reason, have a friend on hand to steady you should you begin to wobble. I don’t want anyone to break a bone by retro-running. In addition, because of the possibility of falling, you shouldn’t do this on a treadmill. Do it on ordinary ground.

DEAR DR. DONOHUE: Everyone tells us to eat fewer calories if we want to lose weight. No one tells us how many fewer calories. Can you give us an idea? – W.R.

ANSWER: First, figure out how many calories it takes to keep your body at its present weight. You can do that by multiplying your weight in pounds by 10 (in kilograms by 22). That figure is the number of calories you must provide your body to keep it at its current weight. This number represents what is called the resting metabolic rate — the calories burned from a day and night of inactivity.

If a person is moderately active, then multiply the resting metabolic rate by 25 percent and add that number to the resting metabolic rate to obtain the daily calorie need.

If a person is extremely active and engaged in hard physical activities, that person multiplies the resting metabolic rate by 50 percent and adds it to the resting metabolic rate to get the total daily calorie requirement.

To lose one pound a week, decrease your calculated calorie needs by 500 calories.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.