DEAR DR. DONOHUE: I am 54 years old, weigh 140 pounds and am 5 feet 10 inches tall. Since college I have, daily, spent an hour swimming laps.

Recently I was diagnosed with osteopenia. My doctor told me I am “totally wasting time by swimming.” She said I should start jogging and lifting weights. I love swimming. What should I do? – S.N.

ANSWER: You should continue to swim. Swimming is excellent exercise. It’s great for the heart. It keeps blood pressure down. It controls weight. (Have you ever seen an obese long-distance swimmer?) It builds muscles. Many swimmers have gigantic shoulders. But it’s not the greatest exercise for osteoporosis.

You don’t have osteoporosis. You have osteopenia, a step on the road to osteoporosis. So you do want to get in some bone-strengthening exercise. Stressing bones makes them stronger and makes them fill with calcium. Weight-bearing exercise is the exercise that does that. Walking is weight-bearing. Bones are propping up the body’s weight in the face of gravity, which wants to draw the body down to the ground. In swimming, the force of gravity is not as great, so bones are not stressed as much as they are on land.

Walking is good. Brisk walking and jogging are better. The forces on bone are twice what they are during walking. Best is weightlifting with barbells, dumbbells and weight machines. Weightlifting is the ultimate weight-bearing exercise.

Even older, fragile people can lift weights. They can start out with light weights, and they can start out doing so in a chair. Simply lifting a soup can over the head counts as weightlifting. A soup can weighs about 1 pound and 3 ounces – at least the one I looked at does. A series of overhead lifts with a soup can is a good way to get more calcium into the arms and shoulders. As for the legs, weight-bearing leg exercises can be done while seated in a chair too. Ankle weights are available at all sports stores. Attaching light ankle weights to the lower legs can stress the leg bones and strengthen them. Just move the legs straight out. You are not in the fragile, elderly category; you can do more.

A word to high-school students is worth some newspaper space. The high-school years are the years when bones are avid for calcium and are the most responsive to weight-bearing exercise. Don’t let this opportunity pass you by.