DEAR DR. DONOHUE: I am a woman in her mid-60s who’s been told that she’s on the verge of osteoporosis. My doctor told me to take calcium, vitamin D and do weight-bearing exercise. I joined a gym. It has a machine that transfers to the body vibrations that are supposed to help osteoporosis. What do you think of this? And what does “weight-bearing” mean? — H.A.

ANSWER: Vibration machines are somewhat new to the sports world. These machines have a platform that oscillates. You stand on the platform. Motion of the platform sends vibrations through the body that are said to stimulate bone formation and strengthen muscles. Seven studies have examined these claims, with conflicting results. A recent study performed in Canada questions the value of this kind of therapy for osteoporosis. More than that, I cannot tell you.

These machines are also used to rehabilitate muscles and tendons. A conclusive study of this claim has yet to be published.

“Weight-bearing” indicates that your bones, including backbones, are supporting body weight during an exercise. That puts stress on the bones, and they do grow stronger. Walking is a weight-bearing exercise. Swimming isn’t. The buoyancy of the water relieves bones of their job of propping up body weight.

Weightlifting is the ultimate weight-bearing exercise. Telling most women to lift weights usually provokes an unfavorable response. They think they’re going to develop huge muscles. They won’t, unless they have deliberately set about to build large muscle mass.

Weightlifting definitely strengthens bones. Your gym should have a staff of trained personnel who can devise a program for you and teach you the mechanics of proper lifting.

DEAR DR. DONOHUE: I recently started exercising at a local gym. I read that it’s a good idea to take a protein supplement after exercising. In stores, there are several different types of protein, like soy and whey. Which is better? When do I take it? — P.

ANSWER: If you take in .36 grams of protein for every pound of your body weight, you’re getting enough of it. For a 150-pounder, that would be 55 grams of protein a day. It’s not hard to get all the protein you need through food. Four ounces of chicken, beef and fish have around 30 grams of protein. Four ounces is a small serving. Half a cup of cooked beans has 8 grams. An egg has 7, and 2 tablespoons of peanut butter have 8 grams. Serious and devoted bodybuilders can double their daily protein intake.

If you want a supplement, that’s OK too. The only thing I would look for on the amino acid content of the supplement is the amino acid leucine. It’s said to enhance protein synthesis in the body.

Take a protein meal or protein supplement within an hour after exercising.

DEAR DR. DONOHUE: The bicycle exercise for abdominal muscles is great, and it’s easy to do. How many pedaling motions should I do a day? — M.B.

ANSWER: For other readers, the bicycle exercise is done by lying on the floor with hands at the sides of the head. Bend the knees to a 45 degree angle so the legs are raised off the ground. Start pedaling as you would when riding a bike.

You can either count the pedaling motions, or you can time yourself. For those starting out, 10 to 15 pedals at one time are enough. If you prefer timing the exercise, one full minute of pedaling is a good start. Rest and repeat the exercise. As you get the hang of it, you’ll find it easier to perform. Do as many pedal motions as is comfortable. The same advice goes for the time of pedaling if that’s your preferred method. Stop pedaling if it becomes painful.

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. Readers may also order health newsletters from www.rbmamall.com.