DEAR DR. DONOHUE: I’m a high-school senior, and I plan to major in some aspect of sports medicine in college. I’ve been reading about the different kinds of muscle fiber, and wonder if you’d discuss this. — J.K.

DEAR DR. DONOHUE: I have read that women over 65 have virtually no fast-twitch muscle fibers left. That’s the reason our metabolism slows. Is this true? Is there no hope of rebuilding them? Is this the reason why I cannot lose weight? — S.S.

ANSWER: These two letters focus on a topic that’s an unknown to most readers — fast-twitch and slow-twitch muscle fibers.

Slow-twitch muscle fibers contract more slowly and are dependent on oxygen for generating fuel for power production. They’re the kind of muscle fibers that marathoners have in abundance. They resist fatigue and keep people moving for long periods of time.

Fast-twitch muscle fibers are the opposite. They generate power for brief intervals. They also tire quickly. Sprinters are the prototypes of people endowed with lots of fast-twitch muscles.

The only direct way to know if an individual has more of one fiber than the other is through a muscle biopsy. Not many are eager to have that done. People instinctively gravitate to endeavors that favor their kind of muscle fiber. Training doesn’t convert one type into another. Genes determine which is the predominant type.

S.S., both types of muscle fiber shrink with age. Fast-twitch fibers suffer a greater depletion. Exercise can slow the loss of both. You are going down the wrong road when you equate body metabolism only with muscle fiber type. Muscles make a contribution to metabolism, but they are only part of the picture. Hormones, genetics and other elements of body composition are involved with metabolism — the production and utilization of body energy and body chemistry. Exercising, both weightlifting and aerobics (jogging, biking, swimming), rev up body metabolism and make a contribution to calorie burning and weight loss.

DEAR DR. DONOHUE: Back in the late 1980s, I was into bodybuilding and even won some competitions. Over the years, I let myself go and am out of shape.

Last year I started exercising again and have lost 70 pounds. I do pretty much the same routine I did in the ’80s. I feel great and am toning up, but I don’t seem to be getting any bigger. Is it possible for me to get back to the same size I was 25 years ago? I am 52 years old now. — K.Y.

ANSWER: Fifty-two isn’t old age. It isn’t your 20s, either — the peak of testosterone production and the best time for muscle growth. So you have to be realistic about this.

You most likely won’t attain the same muscle growth you had 25 years ago, but you can achieve muscle enlargement through dedication to training and to what you eat. Don’t even think about taking the steroid (male hormone) route.

DEAR DR. DONOHUE: I am a 62-year-old male and exercise regularly. I had a stress test during an asthma episode, and the doctor said my maximum heart rate should be 130. My family doctor, on the other hand, says whatever feels right is OK to do. I run 10 to 12 miles a week and lift weights for two and a half hours a week. While running on the treadmill, my heart rate reaches 147 without my feeling any different. How should I proceed with my exercise and heart rate? — R.V.

ANSWER: It seems to me that a stress test done during an asthma attack doesn’t yield valid results. I’d ask the heart doctor for a repeat.

I like your family doctor’s approach. But I’m reluctant to tell you to ignore the heart doctor’s advice. I’d have to see the stress test.

Why not play things safe and get another stress test? At least talk to the heart doctor about this.

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.

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