DEAR DR. DONOHUE: I have serious doubts about judging weight and health. Everyone talks about the obesity epidemic, but no one speaks for those of us who are overweight by accepted standards but are still in good shape. I am 33. Since grade school, I have exercised hard — and still do. I lift weights and run long distances and do both regularly. I am 5 feet, 8 inches tall and weigh 185 pounds. My BMI is 28, which puts me in the “overweight” category. I am quite muscular, and my waist is 31 inches.

I have the same build as my father and grandfather. My grandfather is 79 and still works his 80-acre farm. What do you say about this? — D.R.

ANSWER: I agree with you. The Mayo Clinic did a study comparing overweight but highly fit people to people with normal weight but low fitness. The overweight but highly fit had a lower risk of dying.

If overweight people have normal cholesterol, normal blood pressure and normal blood sugar, their overall health isn’t impacted by their weight. They are, however, in the distinct minority.

As for BMI, body mass index, it’s a better criterion of body composition (how much of the body weight is fat and how much is muscle and bone) than is weight obtained by stepping on a scale, but it’s not perfect. Heavily muscled individuals come out as being overweight when their weight has no bearing on health or longevity. If people want to calculate their BMI, divide weight in pounds by height in inches squared, and multiply that number by 703. Those proficient with metric measurements, divide weight in kilograms by height in meters squared. Normal values lie between 18.5 and 24.9.

Waist measurement is another reliable criterion of health. For men, the waist should be less than 40 inches (102 cm), and for women less than 35 inches (89 cm). Abdominal fat has a greater negative impact on health than fat in other places.

DEAR DR. DONOHUE: Please explain interval training to me. Several people have told me about it, but I was confused by what they said. They’re in physical education programs and seem to know what they’re talking about. — R.G.

ANSWER: The essence of interval training is to combine short periods of intense exercise with equal periods of reduced-intensity exercise. It applies to all aerobic exercise: swimming, running, biking and skating.

Take running as an example. On an interval program, the runner speeds up for a short time and then slows down for an equal amount of time. This gives the runner a chance to vary the running program and to incorporate very demanding exercise into it. The runner runs all-out for one minute, then slows down to a jog for one minute.

The intervals can vary according to one’s abilities. They can be as short as 10 seconds or as long as four minutes. Don’t start out by trying four minutes. That’s far too demanding. You have to get used to this program.

Interval training helps the body get rid of lactic acid generated by high-intensity exercise. Lactic acid buildup leads to muscle fatigue.

Older people ought not to engage in this without their doctor’s OK.

DEAR DR. DONOHUE: I started training for a marathon last fall. It will be held in late spring. Have you any tips on when I should cut back on my running? How long before the actual race? — J.J.

ANSWER: I don’t have any personal experience. I never wanted to run a marathon.

An expert marathoner suggests reducing the length and time of running about four weeks before a race. The week before, run only 40 percent of your best distance and time, and quit running at least three or four days before the marathon.

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