DEAR DR. DONOHUE: In the past I used to do aerobic exercise, which caused me to perspire a lot. I knew I was working hard. Now I walk briskly, but I no longer get my heart rate up very high. I don’t seem to be in the “zone.”

Since this change, I do not look or feel any different. Am I exercising at a lower fitness level because I no longer perspire? – C.A.

ANSWER: Your question is asked in many ways by many readers. The essence of it is: How intense should exercise be, and how is that intensity measured?

I can’t argue that intense exercise promotes sweating. However, sweating isn’t the best way to gauge how strenuous exercise is. Not all people sweat to the same degree from the same amount of exercise. Other factors influence it – hot weather, a heated room, humidity and on and on.

Heart rate is a better gauge of exercise intensity. You used the word “zone,” so I’ll use it, too. The zone is the heart-rate range that provides the most benefit for heart health. You first determine your maximum heart rate by subtracting your age from 220. Then, to obtain your training zone, take 60 percent to 85 percent of that number.

For a 40-year-old, the maximum heart rate is 180. The lower limit of that person’s training zone is 180 times 60 percent – 106. A 40-year-old’s exercise should raise the heart rate to at least 106 beats a minute. The upper limit is 180 times 85 percent – 153. The fastest a 40-year-old’s heart should beat is 153. Older people should consult their doctors about the safety of exercising at this heart rate.

If swimming is a person’s aerobic exercise, deduct 13 from these target heart rates.

If all this arithmetic is a bit much, then you can rely on “perceived exertion” to be your guide. You judge the intensity of exercise by how it feels to you. The scale runs from “very light” to “very hard” exercise. A person should exercise at a pace that feels “somewhat hard.” That’s exercise that doesn’t make you so breathless that you can’t carry on a conversation.

DEAR DR. DONOHUE: I am 74 and would like to exercise on my small trampoline, which is 1 foot high. How does a trampoline affect my knees? – L.M.

ANSWER: It should affect them very little. When you land on the trampoline, it gives way, so your knees don’t get the kind of shock they would if you were landing on the ground. You’re the best judge of how it affects your knees. If they hurt, stop. If they don’t, continue.

DEAR DR. DONOHUE: My stomach is flabby, really flabby. What can I do? – T.S.

DEAR DR. DONOHUE: I am a 61-year-old woman, 5 feet 5 inches tall, weighing 125 pounds. I have a nice shape. But since I turned 50, I’ve struggled to retain a flat stomach, and hate my little potbelly. I exercise daily, including doing sit-ups, but I still can’t get rid of it. I notice that 99 percent of women over 50 have extra belly weight. Is this a hormonal thing? Should I stop fighting it, like wrinkles? Do I need to cut out my three glasses of chardonnay a week? – B.A.

ANSWER: B.A., you can start looking at men over 50, too. They have the same problem. Fat seems to seek out the abdomen at older ages.

Both of you lie on your backs on the floor. Did your stomachs go away? I’ll bet they did. Your stomachs flattened because your lower backs flattened.

A great part of a protruding abdomen is a pronounced inner curve of the lower back. If you make a conscious effort to flatten the lower back a bit, your stomachs will flatten. Stand with your backs against a wall and try to straighten the inward curve of your lower backs. Bring the back toward the wall. Maintaining that posture keeps the stomach in. This isn’t the only thing you have to do. You have to exercise your abdominal muscles, and you have to watch your calories. You don’t have to give up your three glasses of wine, B.A.

DEAR DR. DONOHUE: I take hydrochlorothiazide for my blood pressure. It has brought it down without giving me any side effects. However, I have learned that it removes potassium from the body. My doctor hasn’t said a word about this to me. Should I be taking potassium, or should I have mine checked? – L.B.

ANSWER: How much hydrochlorothiazide (HydroDIURIL) do you take every day? In the not-so-distant past, the commonly prescribed dose was 25 mg to 50 mg a day. At those doses, potassium loss was somewhat common. Nowadays, the standard dose is 12.5 mg a day. At that dose, potassium balance rarely is disturbed.

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.