DEAR DR. DONOHUE: Please clarify the conflicting advice on how much a person has to exercise for health. I am pretty busy, but I could get in 30 minutes of exercise a day without trouble. Now I read that it takes 60 minutes of exercise. Anything less wastes your time. Can you straighten this out for me? – C.K.

ANSWER: Information on what constitutes enough exercise time is confusing and often conflicting. What makes it worse is that the information comes from respected sources, so whom can you believe?

The American College of Sports Medicine and the Surgeon General suggest that people engage in 30 minutes a day of moderate intensity exercise on most, if not all, days of the week. “Moderate intensity” is defined as exercise that permits people to carry on a conversation with a little bit of effort. Moderate-intensity exercise doesn’t wind people and leave them gasping for air, but it does present somewhat of a challenge to talk.

The latest exercise prescription comes from the National Academy of Sciences. This is the source that advises an hour of daily exercise. A closer reading of what was written gives plenty of leeway in obtaining the hour of exercise. Half of the hour should be exercise of moderate intensity as defined above. The other half-hour is designated as leisure activity — exercise done at a reduced clip. All daily activity counts: climbing stairs, housecleaning and gardening, for instance. The hour of exercise is primarily directed at people who are exercising to lose weight and to keep it off.

The hour of exercise can be broken into 10-minute sessions, so you don’t have to devote a full hour to exercise without any interruption. The same goes for the 30-minute exercise sessions. They can be broken into 10-minute sessions too.

If you haven’t been exercising, check with your doctor before you begin any exercise program.

The newly written pamphlet on aerobics and fitness gives novice exercisers a guide to a sensible program. Readers can obtain a copy by writing: Dr. Donohue – No. 1301, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.50 U.S./$6.50 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

DEAR DR. DONOHUE: I am on the low-carbohydrate diet, and it’s working. I have lost 10 pounds in one month. However, I am on a competitive rowing team, and I don’t have the energy I used to have. Could this be my imagination, or could the diet be draining my energy? – H.P.

ANSWER: Carbohydrates – sugars and starches – are stored in every muscle as glycogen. Glycogen generates energy for intense and usually short bursts of activity. Weightlifting is an example. Rowing is a more protracted form of intense activity, but it depends on a good supply of muscle glycogen, too.

Some exercise experts have found that the low-carbohydrate diet doesn’t provide enough carbohydrates to replenish muscle glycogen.

A low-carbohydrate diet could well be the reason that you feel wiped out performing the kind of exercise you could previously do without exhaustion.

Care to experiment? Go off your diet for a week and see if your energy comes back. Let me know the results. I am sure others would like a proven answer to your question.

DEAR DR. DONOHUE: I am 85 years old. I injured my left thigh muscle in 1988. Lately it has been bugging me really bad. I ride my stationary bike an hour and a half five days a week. I also exercise my legs with weights. I hope you have some advice for me. – J.G.

ANSWER: I am in awe of your program.

I find it hard to implicate an injury that occurred 15 years ago as being the reason why your thigh muscle hurts now. The only way you are going to get to the root of the problem is to pay the family doctor a visit.

DEAR DR. DONOHUE: I am writing because for many years a question in my family has gone unanswered. Does the use of rubbing alcohol on the legs cause blood clots? – J.V.

ANSWER: I can’t believe it does, and I have no reason to explain why it might do such a thing.

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.