DEAR DR. DONOHUE: I am a 32-year-old female, and I exercise on my stationary bike three times a week. I have read that a person burns more fat at a less-than-all-out effort. When I slow down my pedaling, I barely feel that I have exercised. I have also read that a person burns the most calories if he or she pedals at high speeds. Which is true? – I.W.

ANSWER: It is true that low-intensity exercise derives more of its energy from fat than from stored carbohydrates.

For example, when walking a mile in 20 minutes or slower, the energy leg muscles obtain comes mostly from the combustion of fat and oxygen.

A sprinter, however, provides energy for rapidly moving legs mostly from carbohydrates in stored muscle sugar (glycogen).

The inescapable deduction is, therefore, to exercise more leisurely and burn more fat.

However, that is not true. To calculate how much fat is burned, you have to calculate the total number of calories burned.

Say a person is burning 200 calories from exercise at a less-than-all-out effort. Fat provides 60 percent of the calories and carbohydrates 40 percent. The number of fat calories this person burns is 120.

If that person ratchets up the intensity of exercise so a total of 400 calories are burned, the proportion of fat and carbohydrates burned changes. Fat then provides only 40 percent of the total calories, and carbohydrates provide 60 percent. But the total number of fat calories burned is 160 – 40 more than burned during the less-intense exercise.

Furthermore, the more-strenuous exercise is giving the heart a better workout than the less-strenuous form.

DEAR DR. DONOHUE: I have begun a running program, and I want to do it sensibly. I run 1 mile now. I want to run more, but I would like your opinion of how much more is considered safe. If I stay at the 1-mile mark, I know I won’t make much progress. – R.S.

ANSWER: Yours is a most sensible question.

It’s safe to increase running distance by 10 percent each week. Next week you can run 1.1 miles. The week after, go for 1.2 miles.

I wanted to calculate how long it would take you to reach marathon mileage, but it gave me a headache.

DEAR DR. DONOHUE: My wife and I are avid cyclists. She likes to put the pedal to the metal, so to speak. She clips along at 17 to 18 miles an hour over an 18-mile course. I cover the course at a slower pace of 15 to 16 miles an hour. I cannot comfortably ride faster without becoming breathless and winded. My wife is of the opinion that she has to exhaust herself to get a good workout. I believe that if I am having a good time, what’s the point of torturing myself? Can you comment? – R.K.

ANSWER: I agree with you. Exercise does not have to be a torment to get health benefits from it. Overdoing often brings with it injury, burnout and muscle breakdown.

The runner’s rule applies to bikers. A person ought to be able to carry on a conversation while running or biking. If the effort is of such magnitude that it is impossible to talk, the effort is too much.

DEAR DR. DONOHUE: I am a 71-year-old female with no health problems. I stick to a regimen of walking 2.5 to 3 miles a day, and I can do 2 miles in about 30 to 40 minutes.

My son tells me that, unless I get my heart rate up, my walking does not do much good.

Do you agree? I am a bit old for racing. – R.S.

ANSWER: You are doing yourself a world of good. You don’t have to walk any faster or farther or for more time than you are now doing.

The preoccupation with heart rates can sometimes get out of hand. Your body is a good barometer of how fast you should walk.

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.