DEAR DR. DONOHUE: I am a 36-year-old woman who has dieted all her life. I am serious about losing 20 pounds. I can do it if I stop eating, but I want to do it more sensibly so I can keep the weight off. I am 5 feet 4 inches tall and weigh 150 pounds. How do I calculate how many calories I should eat every day? – P.P.

ANSWER: There are many rules to calculate the optimum daily calorie intake. I’ll give you one that is more or less reasonable.

First figure your resting metabolic rate. That gives the number of calories the body uses when it’s at rest. To obtain that number, multiply body weight by 10. For you, the resting metabolic rate burns 1,500 calories a day.

The next step is to factor in the number of calories spent in physical activity. For a sedentary life, multiply the resting metabolic rate by 0.2; for a fairly active life, multiply by 0.3; for an active life, multiply by 0.4; for a very active life multiply by 0.5. The number is added to the number of calories that fuel the resting metabolic rate to yield the total calorie intake. This gives you the number of calories it takes to maintain your present weight.

Let me make you fairly active. Multiply the resting metabolic rate, 1,500, by 0.3. The result is 450. Add it to 1,500 to come up with a daily total of 1,950 – the number of calories it takes to keep you at your present weight.

To lose weight, deduct 500 calories a day from that number, and you should lose one pound of fat every week.

You must get a booklet that lists the calorie content of foods. They’re available everywhere, and they aren’t expensive.

You can skip all this by keeping a daily food diary for one week and adding up how many calories you consume each day. Average the week’s calorie total by dividing by seven. Then deduct 500 calories to arrive at what the daily calorie intake should be to lose one pound of fat a week.

To be successful, you must exercise. That will prevent a drop in the resting metabolic rate that always occurs when a person reduces calorie intake.

DEAR DR. DONOHUE: Are those plastic or rubber sweat suits good for losing weight? I would like to try one if they are safe. – R.K.

ANSWER: All that those suits do is encase a person in a practically airtight cocoon, which makes for drenching sweats. The lost weight is fluid weight. As soon as a person begins drinking again, the weight comes back. This isn’t a good way to lose weight.

Are they safe? They have a potential to dehydrate people and to raise body temperature. I wouldn’t use one.

DEAR DR. DONOHUE: I have a curiosity question. After exercising with weights, my arms are noticeably larger. In about an hour, they have shrunk to their usual size. Why does this happen, and is there any way I can get them to stay large? – Q.S.

ANSWER: More blood flows to exercising muscles. The engorged blood vessels increase the size of the muscle being exercised. When you stop, the engorged blood vessels lose the extra blood, and the muscle shrinks to its normal size.

There is no way you can keep muscles pumped up with blood.

You can permanently enlarge muscles by continuing your exercise program and by eating a diet that provides you with enough energy and protein.

DEAR DR. DONOHUE: Most of my friends are using a supplement to give them bigger muscles. I would appreciate what you have to say about creatine and HMB. – B.D.

ANSWER: Let me add glutamine to your list. All three – creatine, HMB (hydroxymethylbutyrate) and glutamine – are related to amino acids, the basic units of protein. Creatine is the one most studied. Some studies show it improves muscle strength and performance, while others show it doesn’t. Not as much work has been done with the other two. They are not banned substances. They don’t seem to harm. They’re not wonder workers.

DEAR DR. DONOHUE: I have been having an affair with a guy for the past five years. He is 49. Every time we are intimate, his penis bleeds. I ask him what causes it, and he barely comments. He says the skin of his penis is too tight. Doctor, I am really worried, because we sometimes have unprotected sex. Please explain this to me. – L.O.

ANSWER: Your friend might have a foreskin that cannot retract. Then, sexual relations can be painful, and the foreskin might bleed. This is a remediable problem.

If your worry is cancer and whether it could be transmitted to you, stop worrying. You are not going to catch cancer from him.

If you are worried about a sexually transmitted infection, bleeding is not a common sign of those infections.

This man needs to consult a doctor. An answer will come only through an in-person examination.

Many times the ejaculate can contain a small amount of blood, and that is usually harmless. It comes from bleeding of a small, surface blood vessel in the genital tract. Although this is almost always harmless, it too calls for a visit to the doctor for reassurance.

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.