DEAR DR. DONOHUE: I am confused about how much water to drink when exercising in hot weather.

I learned that you should drink all the time during exercise, even when you aren’t thirsty.

Now they tell me that drinking too much affects your brain and can cause death. What’s the story here? -- M.F.

ANSWER: This has to be one of those “it depends” answers.

How much water or any other fluid is needed in hot water depends on how hard is the exercise, how long you do it, how heavily you sweat, how hot it is and how acclimatized you are to heat.

It takes two weeks to acclimatize to heat. After two weeks, less sodium and potassium are lost in sweat.

Formerly, the advice for fluids was to drink frequently even if you didn’t feel thirsty. That’s overkill.

You can let thirst be your guide most of the time. Older people have a somewhat blunted thirst sense, so they might have to remind themselves to drink in exceptionally hot weather.

To stay hydrated during heavy physical activity or exercise, drink 12 to 16 ounces of fluid three to four hours before exercise.

The fluid can be plain water. It’s also a good idea to take a salty snack before exercise -- pretzels, peanuts or crackers.

During activity, drink about every 20 minutes, as much as your thirst tells you.

If your activity lasts longer than two or three hours and if you sweat heavily, then you have to pay attention to your salt intake.

Marathon runners taught us this. A few marathoners died from drinking only water during hot-weather races. Doing so lowers body sodium.

That’s hyponatremia, and it can be serious. Headache, vomiting, swollen ankles and feet, fatigue far out of the fatigue usually felt and disorientation are some of the signs of hyponatremia.

Sports drinks with sodium in them can prevent hyponatremia.

You can make your own replacement fluid by adding 1 tablespoon sugar, 1/8 teaspoon salt and 1 tablespoon orange juice to an almostfilled 8-ounce glass of water. You have to make enough to last for the whole exercise session.

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DEAR DR. DONOHUE: My right ear began to throb so bad that I had to see a doctor.

She said I have otitis externa, and gave me prescriptions for eardrops and pain medicine.

She didn’t say much to me about this. She was in a hurry to see other patients. Will you explain to me what otitis externa is? -- P.P.

ANSWER: It’s an infection of the ear canal, that small tunnel (about 1 inch long) that runs from the external ear to the eardrum. It’s the place where wax forms. Infections of the canal come about for a variety of reasons.

They always hurt, and they sometimes cause a foul drainage.

Be sure to use the eardrops -- which, I’m sure, are antibiotic drops -- for as long as the prescription says to. Don’t stop when the pain leaves, or the infection can return.

I know what you mean about the doctor being busy. Managed care has doctors on such tight schedules that the doctor-patient relationship is often lost.

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DEAR DR. DONOHUE:

Is it possible to get wet macular degeneration from a scratch on the left eye during cataract surgery, or could it have been a cyst that caused wet macular degeneration? I am 85, and my right eye is perfect. I had cataract surgery on that eye also. -- C.

ANSWER: A scratch on the eye isn’t likely to cause macular degeneration. The possibility of cataract surgery leading to macular degeneration is a remote one. More than 6,000 people who had had a cataract removed were followed for five years after the operation. Slightly more people who had the operation developed macular degeneration, dry or wet, in the operated eye than did a similar group of people who had not had an operation. This isn’t proof that cataract surgery leads to macular degeneration. The same risks that cause cataracts also cause macular degeneration. The numbers that do develop it after cataract removal are small. A cause-and-effect relationship has not been proven.

I am not clear what you mean by a cyst causing the degeneration. In what part of the eye was the cyst? I have not seen a link between cysts and macular degeneration.

The retina is the back layer of the eye, the layer that converts incoming images into nerve signals that can be transmitted to the brain so we can see. The macula is a small, round area of the retina where there’s an aggregation of cells that are essential for central vision -- the kind of vision needed to read a paper, watch TV and drive. Dry macular degeneration, accounting for 85 percent to 90 percent of cases, is a wasting away of macular cells. Wet macular degeneration results from a sprouting of blood vessels in that region. Those newly formed blood vessels leak fluid and destroy macular vision. Procedures are available that can halt the progression of wet macular degeneration.

The booklet on macular degeneration explains both kinds and what is available to help those with this common eye problem. Readers can obtain a copy by writing: Dr. Donohue -- No. 701W, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.75 U.S./$6 Canada. with the recipient’s printed name and address. Please allow four weeks for delivery.

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DEAR DR. DONOHUE: My son-in-law insists on lifting his small children by their arms. I am terrified that he will pull their arms out of joint. -- A Worried Grandpa

ANSWER: Lifting young children by grabbing onto their hands or arms can cause the radius (the larger of the two lower arm bones) to slip away from the ligament that keeps it in place. Children who are 5 and older aren’t in danger of this happening.