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

DEAR DR. DONOHUE: I carry the diagnosis of congestive heart failure. Medicines have made me feel pretty good. My wife thinks that I should do nothing. If I do much more than sit, she is all over me. A little bit of activity isn’t dangerous, is it? If I don’t do something, I am going to turn into a blob of fat. — R.K.

ANSWER: You’re in a boat with 5 million other Americans who have chronic heart failure. Heart failure means the heart doesn’t pump enough blood with each beat to supply the body with oxygen. Shortness of breath on slight exertion is a principal sign.

Rest used to be the rule for heart-failure patients. Too much rest, however, deconditions the body and makes it even more difficult for a heart-failure patient to do things that are part of everyday life, like walking and a few household chores.

You have to ask your doctor what limits you should be bound by. If there’s a supervised exercise program for heart-failure patients in your area, join it. Many hospitals sponsor such programs. I encourage you to become active. You’ll find regular exercise permits you to do more than you believed you could do.

DEAR DR. DONOHUE: I’m 61, and I don’t take medicines. I feel good. I have been exercising for two years. I would like to increase my activity. What’s considered a safe way to do so? — L.P.

ANSWER: Follow the 10 percent rule. It’s safe to increase exercise by 10 percent each week. Increase means increasing exercise speed, exercise duration, the number of repetitions you lift a weight or the number of pounds you lift. Don’t increase all aspects. Take one at a time. If you jog, increase either the distance or the time by 10 percent. One week make it distance; the next, speed.

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.