DEAR DR. DONOHUE: What am I supposed to drink when exercising in hot weather? I am totally confused by the conflicting information I get. Some say water is all you need. Then I’m told that drinking only water will make your brain flat-line. If water is all right, can it be cold water? They tell me cold water makes your stomach cramp. Should I be taking salt tablets? — L.O.

ANSWER: Lots of confusing information circulates on how best to stay hydrated in hot weather. Everyone agrees you have to take fluids. Where disagreement comes in is deciding how much to drink and what kind of fluid to drink. If a person loses 2 pounds of body weight from exercising in the heat, that signals serious dehydration and can have serious consequences.

Exercise in hot weather promotes sweating in order to keep the body cool. With only moderately intense exercise, people can lose as much as a liter (a little more than 1 quart) of fluid in one hour. More strenuous exercise can bring a loss of 3 liters in one hour

You have to make up those fluid deficits. Water is fine if exercise lasts one or two hours. Cold water is quite acceptable if it’s your preference. It does not cause cramping. During exercise, replace fluid losses every 15 minutes if possible. Don’t gulp an entire liter. One 8-ounce glass every 15 minutes is appropriate.

Extended exercise lasting more than three or four hours requires rehydrating with fluids containing some salt. Drinking only water for prolonged exercise can drop blood sodium to low levels. A profound drop in sodium brings on a headache, vomiting, swollen hands and feet, and mental confusion. The brain is affected. I have never heard of a death, but I have heard of people requiring hospitalization. This is a big issue for marathoners or people doing hard, physical labor.

Salt tablets are not recommended by most. They irritate the stomach.

DEAR DR. DONOHUE: I am not used to heat. I spend most of the time indoors with air conditioning. I volunteered to be a mentor to teenagers at a camp in the South. The time will be spent playing all sports, morning and afternoon. How long does it take to become acclimatized to the heat? I am not looking forward to this. — C.A.

ANSWER: Acclimatization to heat takes about two weeks. The body learns to sweat more quickly and to lose less salt in the sweat. Sweating is a way to cool the body.

The brain becomes more sensitized to the need for fluid. It sends out thirst signals earlier than it would without any training.

Can you get to the camp a week early? You can get yourself pretty well used to heat in that time. For the first two days, take things easy. Run and exercise early in the morning or late in the evening, when the temperature is lowest. You should be able to manage a half-mile run in those first two days. After that you can step up the pace, distance and time.

DEAR DR. DONOHUE: Do you think it’s right to involve young children in structured athletic training? My husband has our 4-year-old daughter practicing tennis for an hour every day. She doesn’t complain, but she never complains about anything. I believe my husband is using our daughter to fulfill his unfulfilled dreams of becoming a tennis pro. — H.B.

ANSWER: I don’t think it wise to put a child of that age into regimented physical training. Children that young are not equipped physically or mentally to devote themselves to a demanding program. She could injure herself, and she could develop a distaste for tennis.

When your daughter expresses for herself a desire to learn tennis and when she has grown older, then your husband can coach her — if she wants him to do so.

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.