Many people believe they are supposed to drink eight glasses of water a day, or about two liters. Why? Because that is what they have been told all their life. But a new report offers some different advice. Experts say people should obey their bodies; they should drink as much water as they feel like drinking.

The report says most healthy people meet their daily needs for liquid by letting thirst be their guide. The report is from the Institute of Medicine, part of the National Academies. This organization provides scientific and technical advice to the government and the public.

The report contains some general suggestions. The experts say women should get about two-point-seven liters of water daily. Men should get about three-point-seven liters. But wait -- in each case, that is more than eight glasses.

There is an important difference. The report does not tell people how many glasses of water to drink. In fact, the experts say it may be impossible to know how many glasses are needed to meet these guidelines. This is because the daily water requirement can include the water content in foods.

People do not get water only by forcing themselves to drink a set number of glasses per day. People also drink fruit juices and sodas and milk. They drink coffee and tea. These all contain water. Yet some also contain caffeine. This causes the body to expel more water. But the writers of the report say this does not mean the body loses too much water.

As you might expect, the Institute of Medicine says people need to drink more water when they are physically active. The same is true of those who live in hot climates. Depending on heat and activity, people could need two times as much water as others do.

All this, however, does not answer one question. No one seems sure why people have the idea that good health requires eight glasses of water daily.

It may have started with a misunderstanding. In nineteen-forty-five, the National Academy of Sciences published some guidelines. Its Food and Nutrition Board said a good amount of water for most adults was two-point-five liters daily. This was based on an average of one milliliter for each calorie of food eaten.

But that was only part of what the board said. It also said that most of this amount is contained in prepared foods.