Washington ? Just because your doctor tells you to drink eight glasses of water daily doesn’t mean you should, according to researchers at the Indiana University School of Medicine.

Doctors often fall for the same health myths that their patients do, Drs. Rachel Vreeman and Aaron Carroll report in the Christmas-New Year’s issue of the British Medical Journal. Among seven myths they cite is the eight-glasses-of-water one.

“There is no medical evidence to suggest that you need that much water,” Vreeman concluded after their intensive review of medical research on the subject.

She and Carroll trace the misperception to a 1945 recommendation by the Nutrition Council that Americans consume the equivalent of eight glasses of fluids daily. Lost over the years, they concluded, was the council’s note that the 64 ounces called for included water contained in coffee, soda, fruits and vegetables.

Based on informal polls of their colleagues at the Indianapolis-based medical school, other widespread but unjustified convictions included:

¢ People use only 10 percent of their brains. Lots of evidence refutes that common belief, they say. Most compelling is the finding that damage to almost any area of the brain has powerful effects on thinking, bodily functioning and behavior.

¢ Hair and fingernails keep growing after death. According to forensic anthropologist William Maples, whom the researchers cite: “It’s a powerful, disturbing image, but it is pure moonshine.” What does happen, Maples found, is that dehydration after death sometimes causes skin around the hair or nails to shrink.

Vreeman and Carroll also take on claims, endorsed by some doctors they quizzed, that reading by dim light ruins eyesight; eating turkey makes you sleepy; shaved hair grows back faster, darker or coarser; and cell phones seriously disrupt with hospital electronics.

In each case, Carroll said, “we’re not saying that it’s a lie, but that at best nobody knows. At worst, it’s not true.”

In the nobody-really-knows category, they said, are the effects of dim light on vision and the extent to which cell phones might interfere with medical devices.

The pair’s debunking research got started, Carroll said, after he heard a doctor caution in a pre-Halloween radio interview against strangers poisoning kids with candy.

When he mentioned the incident to Vreeman, she was unaware that death-by-poisoned-Halloween-candy was a myth. But she knew that some of Carroll’s health-related beliefs were myths, too.

From raising the matter with other doctors, they found that everyone “believed at least one thing was true that we knew from our list (of seven beliefs above) was untrue or unproven.”

Certainly, some doctors know that all seven are false, Vreeman noted. “Our purpose was simply to remind physicians that to stay current with medical knowledge means not just adding new information, but looking into your head and re-examining what you already believe.”

The pair, who work in children’s health services research at the university, hope to expand their work into a book that examines 100 common health myths. Here are two they’ll bust:

¢ Yes, chewing gum is indigestible. But like everything else, it doesn’t get stuck in the digestive tract.

¢ If kids are hyperactive, it’s not the sugar that’s doing it.