Popular culture is loaded with myths and half-truths. Most are harmless. But when doctors start believing medical myths, perhaps it’s time to worry.

In the British Medical Journal this week, researchers looked into several common misconceptions, from the belief that a person should drink eight glasses of water per day to the notion that reading in low light ruins your eyesight.

“We got fired up about this because we knew that physicians accepted these beliefs and were passing this information along to their patients,” said Dr. Aaron Carroll, assistant professor of pediatrics at the Indiana University School of Medicine. “And these beliefs are frequently cited in the popular media.”

And so here they are, so that you can inform your doctor:

Myth: We use only 10 percent of our brains.

Fact: Physicians and comedians alike, including Jerry Seinfeld, love to cite this one. It’s sometimes erroneously credited to Albert Einstein. But MRI scans, PET scans and other imaging studies show no dormant areas of the brain, and even viewing individual neurons or cells reveals no inactive areas, the new paper points out. Metabolic studies of how brain cells process chemicals show no nonfunctioning areas. The myth probably originated with self-improvement hucksters in the early 1900s who wanted to convince people that they had yet not reached their full potential, Carroll figures. It also doesn’t jibe with the fact that our other organs run at full tilt.

Myth: You should drink at least eight glasses of water a day.

Fact: “There is no medical evidence to suggest that you need that much water,” said Dr. Rachel Vreeman, a pediatrics research fellow at the university and co-author of the journal article. Vreeman thinks this myth can be traced back to a 1945 recommendation from the Nutrition Council that a person consume the equivalent of 8 glasses (64 ounces) of fluid a day. Over the years, “fluid” turned to water. But fruits and vegetables, plus coffee and other liquids, count.

Myth: Fingernails and hair grow after death.

Fact: Most physicians queried on this one initially thought it was true. Upon further reflection, they realized it’s impossible. Here’s what happens: “As the body’s skin is drying out, soft tissue, especially skin, is retracting,” Vreeman said. “The nails appear much more prominent as the skin dries out. The same is true, but less obvious, with hair. As the skin is shrinking back, the hair looks more prominent or sticks up a bit.”

Myth: Shaved hair grows back faster, coarser and darker.

Fact: A 1928 clinical trial compared hair growth in shaved patches to growth in non-shaved patches. The hair which replaced the shaved hair was no darker or thicker, and did not grow in faster. More recent studies have confirmed that one. Here’s the deal: When hair first comes in after being shaved, it grows with a blunt edge on top, Carroll and Vreeman explain. Over time, the blunt edge gets worn so it may seem thicker than it actually is. Hair that’s just emerging can be darker too, because it hasn’t been bleached by the sun.

Myth: Reading in dim light ruins your eyesight.

Fact: The researchers found no evidence that reading in dim light causes permanent eye damage. It can cause eye strain and temporarily decreased acuity, which subsides after rest.

Myth: Eating turkey makes you drowsy.

Fact: Even Carroll and Vreeman believed this one until they researched it. The thing is, a chemical in turkey called tryptophan is known to cause drowsiness. But turkey doesn’t contain any more of it than does chicken or beef. This myth is fueled by the fact that turkey is often eaten with a colossal holiday meal, often accompanied by alcohol — both things that will make you sleepy.

Myth: Mobile phones are dangerous in hospitals.

Fact: There are no known cases of death related to this one. Cases of less-serious interference with hospital devices seem to be largely anecdotal, the researchers found. In one real study, mobile phones were found to interfere with 4 percent of devices, but only when the phone was within 3 feet of the device. A more recent study, this year, found no interference in 300 tests in 75 treatment rooms. To the contrary, when doctors use mobile phones, the improved communication means they make fewer mistakes.

“Whenever we talk about this work, doctors at first express disbelief that these things are not true,” said Vreeman said. “But after we carefully lay out medical evidence, they are very willing to accept that these beliefs are actually false.”

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