Medical Myths Even Doctors Believe

The researchers explored various myths including:

• People only use 10 percent of their brains
• Hair and fingernails continue to grow after we die
• Eating turkey makes you sleepy
• People should drink at least eight glasses of water a day
• Shaving hair cause it to grow back faster or coarser
• Reading in dim light ruins your eyesight

These beliefs are commonly accepted, not only by the general population, but also by many physicians. The authors' surprising findings, when they reviewed medical literature -- all the beliefs were unproven or untrue.

"We got fired up about this because we knew that physicians accepted these beliefs and were passing this information along to their patients. And these beliefs are frequently cited in the popular media. We didn't set out to become myth busters," said co-author Aaron Carroll, M.D., M.S., assistant professor of pediatrics and a Regenstrief Institute, Inc. affiliated scientist.

"Whenever we talk about this work, doctors at first express disbelief that these things are not true. But after we carefully lay out medical evidence, they are very willing to accept that these beliefs are actually false," said co-author Rachel Vreeman, M.D., a pediatrics research fellow.

The first belief they explored -- people should drink at least eight glasses of water a day. This advice has been promoted as healthful as well as a useful dieting or weight control strategy.

"When we examined this belief, we found that there is no medical evidence to suggest that you need that much water," said Dr. Vreeman. She 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. But an important part of the Council's recommendation has been lost over the years -- the large amount of fluid contained in food, especially fruits and vegetables, as well as in the coffee and soda people drink each day should be included in the recommended 64- ounce total. Drinking excess water can be dangerous, resulting in water intoxication and even death, the study authors note.

Dr. Vreeman and Dr. Carroll also explored the popular belief that we use only 10 percent of our brains. Frequently cited by everyone from physicians to comedians (Jerry Seinfeld) and erroneously credited to Albert Einstein, the authors found that there are a lot of ways to disprove this belief. 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 of the brain. Metabolic studies of how brain cells process chemicals show no nonfunctioning areas.

Dr. Carroll and Dr. Vreeman think this myth probably originated with self improvement experts in the early 1900s who wanted to convince people that they had yet not reached their full potential. With the help of these self proclaimed experts (perhaps the descendents of snake oil salesmen), one could tap into the 90 percent of the brain supposedly not being used.

Finger nails and hair grow after death. Most physicians the study authors queried initially assumed this belief was true. But when they thought about it, they knew it couldn't possibly be valid. This myth likely persists because of an optical illusion -- and because it's just so creepy.

"As the body's skin is drying out, soft tissue, especially skin, is retracting. 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," said Dr. Vreeman.

The two researchers also explored another frequently held belief related to hair -- that shaved hair grows back faster, coarser and darker. They found a 1928 randomized clinical trial which 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.

So why has this myth persisted almost 80 years after it was disproved and in the light of more recent studies which confirm the 1928 work? Again, optical illusions may be partially responsible, according to the study authors. When hair first comes in after being shaved, it grows with a blunt edge on top. Over time, the blunt edge gets worn so it may seem thicker than it actually is. Sun naturally bleaches hair over time, so hair which is just emerging after an individual has shaved seems darker, but actually is no darker than any new hair growth.

Even before Abe Lincoln was told that reading in dim candle light was bad for his eyes, people have believed that reading in dim light causes permanent eye damage. Generation after generation of parents have warned children (future physicians included) caught reading with flashlights under their blankets that reading in poor light ruins your vision. The study authors found no evidence that reading in dim light causes permanent eye damage. It is true, they say, that eye strain can occur in dim light, causing some temporarily decreased acuity, but after rest the eyes return to their full potential.

Both Dr. Vreeman and Dr. Carroll admit that they believed that eating turkey makes one especially drowsy. Scientific evidence supports that tryptophan, which is contained in turkey, can cause drowsiness. But they found that turkey doesn't contain an exceptional amount of tryptophan. In fact, turkey, chicken, and ground beef contain about the same amount of tryptophan and protein sources like pork and Swiss cheese contain more tryptophan per gram than turkey.

So why do people believe eating turkey makes them sleepy but never feel that way after a burger or pork chop? Again, this is a belief that the media, and other pop culture icons frequently repeat, popularizing the myth in spite of the easily accessible data on percentage of tryptophan in various products. Most likely, according to the study authors, this myth gained credence because turkey is often consumed during large, heavy dinners. It's the total content of the meal that makes one feel lethargic.

Why have these myths been perpetuated? Dr. Carroll and Dr. Vreeman believe it's because they are told by experts. "Until someone takes the time to question and look for evidence, medical myths will remain accepted beliefs. We have tried to show through science and research why certain widely accepted beliefs are untrue. To some extent this is what we do in our regular jobs every day, teaching medical students and residents (physicians in training) to question what they hear," said Dr. Carroll.

"This study was a light-hearted way to remind people that first they might believe things because they have heard them from other people, especially from experts; and second, staying current is not just a matter of adding new knowledge onto the things we already believe. Sometimes you really have to look back on things already in your head or things you already believe to be true and reinvestigate or investigate for the first time whether those things are true," said Dr. Vreeman.

Dr. Vreeman and Dr. Carroll, who are with Children's Health Services Research in the IU School of Medicine's Department of Pediatrics, are currently working on a book examining dozens of other popular medical beliefs to see if they are true, false or unproven.

So was your mother correct when she said your chewing gum would stay in your stomach for seven years? Dr. Carroll and Dr. Vreeman, both pediatricians with Riley Hospital for Children, one of the nation's leading centers of excellence in pediatric medicine, will let you know in 2008.

This research, entitled "Medical Myths Even Doctors Believe" is published in the Christmas issue of the British Medical Journal.