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 / Updated  / Source: TODAY
By Tara Parker-Pope

Turkey makes you drowsy. Dim light ruins your eyes. Drink at last eight glasses of water a day. These are some of the medical myths that even doctors believe, reports the British Medical Journal.

Researchers from the Indiana University School of Medicine made a list of common medical beliefs espoused by physicians and the general public. They included statements they had heard endorsed by doctors on multiple occasions. The result is a seven-item list of medical and health myths that are widely repeated by doctors and in the media, all of which either aren't true or lack scientific evidence to support them.

The study authors, Dr. Rachel C. Vreeman and Dr. Aaron E. Carroll, said that while doctors realize good medicine requires them to constantly learn new things, they often forget to re-examine their existing medical beliefs.

"These medical myths are a lighthearted reminder that we can be wrong and need to question what other falsehoods we unwittingly propagate as we practice medicine,'' wrote Dr. Vreeman and Dr. Carroll.

Here are the seven medical myths they identified:

1. People should drink at least eight glasses of water a day
The article authors found no scientific evidence for this advice, although they found several unsubstantiated recommendations in the popular press. The source may be a 1945 article from the National Research Council, part of the National Academy of Sciences, which noted that a "suitable allowance" of water for adults is 2.5 liters a day, although the last sentence noted that much of it is already contained in the food we eat.

"If the last, crucial sentence is ignored, the statement could be interpreted as instruction to drink eight glasses of water a day,'' Dr. Vreeman and Dr. Carroll noted. "Existing studies suggest that adequate fluid intake is usually met through typical daily consumption of juice, milk and even caffeinated drinks.''

2. We use only 10 percent of our brainsThe belief that we use only 10 percent of our brains has persisted for nearly a century, the authors noted. Sometimes the claim is attributed to Albert Einstein, but no reference or statement has ever been recorded. The study authors found references to this myth as early as 1907 and noted that it's often repeated by people advocating the power of self-improvement.

However, the authors said that evidence from studies of brain-damaged people, imaging and metabolic studies and other brain research shows that people use much more than 10 percent of their brains. "Numerous types of brain imaging studies show that no area of the brain is completely silent or inactive,'' wrote the authors. "Detailed probing of the brain has failed to identify the 'non-functioning' 90 percent."

3. Hair and fingernails continue to grow after deathThe claim has been repeated in movies and talk-show monologues, but it's not true. The growth of hair and nails requires "a complex hormonal regulation" that stops after the body dies. The reason for the long-held belief may be that dehydration of the body after death, and subsequent shrinking of soft tissue, can create the illusion of growth of hair and nails.

4. Shaving hair causes it to grow back faster, darker or coarserThis common belief is often repeated in the media and reinforced when coarse stubble appears on the body after shaving. A 1928 clinical trial showed that shaving had no effect on hair growth, a finding confirmed by more recent studies. When hair grows back after shaving, it seems coarse because it doesn't have the fine taper of unshaved hair. It seems darker because it hasn't been exposed to the sun like the previously unshaved hair.

5. Reading in dim light ruins your eyesight
The idea that dim light ruins eyesight probably has its origins in eye strain, said the study authors. Bad lighting makes it hard to focus, makes you blink less and leads to dry eyes, particularly if you're squinting. So reading in dim light is uncomfortable, but it doesn't cause permanent damage.

6. Eating turkey makes people especially drowsy
This myth stems from the fact that turkey contains tryptophan, an amino acid found in proteins and essential to the human body. Scientific studies show that sleep and mood are affected by tryptophan.

However, turkey does not contain an exceptional amount of tryptophan. Chicken and beef contain about the same amount, and pork and cheese contain more tryptophan per gram than turkey. Because turkey is consumed with other foods, absorption of tryptophan from turkey is minimal, noted the authors. The myth likely stems from the fact that everyone feels drowsy after eating a large meal because the body is using energy to digest food and blood flow and oxygenation to the brain decrease. Large meals in the United States usually occur around Thanksgiving and Christmas, holidays during which turkey is often served.

7. Cell phones create considerable electromagnetic interference in hospitalsAnecdotal reports persist that cell phones create false alarms on monitors and malfunctions in infusion pumps. After publication of a medical journal article citing more than 100 reports of suspected electromagnetic interference with medical devices before 1993, the Wall Street Journal published a front-page article on the topic. As a result, many hospitals banned the use of cell phones, perpetuating the belief.

But the study authors found no evidence to support it. At the Mayo Clinic in 2005, in 510 tests performed with 16 medical devices and six mobile phones, the incidence of clinically important interference was 1.2 percent. A 2007 study that examined cell phones "used in a normal way" found no interference of any kind during 300 tests in 75 treatment rooms. In contrast, a large survey of anesthesiologists found that use of cell phones by doctors was associated with a 22 percent reduction in medical error resulting from delays in communication.

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