Medical myths - that even doctors believe

Eating turkey makes you drowsy. You should drink at least eight glasses of water a day. Shaving causes hair to grow back faster. Myth or reality? They're myths, say US researchers who looked at seven commonly held medical beliefs espoused by physicians and the general public, and found that they are either unproven or untrue.They used Google and Medline - a database of medical literature - to search for evidence to support or refute each of the popular notions.

Here's what their online investigation, published in the British Medical Journal, concluded:

Myth: Eating turkey makes people especially drowsy.

Reality: Any big meal or holiday feast can make people sleepy by decreasing blood flow to the brain. Wine also doesn't help. Turkey isn't all that rich in tryptophan, the chemical linked to sleepiness after eating turkey. Chicken and ground beef contain similar amounts, and pork and Swiss cheese contain even more tryptophan per gram.

Myth: Drink at least eight glasses of water per day.

Reality: There's no evidence that you have to drink that much water to assure adequate fluid intake. Most of us get plenty of water from food and other drinks including juices, milk and even caffeinated drinks like coffee and tea. Drinking water when you're thirsty makes sense, but eight glasses can be too much for many people. And drinking too much water can be unhealthy. Read Water - how much should you drink.

Myth: We use only 10 per cent of our brains.

Reality: This belief, which dates back at least 100 years, doesn't stand up to studies of patients with brain injuries, which suggest that damage to almost any area of the brain has specific and potentially lasting effects. Numerous brain-imaging studies have shown that no area of the brain is completely inactive.

Myth: Hair and fingernails continue to grow after death.

Reality: The belief may be caused by an optical illusion because after death, skin and other soft tissue dries out. When the shrunken skin pulls back and retracts, it makes fingernails or hair more obvious or prominent.

Myth: Reading in dim light ruins your eyesight.

Reality: Dim lights may contribute to difficulty focusing, leading to dry eyes and eye strain, but it's unlikely to cause a permanent change in the function or structure of the eyes. Once lighting conditions improve, these side effects don't persist for long.

Myth: Shaving causes hair to grow back faster or coarser.

Reality: Newly sprouted hair looks dark because it has had minimal exposure to sun or chemicals, and it seems coarse because shaved hair lacks the finer taper seen at the ends of unshaven hair. Read Top NewYou tips for getting a close shave.

Myth: Mobile phones are dangerous in hospitals.

Reality: Tests in Europe found minimal interference and only at distances of less than one metre, though equipment malfunctions have occasionally occurred. Research actually suggests that allowing physicians to use mobiles inside hospitals reduces the risk of injury because doctors can be immediately contacted when there is a medical emergency.

"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," says Dr Rachel Vreeman of the University of Indiana who co-authored the article.

"Until someone takes the time to question and look for evidence, medical myths will remain accepted beliefs," says co-author Dr Aaron Carroll. "We have tried to show through science and research why certain widely accepted beliefs are untrue."

The researchers plan to expand their work into a book that examines 100 common health myths. So was your mother correct when she said your chewing gum would stay in your stomach for seven years? They'll let you know next year.

For more myth-busting information, watch the New You video Colds & Flu - separating fact from fiction.