They are among medicine's most widely held beliefs – drinking eight glasses of water a day is essential for health, shaving hair makes it grow back coarser, reading in a dim light ruins eyesight. Yet despite their popularity, they are myths.
Medicine is littered with false beliefs because doctors assume that if they have been held for long enough they do not need re-examination. The power of belief is one of the most important reasons why medicine works, underlying the placebo effect and providing a key weapon in the doctors' arsenal, and no professional is keen on undermining it.
In a review of widely held medical beliefs – by public and professionals alike – two US doctors selected seven for critical examination and searched for evidence to support or refute them. The results suggest the beliefs are built on sand.
Rachel Vreeman of the Indiana University School of Medicine and Aaron Carroll of the Regenstrief Institute, Indianapolis, say they could find no evidence to confirm the beliefs, or there was evidence that proved them wrong.
Writing in the Christmas issue of the British Medical Journal, they say: "Physicians would do well to understand the evidence supporting their medical decision-making. They should at least recognise when their practice is based on tradition, anecdote or art. While belief in the described myths is unlikely to cause harm, recommending medical treatment for which there is little evidence certainly can."
Not to be believed
Drink at least eight glasses of water a day
This advice derives from a 1945 recommendation that adults should consume 2.5 litres of the stuff daily, or one millilitre for every calorie consumed. The crucial part of the recommendation – "most of this quantity is contained in prepared foods" – is often ignored. Drinking too much water can be dangerous.
Reading in dim light ruins your eyesight
It causes eye strain, difficulty in focusing and dries the eyes because of reduced blinking while squinting but there is no evidence it causes lasting damage.
We only use 10 per cent of our brains
Studies of patients with brain damage suggest damage to any area of the brain has lasting effects. Brain imaging studies have shown no area of the brain is completely inactive and despite "detailed probing" the non-functioning 90 per cent has not been located.
Hair and fingernails continue to grow after death
They don't. It is an illusion caused by the dehydration of the body which results in the skin retracting. This gives the appearance of increased length to the hair and nails – actual growth requires hormonal regulation which is not sustained after death.
Shaving causes hair to grow back faster or coarser
Shaved hair lacks the finer taper seen at the end of unshaved hair. It has also not been lightened by the sun, so it appears darker.
Mobile phones are dangerous in hospitals
Many hospitals still ban them, despite the lack of evidence that they interfere with electronic equipment, except in rare instances and at close quarters. Technological improvements have reduced the risk further.
Eating turkey makes people drowsy
This myth is based on the assertion that turkey contains high amounts of tryptophan, an amino acid involved in sleep. But turkey contains no more than chicken or beef. Its sleep-inducing effects are due to the quantities eaten.