Many common beliefs about what is good and bad for you are untrue, they are “medical myths”, said US researchers writing in a leading medical journal
Even doctors are duped, said authors Rachel C Vreeman, fellow in children’s health services research at the Indiana University School of Medicine, and Aaron E Carroll, assistant professor of paediatrics at the Regenstrief Institute, both in Indianapolis, writing in the 22nd December Christmas issue of the BMJ.
Should people drink at least eight glasses of water a day? Does reading by a dim light damage eyesight? Does shaving make hair grow back thicker?
These are three of the seven beliefs held by members of the public and doctors that Vreeman and Carroll selected for research and critical review. They noted the quality of the evidence they found, and also when there was no evidence one way or the other. The seven beliefs are:
- People should drink at least eight glasses of water a day.
- We use only 10% of our brains.
- Hair and fingernails continue to grow after death.
- Shaving hair causes it to grow back faster, darker, or coarser.
- Reading in dim light ruins your eyesight.
- Eating turkey makes people especially drowsy.
- Mobile phones create considerable electromagnetic interference in hospitals.
Vreeman and Carroll’s found the evidence about these beliefs ranged from uproven to untrue.
Daily Fluid Intake
There is no evidence to support the need to drink eight glasses of water a day they said. On the contrary, studies suggest that most people get enough fluid every day from juice, milk, and even drinks containing caffeine. And there is clinical evidence that drinking too much water is dangerous and can lead to death.
The myths’s origin may have been a recommendation in 1945 which said 2.5 litres was a suitable fluid intake for adults and that most of this comes in prepared food. If the last part of the recommendation is omitted, it could imply the fluid intake should be in addition to normal food, suggested the researchers.
Use of the Brain
Regarding use of the brain, Vreeman and Carroll found that imaging studies showed that no area of the brain is completely inactive. “Even micro-level localisation, isolating the response of single neurones, reveals no gaps or inactive areas,” they wrote.
And research on patients with brain damage also goes against the belief that only 10 per cent of the brain is used. It shows that damage to any part of the brain has specific and lasting effects.
The myth has been around for about 100 years suggested the researchers, and some people have claimed Albert Einstein was the source, but there are no records to support this. Self-improvement proponents are some of the most frequent users of the myth.
Hair and Nails after Death
Growth of hair and nails depends on a complicated set of interrelated hormonal processes being active, which of course cease on death, so that belief is a myth too.
But, said the researchers, it could have origins in an optical illusion, because the skin dries, shrinks and retracts after death, giving the impression that nails and hair have become a bit longer.
Hair Thickness After Shaving
A similar optical illusion occurs about hair grown after a shave. When hair is allowed to grow longer it tapers at the end, and so appears slightly thinner than the shaft as it comes out of the scalp or skin. However, when the hair is shaved, it grows at the same thickness as the shaft that was there before, but because it has not tapered it appears thicker and coarser.
Also, the hair that comes out of the skin is already dead, and shaving does not affect the live hair growing under the skin. As the hair grows longer and gets “older” it is exposed to more sunlight and chemicals in the air, which is probably what gives it a slightly lighter colour, suggested the researchers.
This myth is propagated by the popular media, wrote Vreeman and Carroll.
Reading in Dim Light
The fear that reading in dim light is bad for your eyes probably originates in the feeling of eye strain, said the researchers, because poor light makes it hard to focus and reduces the rate of blinking leading to dry eyes.
However, eye specialists writing in a range of patient information guides say that reading in dim light does not damage your eyes, but it can lead to eye strain with some temporary bad effects. It is not likely to cause permanent change to the way the eyes work they said.
The researchers did find one article on short sightedness that concluded reading in dim light could damage eye growth. The article cited epidemiological evidence about the higher rate of short sightedness in academics. The researchers referred to many experts who refute the opinion that reading in dim light causes damage to the eyes.
Eating Turkey
The idea that eating turkey makes people especially drowsy could be due to the fact that one of the most commonly known facts about amino acids in food is that turkey contains tryptophan, which is known to be involved in sleep and mood control and can cause drowsiness. However, turkey contains no more tryptophan than many other forms of meat such as chicken and minced beef. In fact some sources of protein such as cheese and pork contain more tryptophan per gram than turkey, wrote the researchers.
Also, many experts say that the effect of tryptophan in turkey is probably reduced by the fact it’s eaten with other foods. The more likely explanation is that turkey is often eaten as part of a large solid meal, for instance at Christmas with stuffing, sausages and various other foods, and followed by Christmas pudding and brandy butter. Add to this the probability that wine is often consumed at the same time and it is not surprising that the myth has caught on. Eating a large solid meal like this decreases oxygen to the brain which can lead to drowsiness said the authors.
Mobile Phones in Hospitals
Again, Vreeman and Carroll found scant evidence to substantiate the myth that mobile phones cause substantial interference with hospital equipment. They tracked down one journal article that listed 100 reports of suspected electromagnetic interference in medical equipment from mobile phones before 1993, which the Wall Street Journal made into front page news, after which hospitals banned the use of mobile phones.
But there is little evidence to support this policy said the researchers. In the UK early studies showed mobile phones interfered with as few as 4 per cent of the equipment and only when within one metre, while less than 0.1 per cent showed serious effects. Rigourous testing at a number of other laboratories and medical centres have also come up with very small percentages and again only when within 1 metre of the equipment.
A more recent study carried out this year found no interference in 300 tests in 75 treatment rooms, and in contrast the authors give an account of a survey of medical staff where use of mobile phones to stay in touch with each other was linked to reductions in risk of medical errors and injury resulting from delays in communication.
Vreeman and Carroll concluded that:
“Despite their popularity, all of these medical beliefs range from unproved to untrue.”
Conceding that their review was not systematic of all the possible myths around, or of all the evidence relating to them, they did find a large number of references to show that some of the myths had no supporting evidence and others even had evidence to disprove them.
“Physicians would do well to understand the evidence supporting their medical decision making,” suggested Vreeman and Carroll.
“They should at least recognise when their practice is based on tradition, anecdote, or art,” they said, and:
“Speaking from a position of authority, as physicians do, requires constant evaluation of the validity of our knowledge.”
“Medical myths.”
Rachel C Vreeman, Aaron E Carroll.
BMJ 2007;335:1288-1289 (22 December)
doi:10.1136/bmj.39420.420370.2
Sources: Journal Article, BMJ press release.
Written by: Catharine Paddock