Many widely held medical beliefs are myths based on frequent repetition rather than any real evidence. Sometimes even doctors are duped, say two US experts in the Christmas issue of the British Medical Journal.

They scoured the medical literature for evidence to support seven widely held beliefs, and found little hard medical evidence to back them up. Some of the beliefs have been disproved.

Rachel Vreeman, of Indiana University, and Aaron Carroll, a professor of paediatrics in Indianapolis, said that they found a large number of references to the conditions during their research.

They produced a list of the most common medical or medicine-related beliefs espoused by physicians and the general public, based on statements they had heard on multiple occasions. They then chose seven for critical review, using Medline, the National Library of Medicine’s database, and the search engine Google to find evidence supporting or rebutting each of these claims.

“Physicians would do well to understand the evidence supporting their medical decision-making,” they concluded. “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.

“Speaking from a position of authority, as physicians do, requires constant evaluation of the validity of our knowledge.”

Eating turkey makes you especially drowsy

Turkey contains the amino acid tryptophan, which is involved in sleep and mood control and can cause drowsiness. Tryptophan helps the body to produce the B-vitamin niacin, which in turn produces serotonin, the mood chemical that acts as a calming agent in the brain and plays a key role in sleep. The myth is that consuming turkey (and the tryptophan it contains) might particularly predispose someone to sleepiness. Casual observation tends to support this belief, since a lengthy snooze after Christmas dinner is a near-universal experience.

But in fact, say the BMJ authors, turkey does not contain an exceptional amount of tryptophan compared with other foods. Turkey, chicken and mince contain similar amounts of tryptophan (about 350mg per 115g) and other sources of protein, such as pork or cheese, contain even more tryptophan per gram than turkey.

The effects of tryptophan in turkey are probably minimised by consuming it with other food, which may limit its absorption, they add.

“Other physiological mechanisms explain drowsiness after meals. Any large meal (such as turkey, sausages, stuffing and vegetables followed by Christmas pudding and brandy butter) can induce sleepiness because blood flow and oxygenation to the brain decrease and meals rich in protein or carbohydrate may cause drowsiness,” they say, adding: “Wine may also play a role.”

Drink at least eight glasses of water daily

This is one of the most deeply entrenched of beliefs among the public – especially the young, who often walk about with a bottle of water to avert the ever-present danger of dehydration.

But finding an authority for the claim is impossible. It is usually bolstered by a secondary claim, which is that tea, coffee or the fluid in food does not count. This is equally baseless.

The two US experts found possible sources for this claim, often repeated by bottled water companies to increase their sales. One, a recommendation from 1945, said that 2.5 litres a day was a suitable water allowance for adults. But it went on to say: “Most of this quantity is contained in prepared foods.”

If this last, crucial sentence is ignored, say the authors, “the statement could be interpreteed as instruction to drink eight glasses of water a day”.

In fact, they add, studies suggest that an adequate fluid intake is usually met through typical daily consumption of juice, milk and caffeinated drinks such as tea, coffee or colas.

“But drinking excess amounts of water can be dangerous, resulting in water intoxication, hyponatraemia [low salt levels] and even death.”

One academic, Heinz Valtin, of Dartmouth Medical School, New Hampshire, has tried to scotch the myth, without success. In the American Journal of Physiology he concluded that it had no basis at all. Nor is it true, he says, that caffeinated drinks do not count. They do, and so do weak alcoholic drinks such as beer, in moderation. For healthy adults living in a temperate climate leading sedentary lives – just the kind of people never seen without a plastic bottle – the injunction to drink more water is nonsense.

Shaving causes hair to grow back faster

This belief was disproved as long ago as 1928, as a result of some pioneering studies by Mildred Trotter at Washington University School of Medicine.

Dr Trotter asked three girls to shave their legs from knee to ankle twice a week for eight months and observed their hair growth under the microscope. There was no increase in the diameter or colour of the hair, proving that hair does not become coarser as a result of shaving.

Nor, says the BMJ, does it grow any more rapidly. Shaving removes the dead part of the hair, not the living section below the skin, so it is unlikely to affect either the rate or type of growth.

Shaving does have one effect that could lead to the belief. Shaved hair lacks the finer taper seen at the ends of unshaven hair, giving an impression of coarseness. Shaving also leaves a long, angled cut on the end of the hair, providing a greater cross-section, which leads to the five o’clock shadow.

In addition, follicles hold short, shaved hairs more erect than longer hairs. The reason that new hairs appear darker than existing ones is that they have not been exposed to the lightening effects of the sun.

Reading in dim light ruins your eyesight

The BMJ authors suggest that any problems caused by reading in low light are more likely to be eye strain, which can occur in poor light and leads to difficulty in focusing. Reading in low light also decreases the rate of blinking, which can lead to dry eyes and soreness.

When the light improves and the eyes are rested, there are rarely any permanent effects. Even patients who suffer from the glandular disease Sjögren’s syndrome, and have decreased visual sharpness in low light, find that their eyes’ performance improves when they stop reading.

At least one review points to the increased incidence of short sight in people who are highly educated, suggesting that increased use of the eyes, such as reading in dim light or holding books close to the face, can lead to impaired growth of the eye and refractive error. However, the BMJ authors are sceptical. “Reading conditions used to involve less light, relying on candles or lanterns, so increased rates of myopia over the past several centuries do not necessarily support [the hypothesis] that dim reading conditions are to blame,” they conclude.

Mobile phones are dangerous in hospitals

Most hospitals still ban the use of mobile phones in the belief that they can affect medical instruments and risk patient safety.

The scare was triggered in 1993 by a journal that cited more than 100 reports of suspected electromagnetic interference with medical devices. When this appeared on the front page of The Wall Street Journal, hospitals around the world banned mobiles. Subsequent studies in Britain showed that mobile phones could interfere with only 4 per cent of devices, and only at a distance of less than a metre, while 300 tests done this year in 75 treatment rooms found no interference.

The ban is probably unjustified and possibly detrimental. A study among anaesthetists showed that the use of mobiles by doctors reduced by 20 per cent the risk of medical error that resulted from communication delays.

Hair and fingernails still grow after death

The stuff of science fiction horror stories and morbid jokes, this belief is, nevertheless, groundless.

It appears to have had a literary origin, in the celebrated First World War novel, All Quiet on the Western Front, where the author describes a friend’s fingernails continuing to grow “in corkscrews” after his death.

Johnny Carson made a joke of it. “For three days after death, hair and fingernails contine to grow, but phone calls taper off,” he remarked. But the American forensic anthropologist William Maples, author of Dead Men Do Tell Tales, said: “It is a powerful, disturbing image, but it is pure moonshine. No such thing occurs.”

This myth does have some basis, however, the authors explain in the BMJ. “As Maples and numerous dermatologists explain, dehydration of the body after death and drying, or desiccation, may lead to retraction of the skin around the hair or nails.

“The skin’s retraction can create an appearance of increased length, or of greater prominence, because of the contrast between the shrunken soft tissues and the nails or hair. The actual growth of hair and nails, however, requires a complex hormonal regulation not sustained after death.”

We use only 10 per cent of our brains

This belief is sometimes attributed to a remark by Einstein, a man so wise that almost anything he said could be taken as gospel.

In fact, say the authors, Einstein never said it, and it isn’t true.

But the belief does at least seem to be contemporary with Einstein, arising as early as 1907, and promoted by propagandists who believed that everybody had unrealised latent abilities that could be tapped into, given the will.

It also seems to be substantiated by occasional stories of people who have led normal and even successful lives but, when subjected to a brain scan, appear to have almost empty skulls.

However, the BMJ authors say that many studies of the brain, including those involving brain damage, brain scanning, microstructural analysis and metabolic studies, show that we use much more than 10 per cent of our brain.

“Studies of patients with brain injury show that damage to almost any area of the brain has specific and lasting effects on mental, vegetative and behavioural capabilities,” say the authors. “Many types of brain imaging studies show that no area of the brain is completely inactive . . . detailed probing of the brain has failed to identify the “nonfunctioning” 90 per cent.

“Even micro-level localisation, isolating the response of single neurones, reveals no gaps or inactive areas. Metabolic studies, tracking differential rates of cellular metabolism in the brain, show no dormant areas.”

. . . or talking in the prescribed manner?

The secret tribal language in which doctors have long communicated with each other is being enriched by the electronic media and urban slang (Nigel Hawkes writes).

Paul Keeley, a consultant in the Department of Palliative Medicine at Glasgow Royal Infirmary, has provided British Medical Journal readers with a sample of new words that he says are in use on the wards:

Adminosphere The pleasantly decorated and furnished offices of trust management, or the dean

Administrivia The flurry of pointless e-mails and paperwork that emanate from the adminosphere

Disco biscuits The clubbers’ drug Ecstasy. As in: “The man in cubicle three looks like he’s taken one too many disco biscuits”

Hasselhoff Term for a patient who turns up at accident and emergency with an injury for which there is a bizarre explanation. Source: Baywatch actor David Hasselhoff, who hit his head on a chandelier while shaving. The broken glass severed four tendons and an artery in his right arm

Agnostication Describes the usually vain attempt to answer the question “How long have I got, doc?”

Blamestorming Apportioning of blame after some particularly egregious error has occurred

404 moment The point in a doctor’s ward round when medical records cannot be located. Derivation is world wide web error message, “404 – document not found”

Testiculation Gesture typically used by hospital consultant “when holding forth on subject on which he or she has little knowledge”. The gesture is of an upturned hand with outstretched fingers pointed upwards, clutching an invisible pair of testicles

Mactilt The lateral movement of the head to an angle of 45 degrees by a palliative care nurse or specialist. It is supposed to indicate sympathy and understanding. Coinage inspired by Macmillan Nurses

Father Jack A confused, elderly patient whose high-pitched exclamations and attempts to get out of bed are responsible for insomnia on wards. The expression is derived from the character in the Father Ted sitcom, who would sit in the corner of the room shouting “Drink”, “Feck”, “Arse”, and so on

Fonzie An unflappable middle-grade doctor who remains cool in an emergency. This, says Dr Keeley with a straight face, is derived from The Fonz, a character in the US sitcom Happy Days, and immortalised in dialogue in the later scenes of Quentin Tarantino’s film Pulp Fiction:
Yolanda – “You don’t hurt him!”
Jules – “Nobody’s gonna hurt anybody. We’re gonna be like three little Fonzies here. And what’s Fonzie like? Come on, Yolanda! What’s Fonzie like?”
Yolanda – “Cool?”
Jules – “What?”
Yolanda – “He’s cool.”
Jules – “Correctamundo! And that’s what we’re gonna be. We’re gonna be cool.”

Explore Health

• Expert Advice

• Health Features

• Mental Health

• Alternative Medicine

• Child Health

• Health Club