Medical myths
Rachel C Vreeman, fellow in children’s health services research, Aaron E Carroll, assistant professor of paediatrics
Sometimes even doctors are duped, say Rachel C Vreeman and Aaron E Carroll
Physicians understand that practicing good medicine requires the constant acquisition of new knowledge, though they often assume their existing medical beliefs do not need re-examination. These medical myths are a light hearted reminder that we can be wrong and need to question what other falsehoods we unwittingly propagate as we practice medicine. We generated a list of common medical or medicine related beliefs espoused by physicians and the general public, based on statements we had heard endorsed on multiple occasions and thought were true or might be true. We selected seven for critical review:
- People should drink at least eight glasses of water a day ( I heard this one like a 100 times!!! )
- We use only 10% of our brains
- Shaving hair causes it to grow back faster, darker, or coarser
- Reading in dim light ruins your eyesight
We used Medline and Google to search for evidence to support or refute each of these claims. Because "proving a negative" can be challenging, we noted instances in which there was no evidence to support the claim.
People should drink at least eight glasses of water a day
The advice to drink at least eight glasses of water a day can be found throughout the popular press. One origin may be a 1945 recommendation that stated: A suitable allowance of water for adults is 2.5 litres daily in most instances. An ordinary standard for diverse persons is 1 millilitre for each calorie of food. Most of this quantity is contained in prepared foods. If the last, crucial sentence is ignored, the statement could be interpreted as instruction to drink eight glasses of water a day.
Another endorsement may have come from a prominent nutritionist, Frederick Stare, who once recommended, without references, the consumption "around 6 to 8 glasses per 24 hours," which could be "in the form of coffee, tea, milk, soft drinks, beer, etc." The complete lack of evidence supporting the recommendation to drink six to eight glasses of water a day is exhaustively catalogued in an invited review by Heinz Valtin in the American Journal of Physiology.
Furthermore, existing studies suggest that adequate fluid intake is usually met through typical daily consumption of juice, milk, and even caffeinated drinks. In contrast, drinking excess amounts of water can be dangerous, resulting in water intoxication, hyponatraemia, and even death.
We use only 10% of our brains
The belief that we use only 10% of our brains has persisted for over a century, despite dramatic advances in neuroscience. In another extensive expert literature review, Barry Beyerstein provides a detailed account of the origins of this myth and the evidence disputing it. Some sources attribute this claim to Albert Einstein, but no such reference or statement by Einstein has ever been recorded. This myth arose as early as 1907, propagated by multiple sources advocating the power of self improvement and tapping into each person’s unrealised latent abilities.
Evidence from studies of brain damage, brain imaging, localisation of function, microstructural analysis, and metabolic studies show that people use much more than 10% of their brains. Studies of patients with brain injury suggest that damage to almost any area of the brain has specific and lasting effects on mental, vegetative, and behavioural capabilities. Numerous types of brain imaging studies show that no area of the brain is completely silent or inactive. The many functions of the brain are highly localised, with different tasks allocated to different anatomical regions. Detailed probing of the brain has failed to identify the "non-functioning" 90%. Even micro-level localisation, isolating the response of single neurones, reveals no gaps or inactive areas. Metabolic studies, tracking differential rates of cellular metabolism within the brain, reveal no dormant areas.
Shaving hair causes it to grow back faster, darker, or coarser
Another common belief is that shaving hair off will cause it to grow back in a darker or coarser form or to grow back faster. It is often reinforced by popular media sources and perhaps by people contemplating the quick appearance of stubble on their own body.
Strong scientific evidence disproves these claims. As early as 1928, a clinical trial showed that shaving had no effect on hair growth. More recent studies confirm that shaving does not affect the thickness or rate of hair regrowth. In addition, shaving removes the dead portion of hair, not the living section lying below the skin’s surface, so it is unlikely to affect the rate or type of growth. Shaved hair lacks the finer taper seen at the ends of unshaven hair, giving an impression of coarseness. Similarly, the new hair has not yet been lightened by the sun or other chemical exposures, resulting in an appearance that seems darker than existing hair.
Reading in dim light ruins your eyesight
The fearful idea that reading in dim light could ruin one’s eyesight probably has its origins in the physiological experience of eye strain. Suboptimal lighting can create a sensation of having difficulty in focusing. It also decreases the rate of blinking and leads to discomfort from drying, particularly in conditions of voluntary squinting. The important counterpoint is that these effects do not persist.
The majority consensus in ophthalmology, as outlined in a collection of educational material for patients, is that reading in dim light does not damage your eyes. Although it can cause eye strain with multiple temporary negative effects, it is unlikely to cause a permanent change on the function or structure of the eyes. Even in patients with Sjögren’s syndrome (an autoimmune disease that features inflammation in certain glands of the body), decreased functional visual acuity associated with strained reading improves when they stop reading.
One review article on myopia concludes that increased use of one’s eyes, such as reading in dim light or holding books too close to the face, could result in impaired ocular growth and refractive error. The primary evidence cited was epidemiological evidence of the increased prevalence of myopia and the high incidence of myopia in people with more academic experience. The author notes that this hypothesis is just beginning to "gain scientific credence." In the past reading conditions involved even less light, relying on candles or lanterns, so increased rates of myopia over the past several centuries does not necessarily support that dim reading conditions are to blame. In contrast to that review, hundreds of online expert opinions conclude that reading in low light does not hurt your eyes.
Conclusions
Despite their popularity, all of these medical beliefs range from unproved to untrue. Although this was not a systematic review of either the breadth of medical myths or of all available evidence related to each myth, the search methods produced a large number of references. While some of these myths simply do not have evidence to confirm them, others have been studied and proved wrong.
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. Speaking from a position of authority, as physicians do, requires constant evaluation of the validity of our knowledge.
Contributors and sources
RCV and AEC are both health services researchers, whose research focuses on examining health policy and professional practices. They have both studied and reported widely on the most effective ways to improve children’s health. This article arose from discussions of how seldom physicians pause to examine the beliefs that they already hold as true. Both authors were responsible for the study concept and design, acquisition of data, and analysis and interpretation of data. RCV was responsible for the drafting of the manuscript and critical revision and is guarantor.
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