ROBERT SIEGEL, Host:
Surely, you've heard that shaving makes your hair grow back faster and coarser, and that we only use about 10 percent of our brains. There is a lot of homegrown medical wisdom out there, and it's tough to separate some of the myths from the real science.
A family physician and former Assistant Surgeon General Douglas Kamerow is here to debunk some of those common beliefs.
DOUGLAS KAMEROW: I used to work for the medical journal, BMJ, which is published in London. It was formally called the British Medical Journal. One of the things I liked best about the BMJ is its humor and irreverent style.
A great example of this is a recent article by two Indiana University doctors, Rachel Vreeman and Aaron Carroll. They critically examined some strongly-held medical beliefs that you may have heard from your mother or even from your doctor. They're not quite true.
For example, reading in dim light ruins your eyesight. If you spend your childhood reading with a flashlight under the covers, you can rest assured. Although poor lighting can cause glinting and dry eyes temporarily, there's no evidence that any permanent damage is done. When you think about it, lighting has only improved since the days when everyone read by candlelight, and vision hasn't gotten any better. In fact, near-sightedness is more common these days with better lighting.
Or how about the old saw that we only use 10 percent of our brains? This one, apparently, has been around since the turn of the last century, and it's been closely attributed to Albert Einstein. Modern medical imaging, however, shows that we use much more than 10 percent of our brains. Essentially, no areas of the brain are completely inactive, and studies of people with brain injuries have found that damage to virtually any part of the brain is associated with specific and lasting negative effects.
Okay, well, here's one that everyone knows is true. Shaving causes hair to grow back faster or coarser. If you're a woman, I guarantee someone has told you this at least once. For this one, we have strong evidence that it's a myth. Clinical trials have proven that shaving has no effect on the rate of hair growth or on the thickness of the hair that grows back. What unshaven hair does have is a finer taper at the end that may make it look less coarse. And newly grown hair looks darker because the sun hasn't had a chance to bleach it.
Speaking of hair, have you ever been told that hair and fingernails continue to grow even after someone dies? What's going on here, it turns out, is that post- mortem drawing of the skin can cost it to retract around the hair and nails, making it look like they're growing. But there's no way that the hair and nails actually continue to grow without the ongoing delivery of nutrients that a beating heart delivers.
Finally, how many diet articles and books have you read that advocate drinking at least eight glasses of water a day? No one knows exactly where this one came from, but it might be from a leading nutritionist in 1970s. It turns out that there's no such requirement. Thirst is the best guide for how much you need to drink. An drinking liquids in excessive amounts can actually be dangerous as we see in the occasional marathon runner who over drinks, gets water intoxication, and dies.
Bottom line, don't believe everything your mother or maybe even your doctor tells you.
SIEGEL: That's family physician Douglas Kamerow, a former assistant surgeon general and he's a health service researcher and columnist. He lives in Maryland. A link to the full BMJ article on medical myths is available at the Web at npr.org.