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Modern wives' tales
Today's medical myths make the rounds online and around the water cooler.
By Ben Cramer | Prevention

The infamous Double Dip


That food's OK! Five-second rule!


Does cracking your knuckles cause arthritis?


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Published: 9/8/2008 12:05 AM

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Does reading in low light really hurt your eyes? How about sitting too close to the TV? (No, and no.) Why are you better off drinking exactly eight glasses of water per day? (You're not.)

Thanks to quack culture, the Internet and well-intentioned but poorly informed relatives, it's become harder than ever to separate fact from fiction. Whatever its origin, misleading health information can cause unnecessary anxiety and distract you from wellness habits that truly deserve your energy and attention. Here's a dissection of five modern wives' tales that will give you all the authority you need to refute your brother's latest forwarded e-mails.

5-second rule

It's safe to follow the "5-second rule" for food dropped on the floor.

Verdict: Fiction.

It's probably not even safe to follow a one-second rule: The transfer of bacteria from a contaminated surface to food is almost instantaneous - or, at the very least, quicker than your reflexes.

In a recent study, Clemson University food scientist Paul Dawson and his students contaminated several surfaces (ceramic tile, wood flooring, and carpet) with salmonella. They then dropped pieces of bologna and slices of bread on the surfaces for as little as five seconds and as long as 60 seconds. After just five seconds, both food types had already picked up as many as 1,800 bacteria (more bad bugs adhered to the moisture-rich bologna than the bread); after a full minute, it was up to 10 times that amount.

Bottom line: There are 76 million cases of food-borne illness annually in the United States, according to the CDC - so unless you're the only family on the block that sterilizes their floors on an hourly basis, you should refrain from eating dropped food.

"Let's not forget what comes into contact with floors - people bring animal feces on their shoes into their homes," Dawson says.

And don't assume that countertops are clean. Dawson's team also found that the salmonella actually survived as long as 4 weeks on the test surfaces. As the recent tomato-related illnesses nationwide showed, "raw fruits and vegetables are as frequently the perpetrators of salmonella transfer as poultry," Dawson says.

Knuckle down

Cracking your knuckles can cause arthritis.

Verdict: Fiction.

If you're suffering from osteoarthritis in your hands, it certainly has nothing to do with this nervous tic. One study at the former Mount Carmel Mercy Hospital in Detroit compared 74 people (age 45 and older) who had been chronic knuckle crackers for decades with 226 who always left their hands alone; researchers found no difference in the incidence of osteoarthritis between the two groups.

But there are reasons to stop this annoying habit: The same study found knuckle crackers to be far more likely to have weaker grip strength and greater hand swelling, both of which can limit dexterity. As for osteoarthritis, that's more likely due to genetics and increasing age.

Bottom line: Try turning your nervous energy into a less harmful habit that occupies your hands (such as doodling). If a different activity doesn't get you to stop, try putting a large rubber band around your wrist and every time you catch yourself cracking your knuckles, pull it back and let it snap as a reminder that your habit really can be harmful. Most important, get to the bottom of what's causing your nervousness in the first place - you may crack your knuckles more often at work than at home, for example - and address those sources directly.

Soda and stones

Cola-type soft drinks can damage your kidneys.

Verdict: Fact.

Despite their global popularity, there's nothing remotely healthy about cola beverages: Drinking 16 ounces or more daily (whether diet or regular) doubles your risk of chronic kidney disease, according to a recent National Institutes of Health study of more than 900 people.

The researchers already knew that consuming any type of soft drink - the average American adult guzzles 59 gallons' worth per year - is associated with several risk factors for kidney disease (hypertension, diabetes and kidney stones), but the spike in the cola category was remarkable. Experts suspect that the ingredient phosphoric acid may be the culprit; it's been repeatedly linked to "urinary changes that promote kidney stones," say the study authors.

Cola has an additional knock against it: Consumption is associated with significantly lower bone density in women, increasing the risk of osteoporosis and bone fractures, says a separate study.

Bottom line: If you're going to indulge in an occasional soda, go for Sprite, 7-Up, ginger ale, and the like - the NIH study found that non-cola drinks didn't have the same impact on the kidneys. But you'll be better off if you skip soda altogether, even the sugar-free varieties: Recent research showed an association between drinking diet soda and weight gain.

Double dip

"Double dipping" spreads germs from one chip to another.

Verdict: Fact.

In a classic episode of Seinfeld, a partygoer accused George Costanza of spreading germs by "double dipping" - swiping a chip into a bowl of dip, taking a bite, and then dipping the same chip again. Having settled the 5 -second rule debate, Clemson University's Dawson decided to do the same recently with this alleged party faux pas.

It turns out that George really was contaminating the other guests: Using Wheat Thins and various dips, Dawson found that a double dip deposited thousands of saliva bacteria into the dip - and of those, 50 to 100 were later transferred through the dip to a clean cracker, presumably destined for another guest's mouth. Still unknown, however, is how long such bacteria can survive in the dip or if they can actually infect another dipper upon ingestion.

Bottom line: You'd better be pretty comfy with your party guests. "Eating from a dip after someone has dipped twice is basically the same as kissing that person," Dawson says.

Be especially wary of thin dips; the study found that the lower the dip's viscosity, the higher the rate of germ transfer from a double dip. For example, a chip's second plunge into a cheese dip is less cause for concern than a watery salsa - thicker dips apparently don't allow errant bacteria to travel as far as thinner varieties.

Finally, think twice about digging into any dip at the end of the night; remnants on the sides or bottom of a bowl are most likely a highly concentrated mash of germs, Dawson says, akin to the last sip in a can of soda.