Health myth or health fact? We investigate 8 common beliefs

Does cracking your knuckles lead to arthritis? Can you get an STD from a dirty toilet?

Cracking your fingers causes arthritis

It’s not the way to make friends in a quiet library or on the bus, but cracking your knuckles won’t give you arthritis, at least not according to the many studies that have focused on addressing this myth.

Arthritis develops when the cartilage within the joint breaks down and therefore the bones to rub together. Joints are surrounded by a synovial membrane, which contains synovial fluid that lubricates them and prevents them from grinding together. When you crack your knuckles, you’re pulling your joints apart. The stretch causes an air bubble in the fluid, which eventually pops, creating that familiar sound.

Cracking your knuckles isn’t necessarily good for you, though. While there’s no proven relationship between the habit and arthritis, persistent cracking can wear down your synovial membrane and make it easier for your joints to crack. It can also lead to hand swelling and weaken your grip.


Going out with wet hair makes you sick

It turns out that leaving the house just after a shower isn’t going to make you sick… unless you’re already sick.

Research from the UK’s Common Cold Centre tested the hypothesis that chilling your body increases your chances of being infected with the common cold virus, also known as acute viral nasopharyngitis. Logically, you would chill yourself if you are freshly scrubbed and leave the house with a head of cold, wet hair. Their results found that, no, it doesn’t. But it can cause the onset of symptoms if the virus is already in your body.

So if you feel a summer cold moving in, blow-dry your hair before leaving the house.

Reading in the dark ruins your eyesight

Poppycock. Reading in dim light can be challenging, to the point of being deeply irritating. It can even give you a headache and result in tired or strained eyes. However, says the UK College of Optometrists, “reading in dim light or in the dark is highly unlikely to cause any permanent damage to your eyes”.

Some studies have found that myopia is more common in highly educated cultures, in which children grow up doing more close work, such as reading, but the connection could simply be that richer populations have better access to diagnosis from eye specialists. Ideally, however, when reading after dark, light should shine directly on to the page, and not come from over your shoulder, thus causing glare.

Dirty toilet seats can transmit STDs

Dark and dingy garage toilets might be your nightmare – or at least the worst part of a summer road trip – but it’s highly unlikely that they’ll give you a sexually transmitted disease (STD). But it’s not impossible...

STDs can be caused by viruses, bacteria, or parasites. According to Beth Israel Deaconess Medical Center, only parasitic STDs like crabs or Trichomonas have any real chance of being transmitted by sitting on a dirty toilet seat. And even then, the likelihood is extremely low. Your genital area would need to come into contact with the toilet seat while the parasite is still on it, and alive. Thanksfully, toilet seats don’t provide ideal living situations for parasites.

The advice? Use a toilet seat cover and hover. Or don’t linger.

You need five portions of fruit and veg a day

The five-a-day campaign has been running, in one form or another, in Ireland since the 1990s. You may not know, however, that in this context, one portion equals 80g. What a nice, neat way, someone thought, of packaging the World Health Organisation’s daily recommended fruit-and-veg dose of 400g. But while the tagline has been taken into the nation’s bosom, obesity has continued to rise, and fresh-produce consumption has declined.

The cost of fresh food has risen, and lower-income households eat the least fruit and veg. Food peddlers, meanwhile, cause mass confusion by flogging five-a-day items in random portion sizes, and by only flagging up the five-a-day eligibility of more expensive, or less healthy, processed foods rather than basic, cheaper fresh ingredients.

As far as we know, though, the advice is actually sound. Some of the original hypotheses – about the extent of the cancer protection that fruit and veg offers – have been rubbished, but it is generally agreed that fruit and veg is nutritious, provides fibre, and takes up room on the plate that might otherwise accommodate a deep-fried Mars bar. (That said, there is no magic superfood, and other unprocessed foods are good for us, too.) In 2014, a study by University College London suggested that seven portions a day were necessary, but, soon after, a much larger study found no evidence that more than five portions a day would give further protection against some cancers and heart disease. Phew.

You need to drink eight glasses of water a day

No one knows where this dictum originated. A 1945 US Food and Nutrition Board document once said that we need 2.5 litres a day, but it also said that much of this can be obtained from food. In any case, how much we need fluctuates, on any given day, according to how active we’re being, what we’re eating, whether we’re ill, and the weather. This is why our bodies handily tell us when we need more water (although old age can stymie thirst signals). Don’t listen to anyone who tells you you’re already dehydrated when you feel thirsty. Someone made that up.

In 2011, Margaret McCartney, a GP, wrote to the BMJ to highlight the lack of evidence for hydration advice, including the NHS’s more modest recommendation of six to eight glasses (or 1.2-1.9 litres) a day. She namechecked an initiative called Hydration 4 Health, which promotes the benefits of drinking extra water to the public and to doctors. Hydration 4 Health recommends two litres for men and a little less for women (1.6 litres). It is sponsored by the French company Danone, which owns Evian, Badoit and Volvic mineral waters.

A review study published in the Journal of the American Society of Nephrology in 2008 found “no clear evidence of benefit from drinking increased amounts of water”. The potential perks the study investigated included improved kidney function and detoxification, clearer skin, fewer headaches and reduced calorie consumption due to feeling fuller. However, the authors wrote, “although we wish we could demolish all of the urban myths found on the internet regarding the benefits of supplemental water ingestion, we concede there is also no clear evidence of lack of benefit”.

You lose the most body heat from your head

It’s easy not to question this. Heat rises, after all. Roofs need insulation and so do heads. Except, now you mention it, so does any part of the body when it is cold. It is thought the confusion arose from the misinterpretation of an experiment carried out by the US military in the 1950s. It was freezing and only the participants’ heads were exposed to the elements, so, of course, that is where they shed the most heat.

More recent investigations have found that the head loses as much body heat as any other exposed body part. Bad news for the hat industry.

Starve a fever, feed a cold

This goes back a long, long way: it appears in John Withals' dictionary of 1574, and has been linked to a misreading of Chaucerian English in The Canterbury Tales. The original thinking was probably that fasting would cool the body during a fever, whereas eating would warm you up when you have a cold. However, in practice, we should feed both colds and fevers.

Fevers speed up the metabolism and burn more calories, so food is welcome. That said, if you lose your appetite for a few days, bodies are adept at using fat stores for emergency energy. Drinking, however, is essential, and this is one occasion when you should force yourself to drink, even if you don’t feel like it. Fevers and colds speed up dehydration (which will in turn cause mucus to harden, and this you really don’t want to happen).

Additional: Guardian service