Five Health Myths Reinvestigated

Do you really need to eat five daily portions of fruit and veg? Do you lose most heat from your head? Does reading in the dark ruin your eyesight?
Eat five portions of fruit and veg a day
The five-a-day campaign has been running, in one form or another, in the UK since 1993. You may not know, however, that in this context, one portion equals 80g. What a nice, neat way, someone thought, of packaging the WHO’s daily recommended fruit-and-veg dose of 400g.
But while the tagline has been taken into the nation’s bosom – with people often jokingly wondering whether a Terry’s Chocolate Orange, or glass of Fanta, counts as one of their five-a-day – 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 daywould give further protection against some cancers and heart disease. Phew.
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.
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”.
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 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.
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 toa 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).