They're taken as medical fact, but some of the healthy mantras we are forced to swallow are out of date, unreliable or simply not true. Here Michael Mosley debunks the myth behind seven of our most common misconceptions
1 Drinking coffee is bad for you
There have been numerous claims down the years that drinking coffee will increase your risk of succumbing to a whole range of terrible things. These claims, like the ones about the benefits of fruit and vegetables, have been based largely on retrospective case studies.
When scientists observed 130,000 men and women for more than 20 years they found something altogether more surprising. The report, 'The Relationship of Coffee Consumption with Mortality', concluded that "regular coffee consumption was not associated with an increased mortality rate in either men or women". In fact, they found moderate coffee consumption appears to be mildly protective.
Based on this and other studies the most effective 'dose' is two to five cups a day. More than that and any benefits drop off. What it is in coffee that helps, we simply don't know.
The amount of coffee you can drink without side effects such as insomnia may be down to how much of the liver enzyme CYP1A2 you have. The speed with which caffeine is cleared from your body is dependent on levels of CYP1A2 and that is largely down to your genes. Which could explain why I can drink coffee in the evening with no problems, while one cup in the afternoon has my wife twitching.
2 There's no such thing as a healthy tan
The leathery, mahogany look is out of fashion, but is it also true, as some claim, that there is no such thing as a healthy tan? It's certainly true that ultraviolet light causes skin damage, skin ageing and can lead to skin cancer, but the most lethal form, melanoma, is not straightforwardly related to ultraviolet exposure. Melanoma is more common in people who work indoors than in people who work outdoors.
An Australian office worker is more likely to get a melanoma than an Australian farmer.
Although it is not a good idea to binge on sunshine, there is increasing evidence of a huge range of benefits to be got from regular exposure to the sun. The link is vitamin D, most of which we manufacture in our skin. Recent studies have shown that low levels of vitamin D are strongly associated with increased risk of heart disease, stroke, schizophrenia, multiple sclerosis, asthma and at least a dozen cancers. Sunshine is not just good for your body, it's good for your mood.
As the poet Walt Whitman put it, "Keep your face always toward the sunshine – and shadows will fall behind you." Time to get into the garden.
3 A couple of glasses of red wine a day are good for you
You can't open a newspaper without reading a story about a glass of wine being good or bad for you or making no difference at all. So is it true? Well, if you like a drink there is some good news, but it might not be in the quantities you would hope.
The upside of drinking modest amounts of alcohol is that it may protect you against heart disease.
The downsides of drinking alcohol are the increased risk of liver disease and cancer. Dr Peter Scarborough of the University of Oxford says that the downside swiftly displaces the upside and that half a unit (about a quarter of a glass of wine a day) is the optimum amount from a health perspective.
"If you're drinking more than that then you are not at the best level for reduced risk," he says.
4 Eating 'five a day' cuts your risk of cancer
The five-a-day campaign was dreamt up in the fields of California in 1988 and was launched on the back of claims that eating more fruit and vegetables would cut your risk of developing cancer.
Yet when Professor Tim Key, of the Cancer Epidemiology Unit at the University of Oxford, wrote a review in the British Journal of Cancer, he concluded: "The possibility that fruit and vegetables may help to reduce the risk of cancer has been studied for over 30 years, but no protective effects have been firmly established."
The original claims were based on case-control studies in which people with cancer were compared with those without it and were asked to look back at their lifestyle for an explanation. A better way is to do a cohort study, where you collect data about a group of healthy people then follow them for many years. This approach has produced very different results. In the Nurses' Health Study and the Health Professionals Follow-Up Study, for example, it was found that men and women with the highest intake of fruit and vegetables (eight or more servings a day) were no less likely to develop cancer than those who had eaten less.
It is possible that these studies have design flaws, but more likely that some fruit and vegetables protect against some cancers. A report by the World Cancer Research Fund suggests that non-starchy vegetables such as broccoli, cabbage and onions will "probably" protect against cancers of the mouth, oesophagus and stomach. Tomato and tomato-based products "may" reduce the incidence of prostate cancer. That said, eating fruit and vegetables does seem to protect against heart disease and other chronic diseases, and is certainly better than eating junk food.
5 Drinking two litres of water a day will do wonders for you
This is a myth, but where did this figure of two litres come from? It probably dates back to the 1940s when researchers calculated that this was how much water someone's body used in 24 hours. However, the researchers also said (and this gets ignored) that we obtain much of the water we need each day from our food. Drinks such as coffee and tea also count, despite what many people believe.
Drinking eight glasses of water a day could help you lose weight if you drink it very cold. Drinking ice-cold water burns through a few calories simply because you have to raise that water to body temperature. Doing so adds up to an impressive 70 calories. Apart from that, the claim that drinking two litres of water a day is good for us has no evidence to back it up. The boring truth is: just drink when you're thirsty and you'll be fine.
6 Eating eggs will raise your cholesterol
We used to be told not to eat more than a few eggs every week on the ground that eggs contain cholesterol and cholesterol is bad for you. At the time it was widely believed that elevated cholesterol in our blood was caused by large amounts of cholesterol in our food. We now know that's not true and that most of the excess cholesterol in our blood is actually produced by our livers and is a direct response to eating too much saturated fat, not cholesterol. When scrambled, boiled, poached or in a healthy omelette, eggs are a superb source of protein, are rich in vitamins and minerals and make a great start to the day.
7 Smoothies are a health drink
Smoothies have a reputation for being healthy because they are based on fruit, but unfortunately by the time you get rid of the fibre and the peel you have lost many of the potential health benefits. You are left with a big sugar hit. In a survey published this year, researchers revealed that out of 52 commercial smoothies, 41 had more sugar than cola and all had more calories. They are also acidic and the bits cling to your teeth, so dentists are not keen. An apple a day may keep the doctor away, but not when it's peeled, blended, mashed and packaged.
- The Fast Diet, Fast Diet Recipe Book and Fast Exercise are all published by Short Books
- Michael's websites include thefastdiet.co.uk and fast-exercises.com