Medical contradictions: So bad it's good for you...

Does sunbathing give us vital vitamin D – or just cause skin damage? Will alcohol increase longevity, or drive us to an early grave? Clint Witchalls unravels those medical contradictions

A few decades ago, the medical establishment told us that margarine was healthier than butter, eggs were bad – they raised cholesterol levels – and we should brush our teeth vigorously, especially after fizzy drinks. Now we know margarine is full of hydrogenated fats, the cholesterol in eggs has negligible impact on serum cholesterol levels and brushing your teeth vigorously, especially after acidic drinks, erodes tooth enamel, and your gums. Thanks to the medical advice of the 1970s, I now have furred arteries, receding gums and I missed out on many fine omelettes.

Scientists keep telling us that science isn't based on certainty. Findings are always provisional, subject to change when new evidence emerges. I find this odd because I have yet to come across a medical professional who sounds tentative and uncertain. When I had an NHS MOT, the nurse told me I was overweight and I should aim to bring my BMI down from 26 to below 25. When I told her there were several studies that found that being slightly overweight in my middle years may increase my lifespan rather than curtail it, she pooh-poohed me.

Is salt really bad for me? Do the positive effects of moderate drinking outweigh the negative? Can there be such a thing as too much sunlight if you live in soggy old England? Here are a few contemporary uncertainties.

Salt

The prevailing view is that salt causes the body to retain water, which increases blood pressure and leads to heart disease. But the May issue of the Journal of the American Medical Association seems to contradict this. A study led by Dr Jan Staessen, of the University of Leuven in Belgium, followed 3,681 Europeans. The researchers found participants with the lowest salt intake had the highest rate of death from heart disease (4 per cent). An analysis of seven studies on salt and cardiovascular disease, published in the American Journal of Hypertension, found no strong evidence that consuming less salt reduces the risk for heart attacks and strokes in people with normal or high blood pressure. Yet there are earlier studies that contradict these studies. One, published in the British Medical Journal (BMJ) in 2009 found that a high-salt diet increased the risk of stroke by 23 per cent.

Verdict: If you suffer from hypertension or heart disease, go easy on the salt.

Coffee

Sometimes there are so many health pros and cons it's difficult to decide whether it's worth the risk. Drinking coffee may protect against prostate cancer, breast cancer, stroke (especially in women), type-2 diabetes, gout, cancer of the liver, Parkinson's and Alzheimer's disease. On the other hand, it may increase your risk of having a heart attack (people who drink coffee have elevated levels of LDL cholesterol and homocysteine), it causes spikes in blood-sugar levels, it's addictive and it has been linked to rheumatoid arthritis.

I've decided I like coffee based on its taste and the buzz it gives me.

Verdict: There seem to be far more pros to drinking coffee than cons.

Sun

Everybody knows getting sunburned is bad for you. But sunlight is necessary for synthesising vitamin D and a deficiency is linked to heart disease, various cancers, multiple sclerosis and rickets. Research published in The American Journal of Nutrition showed 86 per cent of English people and 92 per cent of Scots are failing to maintain optimum levels of vitamin D.

Writing in the BMJ last month, Dr Des Spence, a GP from Glasgow, argues that current policies on sun exposure are "counterintuitive" and "bad medicine". The rise in the detection of melanoma coincides with the rise in the number of skin biopsies.

Verdict: Try to get 10 to 15 minutes of sun exposure each day.

Fish oil

Omega-3 fatty acids were hailed as a super-supplement. They would stop your kids misbehaving, help to ease depression, make you smarter and prevent cataracts. There is scant evidence to support these claims. However, according to the Mayo Clinic, there is evidence from multiple studies that fish oil "lowers triglycerides, reduces the risk of death, heart attack, dangerous abnormal heart rhythms and strokes in people with known cardiovascular disease". But a study, published in the American Journal of Epidemiology, found a link between a high intake of omega-3 fats and an increased risk of developing aggressive prostate cancer.

Verdict: If heart disease runs in your family, taking a daily supplement of fish oil may be a wise thing.

Alcohol

Hendrikje van Andel-Schipper of the Netherlands lived to 115. She put her longevity down to a pig fat and vodka. Research published in PLoS Medicine earlier this month found that women who drink in moderation are far more likely to reach 70 in good health than heavy drinkers or teetotallers.

Another study, published this month in Atherosclerosis, found that daily moderate drinking – two drinks a day, seven days a week – decreased atherosclerosis in mice. If only the story ended there. Unfortunately, alcohol is also implicated in cancer. In the body, alcohol is converted into a toxic chemical called acetaldehyde, which can damage DNA.

Verdict: There are many health benefits associated with moderate alcohol consumption. The problem is, one glass of red wine a night can easily become two or three.

Fruit juice

A study found that drinking one glass of orange juice a day can increase a person's risk of getting type-2 diabetes by 18 per cent. And the Eastman Institute for Oral Health published a report in 2009 saying that orange juice is more damaging to teeth than over-the-counter whitening products. "The acid is so strong that the tooth is literally washed away," Yan Fang Ren, the leader of the study, says. But researchers at California Davis found that drinking 100 per cent juice reduced the risk for several chronic diseases, including cancer.

Verdict: Water is for drinking. Fruit is for eating.