Should we all get more sun?

Fear of melanoma is enough to prevent many of us from soaking up the rays. But we may be more prone to other diseases if we don't get a short blast of vitamin D every day, says Jerome Burne

Here's a really healthy tip: if you want to lower your risk of breast, prostate and colon cancer, as well as a host of other disorders - diabetes, osteoporosis, hypertension and multiple sclerosis - go out in the sun. Our demonising, factor-40 approach to the sun may well be making us ill. Over the past 20 years, two epidemiologists have made a series of remarkable, and largely ignored, discoveries that links your risk of developing certain disorders with how far from the Equator you live.

Here's a really healthy tip: if you want to lower your risk of breast, prostate and colon cancer, as well as a host of other disorders - diabetes, osteoporosis, hypertension and multiple sclerosis - go out in the sun. Our demonising, factor-40 approach to the sun may well be making us ill. Over the past 20 years, two epidemiologists have made a series of remarkable, and largely ignored, discoveries that links your risk of developing certain disorders with how far from the Equator you live.

In June, Michael Holick, a leading researcher into vitamins and a professor of medicine, dermatology, physiology and biophysics at Boston University School of Medicine, commented that our current fear of the sun might be damaging more people than it saves from melanoma. His new book, The UV Advantage (Prima Lifestyles), due out in the autumn, claims that a relatively brief exposure to sunshine several times a week can ward off a remarkable range of diseases by boosting vitamin D levels. We may all need much more of this vitamin than we are getting at the moment.

The sunshine hypothesis was first put forward by two brothers, Dr Cedric and Dr Frank Garland, both epidemiologists, who noticed in the late 1970s that the National Cancer Institute maps revealed a striking geographic distribution of deaths from colon cancer in America - they were significantly lower in the sunny south-west and higher in the industrialised north-east. The transition point seems to be around the 42nd parallel, which passes through Boston and Chicago - London is 51.5 degrees north. The evidence pointed to the fact that the lower level of sunshine for half the year in the north effectively prevents any syntheses of vitamin D during winter months.

Then, last year, a study in a leading journal made the startling and controversial claim: "The annual number of premature deaths from cancer due to lower UVB exposure was 21,700 for white Americans." The cancers involved included breast, colon, ovary and prostate. "Differences in dietary or smoking habits", the paper concluded, "do not appear to explain the geographic variation in cancer rates in USA."

But it's not only cancers that are involved. The same pattern suggesting a benefit from regular exposure to sunlight has shown up for autoimmune conditions such as multiple sclerosis and rheumatoid arthritis. A study by the Institute of Child Health in London, involving 10,000 children, found that taking vitamin D supplements reduced the risk of insulin-dependent type 1 diabetes by 88 per cent. Women with osteoporosis, also linked with low vitamin D, are more likely to die of cardiovascular disease. Exactly why is not yet clear.

None of these connections makes much sense in terms of the conventional idea of vitamin D, whose job description is essentially to maintain calcium levels and protect our bones. We can get it from some foods - for instance, oily fish such as salmon and mackerel - but the main source is sunlight. The body makes it in the skin in response to ultraviolet B (UVB), and it goes through several stages. The two that are relevant to our story are 25(OH)D - the storage form that is used as a measure of vitamin D levels - and calcitriol (1,25(OH)2D), the active form.

However, the role of vitamin D in preventing cancer begins to make sense in light of two findings. The first is that, while scientists used to think that calcitriol was only made in the kidneys, research by Professor Holick and others has shown that prostate, colon and breast cells can also manufacture it. "The function of vitamin D is to be a modulator of cellular growth," says Holick, "preventing cells from being too active". In other words, it is important for maintaining normal cell growth and possibly for preventing cancer. Gary Schwartz, an epidemiologist at Wake Forest University in the US, has shown that prostate cells make calcitriol. "It's enough to inhibit cell growth," he says. "Male hormones are the gas pedal driving prostate growth, while vitamin D is the brake."

Animal studies have shown that combining synthetic versions of vitamin D with chemotherapy can boost the effectiveness of cancer treatments for "breast type" tumours by 50 per cent. Researchers say they need to use synthetics because high levels of vitamin D are dangerous and can "induce excessive calcium that can affect bone metabolism". But this raises the tricky and controversial question of just how much we should be getting and how much is safe?

The generally accepted RDA is between 200 and 400 IU (or international units, by which Vitamin D is officially measured), but that is based more on custom than on any hard figures. In fact, a number of researchers now believe that we need much more. The conventional advice is to keep your supplement intake to no more than 600 IU per day, which is designed to keep your blood levels at 27.5nmol/L (nanomoles per litre) without any more from sunlight. The risks of overdosing - more than 2,000 IU per day - are said to include increased thirst, nausea, vomiting and the deposit of calcium in blood vessel walls and the kidneys where it can cause serious damage.

But just how real is this danger? Contrast this figure with the amount of vitamin D that we can all produce just by going out in the sun. "An adult with white skin, exposed to summer sunshine while wearing a bathing suit", writes Reinhold Vieth, of the Department of Laboratory Medicine and Pathobiology at the University of Toronto, "generates about 10,000 IU of vitamin D3 in 15 to 20 minutes. That amount is equivalent to the vitamin D in 25 conventional multivitamin pills."

Two points about this are interesting. First, that the process is self-limiting - in other words, staying out in the sun for longer doesn't push the level up any further. Second, that this sort of exposure pushes your blood level of vitamin D up to a whopping 235 nmol/L, nearly 10 times the amount the RDA aims at.

While even vitamin-D campaigners such as Vieth and Holick aren't suggesting that people start supplementing at the 10,000 level, they do agree that an amount of about 1,000 IU might be appropriate for the general population, with more for those at particular risk of low levels. However, some researchers believe that 2,000 IU might be a more appropriate amount as that would give you a blood level of around 100 nmol/L, closer to the amount our hunter-gatherer ancestors would have had living outdoors all day in lower latitudes. If the radicals are right, many of the "diseases of affluence" may be a variation on rickets - the deficiency disease that stunted the bones of the children of poor families during the industrial revolution.

It is worth stressing that this is not mainstream thinking and more research does need to be done. The radicals also accept the dangers of skin cancer and certainly don't advocate throwing away the sun lotion or abandoning sun hats. "The current recommended level is enough to prevent rickets but not enough for optimal health," says Dr Robert Heaney, an osteoporosis expert at Creighton University, who takes 1,000 IU a day himself.

Professor Holick points out that since 90 to 95 per cent of most people's vitamin D comes from casual exposure to sunlight, the best way to raise our blood levels is to sensibly increase our time in the sun. His advice is to expose unprotected skin to sunlight for a few minutes every day.

The length of time depends on a variety of factors, such as what sort of skin you have - the average African-American needs five to 10 times as long in the sun as a Caucasian - what time of the year it is and how far from the Equator you are. Groups particularly at risk include those whose culture requires the covering-up of all exposed skin, and the elderly - your ability to synthesise vitamin D will have halved by the time you are 60.

A simple rule of thumb is: if you have a typical white person's skin, known as type 2, and you live above the 42nd line of latitude, then you should try to be outside for five to 10 minutes between 11am and 3pm during five months of summer. That way you'll build up enough to last through winter. Then, if you are going to stay out in the sun, you can cover up as usual if you feel you need to.

A longer version of this article appears in the August edition of the monthly newsletter 'Medicine Today' ( www.medicine-today.co.uk)