NINE MINUTE stroll in the lunchtime sun that may be key to better health

We all know the rules, drummed i nto us by cancer charities every summer: stay out of the sun in the middle of the day, slap on high-factor sunscreen and don’t overdo the sunbathing.

But according to a study by U.S. scientists published last week, in countries with ‘low solar intensity’ — such as Canada and the UK — ‘the current policy of sun avoidance is creating probable harm for the general population’.

Furthermore, the time we’re meant to avoid the sun — between 11am and 3pm — is precisely when the skin can best synthesise vitamin D in our bodies, they write in the Journal of the american College of Nutrition.

This echoes concerns of some critics, who say the UK’s sun protection advice is wrongly based on data from australia, where the sun is much fiercer.

Vitamin D is essential for health because it helps our bodies absorb the calcium needed for strong bones. Without it, bones can become soft (causing rickets in children) or, as we get older, brittle (causing osteoporosis).

But while we can easily get all the calcium we need from our diet, vitamin D is much harder to come by in food. luckily, our bodies can make enough — if our skin is exposed to sunlight.

The amount of ultra-violet (UV) rays reaching us varies considerably across the UK, depending on the angle of the sun’s elevation. Directly overhead, the rays have a much shorter distance to travel to earth. But when the sun is low in the sky — in the morning, evening or winter — its light has farther to travel through the atmosphere, which absorbs much of the rays before they reach us.

This is why the hours either side of midday are the best time to make vitamin D, as UV rays are at their strongest.

In fact, as the U.S. scientists report, ‘moderate sun exposure [during these hours] — less than the time required to burn — to arms, shoulders, trunk and legs should be There sought rather than avoided’.

is, admits anjali Mahto, a consultant dermatologist at Northwick Park hospital, london, and spokeswoman for the British Skin Foundation, a contradiction between the need to maximise vitamin D and minimise sun harm, complicated by the fact that nobody actually knows how much vitamin D we need.

regardless of this, she says, ‘the amount of time the skin needs to make vitamin D isn’t that long’. It varies, she says, between skin types and racial background — ‘but on the whole, the time taken to burn is longer than the time it takes your skin to make vitamin D’.

Yet even the slightest risk of burning is now anathema to most people, and the figures seem to justify our fears. Over the past decade the number of deaths from malignant melanoma has risen by 33 per cent. This suggests that many people still shun advice about sun protection.

But in 2013 most malignant melanoma deaths were in those aged 60 to 80. What we could be seeing, says David eedy, president of the British association of Dermatologists, are ‘the consequences of sun exposure in the Seventies and eighties, when package holidays became popular’.

So skin cancer cases could be reaching a peak. Meanwhile, tens of millions of Britons don’t get anything like enough vitamin D.

an astonishing 40 per cent have too little vitamin D in their bodies in winter, according to the latest report from the National Diet and Nutrition Survey. Worse, almost a quarter of adults aged 19 to 64 and children between 11 and 18 have less than the recommended minimum levels throughout the year.

‘For people living in latitudes like ours there is a seasonal cycle to vitamin D levels,’ says Professor ann Webb, who lectures in atmospheric physics at t he University of Manchester. ‘But of course, what people get depends on whether they go outside and expose their skin.’

Professor Webb and colleagues have produced a map of ultraviolet l i ght l evels i n the UK. Their conclusion? ‘That from some time in October to some time in March there’s not enough UV for it to be worth your while trying to get exposure anywhere,’ says Professor Webb.

Yet there is evidence that we can build up stores of vitamin D to tide us over — providing we expose enough skin, for long enough, in spring and summer.

But what is ‘enough’ vitamin D? Your levels can be measured by a blood test; current UK guidance is t hat a concentration of 25 nanomoles per litre (nmol/l) or less is too little. In the U.S. the Institute of Medicine regards 30 nmol/l as the minimum safe level for bone health, while other organisations put it at 70 or higher.

One clue to our true needs, say the authors of the new report, lies in studies of the Masai and hadza tribes, who live on or near the equator in east africa.

humans originated there, so tribespeople have ‘daily sun exposure approximating that of ancestral humans’ and their levels of vitamin D are about 114 nmol/l.

In fact, to get through the winter months we should aim to end the summer with a reading of about 100 nmol/l, says Kassim Javaid, a consultant in metabolic medicine at Oxford’s Nuffield Orthopaedic Centre and spokesman for the National Osteoporosis Society.

This was demonstrated by an australian study of 120 people who spent a year in antarctica, where they were deprived of sunlight from March to august.

Those who arrived with normal vitamin D levels were deficient within four months; those with levels twice as high — 100 nmol/l or more — had enough in their systems to last until the sun returned, reports the journal Osteoporosis International.

The key to building sufficient stocks, says Professor Webb, is finding the right level of exposure for your skin, while avoiding burning. ‘The most efficient way to make vitamin D is with short regular exposures — a little bit every day,’ she says. a short lunchtime stroll should do the trick ( without sunscreen or SPF face creams).

Professor Webb and colleagues demonstrated the ‘little and often’ principle with an experiment involving 109 caucasians from Manchester.

They were exposed to short bursts of ‘simulated summer sunlight’ — nine minutes every weekday and 18 minutes on Saturday and Sunday — during winter, while wearing shorts and T-shirts.

This ‘significantly raised vitamin D levels’ to a level sufficient to get them through winter.

and the more skin you expose, the better, says Professor Webb, as you can then either make more vitamin D in a given time, or reduce your time in the strong sun.

however, she found that the longer you stay in the sun in any one session, the less efficient your body gets at making vitamin D: ‘little and often is best. after that, instead of maximising the benefit, BONe you maximise the damage.’

health may not be all we risk by staying out of the sun. The Scientific advisory Committee on Nutrition is reviewing evidence linking disease to vitamin D deficiency, and among those awaiting its conclusions is Oliver Gillie, a scientist and former journalist who has long lobbied for action on vitamin D.

Three years ago, Gillie was diagnosed with chronic lymphocytic leukaemia (Cll), a condition that research has now suggested is linked to low levels of vitamin D. Gillie, 77, blames years of following official advice to avoid the sun.

he believes that for too long sun protection advice in the UK was influenced by studies in australia and parts of the U.S. ‘With hindsight,’ he says, ‘that was unwise.’