Sunshine, sunlight

A suntan became attractive in the 1950s.  Before then, It was seen as a sign simply that the tanned person had worked a lot out of doors or, possibly, as a European, had been employed in tropical countries.  By the 1970s evidence that the sun's rays caused skin cancers began to emerge - specifically, that ultra violet radiation damages DNA in skin cells, leading to three sorts of cancers, including the deadliest form, malignant melanoma (1).

More recent epidemiological research indicates that avoiding sunlight actually increases our risk of colon, prostate, breast, lung and even skin cancers(1).  William Grant concluded that every year at least 23,600 Americans die of cancer through lack of sunshine while about 9,800 die from skin cancer (2).  Grant has more recently revised the number of people dying from inadequate sunlight to 40,000 (1).

In 2000 Rob Faigin reviewed the available medical, physiological and epidemiological research and came to similar conclusions.  He was also able to draw out the practical implications for human health beyond cancer (3).

Physiology
They key link between sunlight and human health is through vitamin D.  The body has two sources of this important chemical: foods such as oily fish and dairy products, and skin cells, which use those same cancer-causing UV rays to convert a precursor chemical into vitamin D₃.  Further processing in the liver and kidneys results in 'active vitamin D'.  Active vitamin D's best-known function is enabling calcium to be absorbed from the gut.  Calcium plays a key role in nerve, muscle and metabolic signaling, as well as cementing bone cells together.  (That is why the most obvious sign of vitamin D deficiency is rickets, the weak bone disease.)  Research since the 1980s has pointed to active vitamin D being an important brake on cell proliferation - so a lack of it could dispose people to cancer, the result of cells multiplying out of control (1).

Rickets appears to be making a comeback in the US (1, 3).  The problem is seen with breastfed infants, as formula milk is fortified with vitamins.  US doctors advise that all breastfed infants should be given vitamin D drops.  However, there may even be benefits from sunshine that are not available from vitamin D pills.  This complements the accepted idea that people who use vitamin supplements instead of eating healthily miss out on hundreds of the beneficial compounds in fruit and vegetables (1).

How much sun exposure?
The 'RDA' for sunlight varies greatly, depending on:
•  your latitude (and altitude)
•  the time of year
•  your skin colour
•  the time of day
This works out at one-quarter of the exposure that would cause redness, two or three times a week.  These variables have been included in Michael Holick's book The UV Advantage, published early in 2004 (1).

Chemical sunscreens
Public health research in Denmark in 2001 has led there to restrictions being placed on sunscreens, particularly when used for children, where they contain hormone disruptors - and they usually do.

References:
1   New Scientist, 9 August 2003, pages 30-33
2   Cancer, 2002, vol 94, p 1867
3   Rob Faigin, Natural Hormonal Enhancement, 2000

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