The natural form of vitamin D is either produced by exposure to sunlight or taken in the diet by eating oily fish, meat or foods to which it has been added artificially.
For thousands of years human skin evolved so that people could absorb enough sunlight to make vitamin D, which strengthens bones. More than 63 reputable scientific studies have shown that vitamin D reduces the risk of developing prostate cancer (especially the more malignant forms) and breast, ovarian and colon cancers, as well as preventing weak bones. Although sun in excess increases the risk of skin cancers the higher the vitamin D levels the lower the incidence of many other malignancies.
However, modern life has upset nature’s delicate balance. The skin that was honed to protect someone from sunburn and skin cancers, not just melanomas but also epitheliomas and rodent ulcers, is now having to cope with life in northern climates where the benefit of sunlight is seasonal. Even if the Sun has not been obscured by low clouds and drizzle it is not effective at stimulating the body to manufacture vitamin D in winter once it is low in the sky. In Britain from October to May the Sun’s rays are filtered by the atmosphere and little vitamin D is manufactured.
It is already known that people with dark skin, if they move away from tropical and equatorial areas, need 30 times as much exposure to the Sun as do the fair-skinned people whose vitamin D manufacturing process has evolved so as to muddle through dank, dark winters. A dark-skinned person would have to drink ten tumblers of milk a day, an impossible and undesirable intention, or take generous helpings of oily fish daily to compensate for the lack of sun. Similarly, fair-skinned, blue-eyed, red or blond-haired northern Europeans are at risk of developing skin cancers if they burn in the sun, especially as children or young adults, as their skin did not evolve to withstand tropical sunrays.
Source - Times