A mdoest amount of sunlight may protect against cancer, according to scientists who have found a link between sunshine and a substance produced naturally in the body which may lower the risk of developing tumours in later life.

Michael Holick, Professor of Medicine at Boston University School of Medicine, has identified a critical enzyme involved in the production of vitamin D in the skin that appears to protect against colon, breast and prostate cancer. "The presence of this enzyme in the colon supports the notion that vitamin D may have a key role in cell growth regulation and cancer protection," Professor Holick told the American Association for the Advancement of Science meeting in Boston.

Vitamin D, which is produced by the skin in the presence of sunlight, is essential for bone development. Professor Holick said that the active form of the vitamin also prevents colon cells from proliferating uncontrollably by stimulating them to become fully mature. He has found evidence that the same process occurs in breast, skin and prostate cells.

These findings may explain why the chances of dying of colon, breast and prostate cancer are greater for people living in colder, northern climates, who are less able to produce enough quantities of vitamin D because of a lack of year-round sunshine, he said.

Studies in the US have shown that death rates from breast, colon and ovarian cancer in Boston and New England were about twice what they were in the sunny southwest of America between 1950 and 1994.

However, over the same period it is well established that rates of skin cancer have soared for northern Europeans because of over-exposure to the sun during hot Mediterranean holidays. Professor Holick said that his research only indicated a benefit from moderate amounts of sunshine at times of the day when the sun was weakest.