It's definitely a puzzle to scientists, but a new study has suggested that the main cause of skin cancer - sunlight - may also help protect against the deadly disease as well as other malignant tumours.

A team of international researchers has carried out the study and found moderately increased exposure to sunlight not only reduces the risk of skin cancer but can also prevent breast, colon and lung cancers.

"Since vitamin D has been shown to play a protective role in a number of internal cancers and possibly a range of other diseases, it is important to study the relative risks to determine whether advice to avoid sun exposure may be causing more harm than good in some populations," according to lead researcher Richard Setlow of Brookhaven National Laboratory.

According to the researchers, solar radiation is a major, if not the main, source of vitamin D in humans. In the presence of sunlight, the body converts certain precursor chemicals to active vitamin D.

But, the researchers have cautioned that too much exposure to sunlight can cause other skin diseases. "As far as skin cancer goes, we need to be most worried about melanoma, a serious disease with significant mortality," Setlow was quoted by the ScienceDaily as saying.

For the study, Setlow and his colleagues in the United States and Norway used a special model to calculate the relative production of vitamin D via sunlight on a group of people depending on how far they live from the Equator.