Sunlight can reduce a man's risk of prostate cancer, a study suggests.
Exposure to the sun has both benefits and risks
Researchers from three US centres found men exposed to a high amount of sun had half the risk of the disease than those exposed to a low amount.
Writing in Cancer Research, they suggest that the protection was a result of the body's manufacture of vitamin D after sun exposure.
But men were warned not to sunbathe excessively because of the risk of developing skin cancer.
Vitamin D is also found in foods such as oily fish.
Experts from the Northern California Cancer Center, the Keck School of Medicine of the University of Southern California, and the Comprehensive Cancer Center of Wake Forest University all worked on the study.
They looked at 450 white patients aged 40 to 79 with advance prostate cancer, from San Francisco.
They compared them with a group of 455 men of similar ages and backgrounds who did not have prostate cancer.
The men were all asked whether their jobs had involved working outside, and if so, how regularly they did this.
The scientists also looked at the difference between pigmentation in underarm skin which is usually not exposed to sunlight, and forehead skin, which is.
To do this, they used a portable reflectometer - a device which measures skin tone by emitting light and assessing the amount that is reflected back, giving a reading on the colour of the skin from 0 to 100.
The difference in scores taken from underarms and foreheads provided an indication of how much exposure to the sun men had experienced.
The risk of prostate cancer was found to be halved in men who had the highest amount of sun exposure - an average of 20 hours a week, or more.
Previous research has shown that the prostate uses vitamin D to promote the normal growth of prostate cells and to inhibit the invasiveness and spread of cancer cells to other parts of the body.
Genes determine how the body processes vitamin D. They control receptors which vary in their ability to bind to the vitamin and therefore influence the behaviour of cells.
DNA tests carried out by the researchers showed the risk of prostate cancer was reduced by up to 65% in men with certain gene variants.
Dr Esther John, of the Northern California Cancer Centre, who led the research, said: "We believe that sunlight helps to reduce the risk of prostate cancer because the body manufactures the active form of vitamin D from exposure to sunlight."
She added that if future studies continued to suggest this link, increasing vitamin D intake from food and supplements might be the safest solution to achieve the right levels.
Chris Hiley, head of policy and research at the Prostate Cancer Charity, warned that while increased exposure to sunlight might decrease the risk of prostate cancer, it also increased the risk of skin cancer.
"Men need more evidence-based research to know how to balance the risks and benefits."
Henry Scowcroft, of Cancer Research UK, also cautioned that more work was needed to weigh up the risks involved.
"For most people, it usually takes just a few minutes of sun exposure for your skin to make a very large amount of vitamin D," he added.