Women who stay out of the sun are increasing their risk of developing breast cancer, a new study suggests.

The safe-tanning messages that are drummed into women each year may help to reduce their risk of skin cancer – but at the cost of increasing their risk of breast cancer.

The majority of vitamin D comes from exposure of the skin to sunlight but many women – exposed less in winter and reluctant to bare themselves in summer because of the dangers – are deficient.

There has been anecdotal evidence to suggest that breast cancer is less common among women who live closer to the Equator, where sunshine is stronger.

However, a new study provides evidence that the lower the levels of vitamin D in a woman’s blood-stream, the greater the risk of her developing breast cancer if she has passed menopause.

Of more than 1,000 women who took part in a trial, those who were given both calcium and vitamin D supplements had less than half the chance of developing breast cancer than those given a placebo (13 cases among 446 women compared with 20 cases among 288 women.

A team from Creighton University in Omaha, Nebraska, enrolled 1,179 women all 55 or older, who had no history of cancer. The women were divided randomly into groups and given either supplements of calcium alone, calcium plus vitamin D, or a placebo for four years.

The researchers were interested primarily in the risk of the women suffering from osteoporosis, but they also looked at cancer risks.

The team, whose study was published in the American Journal of Clinical Nutrition, found that calcium alone also had a protective effect against cancer but it was not as strong.

When the researchers repeated the analysis for those women who were free of cancer after the first year of the study, the results were even more striking. By doing this, the team excluded any cases that would have been present, but undetected, before the trial began.

In this second analysis, the risks were reduced by more than three quarters.

“Our findings of decreased all-cancer risk with improved vitamin D status are consistent with a large and still growing body of epidemiologic and observational data showing that cancer risk, cancer mortality, or both are inversely associated with solar exposure, vitamin D status, or both,” the researchers said.

The findings underscored the value of achieving and maintaining a high concentration of the vitamin, they added.

The Creighton University study follows one published in May in Archives of Internal Medicine that reached similar conclusions.

This earlier one used data from the Nurses’ Study at Harvard, which followed more than 30,000 women for up to 15 years. Their dietary intake of both calcium and vitamin D was calculated from dietary questionnaires. The team, from Brigham and Women’s Hospital and Harvard Medical School, found that high levels of the two nutrients were linked with a 40 percent lower risk of breast cancer in younger women.

The difference was more marked for aggressive cancers. But this study, unlike the Creighton trial, did not find a link among older women.

“Findings from this study suggest that higher intakes of calcium and vitamin D may be associated with a lower risk of developing premenopausal breast cancer,” the authors concluded.

Vitamin D is present in foods including milk, eggs, oily fish, green vegetables and fortified margarines. But a significant part of the vitamin D need is manufactured in the skin by exposure to sunlight.

Earlier studies have linked high levels of vitamin D to reduced risks of other cancers, including of the colon and prostate. But the advice of Cancer Research UK has long been that the risk of skin cancer from overexposure to the sun exceeds benefits achieved through higher vitamin D status.

Not all experts agree. Cedric Garland, of the University of California at San Diego, claimed in the British Medical Journal in 2003 that sun avoidance would increase the risk of cancers overall, especially among those who live at latitudes as far north as Britain.

He recommended 10-15 minutes a day of sun exposure, without sunscreen, to allow adequate synthesis of vitamin D. But this alone is not enough, he suggested, because vitamin D is not stored for long in the body and there is not enough sun during the winter to synthesise it.

He therefore recommended the use of supplements, as in the new trial, to boost levels. But the risks from overdosing are such that these must be taken with caution.