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Vitamin D linked to breast cancer.Roger Hallett/The Globe and Mail

Could a drop in vitamin D levels circulating in a woman's bloodstream predispose her to breast cancer? Some U.S. researchers believe the answer to this question can be found in blood samples collected and stored by the American military.

In total, nine million vials of blood were drawn from members of the U.S. armed forces from 2002 to 2008 as part of a routine monitoring program for infectious diseases. The samples were usually collected every two years.

As might be expected, much of this blood is from men. But a considerable amount – about 900,000 samples – came from female recruits, providing a unique opportunity to examine vitamin D levels in some women before they were diagnosed with cancer.

"I must say the DOD [Department of Defence] had the wisdom to store the samples in freezers so we are able to study them today," said Dr. Cedric Garland, a professor in the department of family and preventive medicine at the University of California, San Diego.

Garland and his colleagues were given access to samples that included blood from about 600 premenopausal women who were diagnosed with breast cancer. In some cases, the blood had been drawn just months before they learned that they had cancer.

The researchers used the samples to measure the amount of vitamin D circulating in the women with breast cancer. These figures were compared with the vitamin D levels of 600 healthy female military recruits who served as a control group.

The analysis, published in the journal Cancer Causes & Control, revealed a trend in which reduced vitamin D levels are associated with a higher risk of cancer, according to Garland. "The most important thing that comes out of this study, in our minds, is that while this is only a trend, over all it was an extremely strong effect during the 90 days preceding the diagnosis of breast cancer," he said.

In other words, a big drop in vitamin D for just a short time could allow some long-simmering tumours an opportunity to take off. And in a matter of months, a tiny inconspicuous tumour can swell to the size of an olive – making it easier to detect in a physical exam or a mammogram.

Garland noted that a cancer may be very slow-growing initially, but eventually the tumour reaches a critical stage when it must tap into a bigger blood supply so that it has the resources to expand exponentially. If it fails to do so, "it goes nowhere – it just burns out or turns into scar tissue," he said.

He is convinced that vitamin D somehow plays a key role in keeping these tumours in check. "Women who were down to 11 nanograms [of vitamin D] per millilitre of blood had double the risk of breast cancer compared to women who were around 48 nanograms per millilitre."

Based on this finding, Garland thinks that women should try to keep their vitamin levels above 48 nanogams per ml. (A lab test is required to get this measurement.) For some patients, that could mean popping supplements containing up to 4,000 international units a day – Health Canada's upper daily limit.

Gillian Bromfield, director of cancer-control policy at the Canadian Cancer Society, has reviewed the study by Garland and his colleagues, and her assessment is that the evidence is not strong enough to make a recommendation of as high 4,000 IUs a day to guard against breast cancer.

Vitamin D is produced naturally by the body when skin is exposed to sunlight. Most Canadians meet their vitamin D needs in the spring and summer simply by going outside. But it's a different story the other half of the year, when the sun's rays are much weaker.

The cancer society recommends that Canadian adults take a daily supplement of 1,000 IU of vitamin D during the fall and winter. The elderly, people with darker skin or those who do not get sun exposure for other reasons may need a supplement year round.

"The recommendation for Canadians to consider taking a vitamin D supplement to possibly reduce their risk of cancer is based on a large body of research showing a possible relationship between low vitamin D intake and increased cancer risk, particularly for colorectal and breast cancers," Bromfield said.

She pointed to the results of a "promising" trial, published in the American Journal of Clinical Nutrition, that indicated a dose of 1,100 IU a day appears to be "clinically significant." Aside from supplements, she noted that most Canadians also ingest some vitamin D from other dietary sources, such as milk and certain types of fatty fish.

Meanwhile, Garland hopes that other studies with eventually confirm the results of his research and show that high doses of vitamin D – 4,000 IUs daily – could help cut the breast cancer risk in half for premenopausal women. "Science is strengthened by replication," he said.