Over one million women worldwide are diagnosed with breast cancer every year, with the highest incidences in the US and the Netherlands. China has the lowest incidence and mortality rate of the disease.
The National Cancer Institute estimates that 13 percent of American women will develop breast cancer during their lives.
Two studies were presented at the meeting, the first (abstract number 4008) by Cedric Garland and Edward Gorham from the University of California, San Diego, reports the results of serum vitamin D levels and incidence of breast cancer for a pool of 1,760 women.
The analysis showed that a serum vitamin D level of 52 nanograms per milliliter was associated with a 50 percent reduction in breast cancer risk. To have such a serum vitamn level would require a daily intake of about 1,000 International Units (IU). The current recommended daily intake in the US is 400 IU.
Both forms of the vitamin, D2 and D3, are hydroxylated in the liver and kidneys to form 25-hydroxyvitamin D, the non-active 'storage' form, and 1,25-dihydroxyvitamin D, the biologically active form that is tightly controlled by the body. Scientists use serum 25- hydroxyvitamin D levels as a measure of vitamin D status.
"There is a strong inverse dose-response relationship between the serum concentration of 25-hydroxyvitamin D and the risk of breast cancer. It's a close fit to a linear model," said Garland.
"There is no substantial downside to a serum level of 52 nanograms per milliliter of vitamin D," explained Gorham. "Such levels are common in sunny climates. There is no known adverse effect of serum levels below 160 nanograms per milliliter."
The scientists echoed calls by other researchers to increase the daily recommended intake of vitamin D3 to 1000 IU from fortified foods and supplements.
Consumption of vitamin D in a diet, without fortified foods or supplements is difficult since no food is naturally rich in vitamin D. Most vitamin D is made in the skin on exposure to sunlight, but some campaigners have advised against too much sun due to increased risk of skin cancer.
The second paper (abstract number 4009) by researchers at the Samuel Lunenfeld Research Institute at Mount Sinai Hospital in Toronto, reports that women who have the highest intake of vitamin D between the ages of 10 and 29, the age range of breast development, are estimated to reduce their risk of breast cancer by 40 percent.
The preliminary findings by Julie Knight and her colleagues are based on interviews of 576 breast cancer patients and 1,135 healthy controls.
The scientists found that women who had worked in an outdoor job, been active outdoors, or consumed cod liver oil or milk during this age range had significantly lower risks of developing breast cancer.
"These outdoor activities included those that didn't involve physical activity and so we believe that this is evidence of a reduction of breast cancer risk, associated with earlier exposure to the sun," said Knight.
Cod liver oil consumption was associated with a 25 percent reduction in the risk, while drinking at least nine glasses of milk every week was associated with a 35 percent reduction.
"What you are exposed to during breast development may be particularly important in determining future breast cancer risk. Current thinking is that exposures during adolescence or before a full-term pregnancy may have a greater effect, as that is when breast tissue is going through the most rapid development," said Knight.
UK researchers recently proposed 10 to 15 minutes of unprotected exposure to the midday sun as a good source of the vitamin. In the US, where over 1.5 million people are diagnosed with skin cancer every year, experts are pushing supplements, claiming recommendations for sun exposure are "highly irresponsible".
Recent studies have shown that sunshine levels in some northern countries are so weak during the winter months that the body makes no vitamin D at all, leading some to estimate that over half of the population in such countries have insufficient or deficient levels of the vitamin.