"This report, while being an observational study, clearly shows that circulating vitamin D levels are lower in patients with advanced breast cancer than in those with early breast cancer," wrote lead author Dr Carlo Palmieri in the Journal of Clinical Pathology (doi.10.1136/jcp.2006.042747).
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.
This observation adds to an ever-growing body of evidence linking vitamin D status with incidence and risk of various cancers, including breast, colorectal and prostate. Indeed, the link between vitamin D intake and protection from cancer is not and dates from the 1940s when Frank Apperly demonstrated a link between latitude and deaths from cancer, and suggested that sunlight gave "a relative cancer immunity."
The authors of the new study recruited 279 women with invasive breast cancer. The disease was in its early stages in 204 women, and advanced in the other 75.
Measuring serum levels of 25-hydroxyvitamin D, the researchers report that women with early stage disease had significantly higher levels of vitamin D (15 to 184 millimoles per litre) than the women in the advanced stages of the disease (16 to 146 millimoles per litre).
Levels of parathyroid hormone were also significantly lower in the early stage breast cancer patients than women with advanced disease, but no difference in serum calcium levels was observed.
Palmieri and his co-workers said that the exact reasons for the disparity was not clear, nor is it known whether the low levels of vitamin D among those with advanced disease are a cause or consequence of the cancer itself, with previous studies reporting that vitamin D sufficiency and exposure to sunlight may reduce the risk of developing breast cancer.
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 next step in this research is to try and understand the potential causes and mechanisms underlying these differences and the precise consequences at a molecular level," said Palmieri.
"We also need to look at the potential clinical implications of monitoring and maintaining high circulating vitamin D levels in breast cancer patients," he said.
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.