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Figure  . Preliminary research suggests breast cancer patients with a vitamin D deficiency fare worse than those with adequate levels of the vitamin.

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Recent research suggests breast cancer patients might fare worse if they suffer from vitamin D deficiency. These results were presented in June at the 2008 annual meeting of the American Society of Clinical Oncology by lead researcher Pamela Goodwin, MD, Professor of Medicine at the University of Toronto and a Senior Investigator at the Samuel Lunenfeld Research Institute at Toronto's Mount Sinai Hospital (J Clin Oncol 26:2008 [May 20 suppl; abstr 511]).

Goodwin and her colleagues measured vitamin 25-OH D levels in the blood of 512 newly-diagnosed breast cancer patients and tracked the progress of their disease over an average of about 12 years. At the beginning of the study, 37.5% of the women had vitamin D levels low enough to be considered “deficient” (<50 nmol/L), and 38.5% had levels that were “insufficient” (50 to 72 nmol/L). Just 24% of the women in the study had “adequate” vitamin D levels (>72 nmol/L).

Compared with women with adequate vitamin D, women with deficient levels had significantly worse distant disease-free survival (hazard ratio [HR] 1.94, P = .02) and overall survival (HR 1.73, = .02). The prognostic significance of vitamin D levels was independent of patient age or weight, tumor stage, or tumor grade.

African American women, who tend to have a poorer prognosis when diagnosed with breast cancer, also tend to have lower levels of vitamin D than White women because darker pigmentation hampers vitamin D synthesis by the skin. Because 95% of the subjects in this study were White, the researchers were not able to investigate the effect of race on the relationship between vitamin D and prognosis.

“The potential for vitamin D to reduce the risk of many chronic diseases, including cancer, heart disease, and multiple sclerosis, is an active and exciting area of research,” says Marji McCullough, ScD, RD, Strategic Director of Nutritional Epidemiology at the American Cancer Society. “If vitamin D is ultimately proven effective, vitamin D supplements would represent an inexpensive and widely available method to improve survival among breast cancer patients.”

In addition to supplements and sun exposure, other sources of vitamin D include fortified milk, orange juice, and cereals. Small amounts occur naturally in oily fish (salmon, tuna, and mackerel), beef liver, cheese, egg yolks, and some mushrooms.

For now, Goodwin cautions that these findings are preliminary and need to be repeated in other studies before firm conclusions about vitamin D and breast cancer can be drawn.

“In order to address whether [improving] vitamin D [status] might change outcomes in breast cancer, we would need a randomized trial,” she says.

Julie Gralow, MD, who moderated an American Society of Clinical Oncology press conference where the study was discussed, agreed. “We have no idea whether correcting vitamin D deficiency will in any way alter these outcomes,” says Gralow, an Associate Professor of Medical Oncology at the University of Washington and the Fred Hutchinson Cancer Research Center.

In advising caution, Goodwin noted that promising findings from observational studies of vitamins often do not carry over when the compounds are tested in clinical trials. For instance, beta carotene once seemed a promising preventive for lung cancer until randomized controlled trials showed it actually raised risk rather than reducing it.

“We don't always know the full biology of vitamin D as it relates to cancer,” Goodwin says. “I would not want my research to lead to women taking huge doses of vitamin D and causing themselves harm.”

Excessive amounts of vitamin D can cause nausea, vomiting, and weakness and can raise blood levels of calcium enough to cause mental confusion and heart rhythm abnormalities, as well as calcium deposits in the kidneys and other tissue. Goodwin's results also suggested that high levels of vitamin D (>120 nmol/L) might actually increase the risk of death in women with breast cancer. However, this trend was not statistically significant, and Goodwin says this question needs to be researched further in a larger study.

Until more is known, Goodwin says she is recommending her patients take the amount of vitamin D currently recommended for bone health. The US Institute of Medicine recommends 200 IU (international units) daily for women aged up to 50 years, 400 IU for women aged 51 to 70 years, and 600 IU for women aged 71 years and older. The agency also recommends intake not exceed 2,000 IU daily.

“If women are considering taking vitamin D supplementation, particularly if it is higher than the levels recommended for bone health, we recommend they consider having their vitamin D levels checked to make sure they're at a healthy level,” Goodwin says.

Goodwin says her team is conducting additional studies of vitamin D in women with breast cancer. She expects results from one, an observational study of patients in Canada and the United States who received chemotherapy, to be available by the end of this year. In collaboration with Dr. Patti Ganz at the University of California Los Angeles, Goodwin's team is also planning another study of Canadian and US breast cancer patients that would determine whether vitamin D deficiency is still common in this population and measure how much levels change when supplements are administered. That study is slated to begin later this year.