Vitamin D and reduced breast cancer risk

A Canadian study reported reduced breast cancer risk among women taking vitamin D supplements. Although earlier information was not comprehensive, other studies had suggested that low vitamin D intake might increase risk. Conducted at a northern latitude of 43°N, with relatively little sun-derived vitamin D, this nutritional study found that women taking more than 400 IU daily had lower odds of breast cancer than women taking no supplements. Lower risk was not established for vitamin D and calcium from food or for calcium supplements alone.

The study examined 3101 breast cancer cases identified through the cancer centre in Ontario women aged 25–75, diagnosed in 2002–2003. A control group of women without a family cancer history was recruited randomly: more than 25,000 Ontario households received calls to obtain 3471 women willing to participate. Ninety percent of participants were white, and mean age was 56. Premenopausal women accounted for 32% of cases and 36% of controls.

All participants were sent questionnaires on lifestyle, diet and medical history. Completion rates were 75% of cases and 80% of controls, providing a large population survey and an unspecified quantity of information. Particular attention was given to calcium and vitamin D intake. Vitamin D supplement use was reported by 13% of cases and 14% of controls.

A significant finding was an adjusted breast cancer odds ratio of 0.76 (95% confidence interval 0.59–0.98) for vitamin D intake over 400 IU daily compared with no supplements. An odds ratio compares the odds of an event in one group with those in another. The source describes an odds ratio of 0.76% as representing 76% of the risk, or a 24% risk reduction. Although premenopausal women were included, too few took vitamin D to provide complete subgroup findings. The reported 0.76 ratio represents all premenopausal and postmenopausal women in the study.

The study emphasized the need for further clinical research linking vitamin D to breast cancer. The investigators lacked information on the women’s earlier vitamin D intake. The assumption was that vitamin D exposure during adolescence was not very important for preventing later breast cancer.