Vitamin D and Breast Cancer Risk

It has long been known that vitamin D is important for calcium absorption and bone health. More recently, vitamin D has been found to modulate breast cancer cell growth and increasingly epidemiologic studies suggest vitamin D may be associated with reduced breast cancer risk. The primary objective of this thesis was to evaluate the associations between vitamin D from all sources (food, supplements and sunlight exposure) and breast cancer. Secondary objectives were focused on methodological issues including the development of a solar vitamin D score and adapting the measurement of vitamin D from foods for use among Canadians. The data source for this study was the “Ontario Women’s Diet and Health Study”, a population-based case-control study of women in Ontario. Cases (n = 3,101) diagnosed between 2002 and 2003 were identified through the Ontario Cancer Registry and controls (n = 3,471) were identified through random digit dialing of Ontario households. Study participants completed mailed risk factor and food frequency questionnaires. Vitamin D intake from supplements (>400 IU/day compared to none) was found to be associated with reduced breast cancer risk (OR = 0.76; 95% CI: 0.59, 0.98). However, total vitamin D intake (from food and supplements) and intake from food alone were not associated with breast cancer risk. Time spent outdoors during 4 periods of life (including adolescence) was associated with reduced breast cancer (e.g., highest versus lowest categories of exposure at age 40 to 59: OR = 0.74; 95% CI: 0.61, 0.88). The novel solar vitamin D score, derived from time spent outdoors, skin color, sun protection practices, and ultraviolet radiation of residence, was also associated with reduced breast cancer risk. In summary, there is some evidence to suggest that vitamin D intake from supplements and determinants of cutaneous vitamin D production are associated with reduced breast cancer risk.Ph