A new study has reported that time spent outdoors may reduce risk of breast cancer through production of vitamin D in the skin. Recent studies have reported that vitamin D may be associated with reduced risk of breast cancer, but most studies have taken only dietary vitamin D intake into account. The study, which was conducted in Ontario, Canada, between 2003 and 2004, included 3,101 women with breast cancer and 3,471 cancer-free controls. The influence of ultraviolet radiation from sun exposure and factors related to vitamin D production in the skin on risk of breast cancer were evaluated.

Time spent outdoors was found to be associated with reduced breast cancer risk during all four periods of life examined, but the benefit was somewhat more pronounced in later years. The age-adjusted risk of breast cancer was approximately 29% lower among women who spent more than 21 hours per week outdoors compared to those who spent six hours or less during the teenage years. The risks were 36% lower for women in their 20s and 30s, 26% lower for women in their 40s and 50s, and 50% lower for women aged 60 to 74 years. Sun protection practices such as use of sun screen and level of exposure to all types of ultraviolet radiation were not associated with risk of breast cancer. Variables specifically related to vitamin D production were found to be significantly associated with reduced risk of breast cancer. These associations did not depend on menopausal status, dietary vitamin D intake, or level of physical activity. The authors conclude that factors related to increased production of vitamin D in the skin may be associated with reduced breast cancer risk.

Please see our article on vitamin D for an in depth discussion of the relationship between vitamin D and breast cancer.