"It is possible that, in more heavily pigmented persons, the sun exposure index usedin this study is a less sensitive measure of past sun exposure and/or that, in these women, such exposure generated less vitamin D (51)."They also looked at whether there were any associations with three polymorphisms in the Vitamin D receptor gene, but didn't find any.
51. Harris SS. Vitamin D and African Americans. J Nutr (2006) 136:1126–9
Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Breast Cancer Risk in a Multiethnic Population.
John EM, Schwartz GG, Koo J, Wang W, Ingles SA.Northern California Cancer Center, Fremont, CA.
Am J Epidemiol. 2007 Oct 12; [Epub ahead of print]
Considerable evidence indicates that vitamin D may reduce the risk of several cancers, including breast cancer. This study examined associations of breast cancer with sun exposure, the principal source of vitamin D, and vitamin D receptor gene (VDR) polymorphisms (FokI, TaqI, BglI) in a population-based case-control study of Hispanic, African-American, and non-Hispanic White women aged 35-79 years from the San Francisco Bay Area of California (1995-2003). In-person interviews were obtained for 1,788 newly diagnosed cases and 2,129 controls. Skin pigmentation measurements were taken on the upper underarm (a sun-protected site that measures constitutive pigmentation) and on the forehead (a sun-exposed site) using reflectometry. Biospecimens were collected for a subset of the study population (814 cases, 910 controls). A high sun exposure index based on reflectometry was associated with reduced risk of advanced breast cancer among women with light constitutive skin pigmentation (odds ratio = 0.53, 95% confidence interval: 0.31, 0.91). The association did not vary with VDR genotype. No associations were found for women with medium or dark pigmentation. Localized breast cancer was not associated with sun exposure or VDR genotype. This study supports the hypothesis that sunlight exposure reduces risk of advanced breast cancer among women with light skin pigmentation.