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Epidemiological data show an inverse relationship between vitamin D levels and breast cancer incidence. This study investigates the relationship of modeled and measured serum 25-hydroxyvitamin D [25(OH)D] levels with age-standardized incidence rates of breast cancer in 107 countries.

The hypothesis being tested is that breast cancer incidence is inversely related to geographically-dependent cutaneous [skin] sunlight exposure. A multiple regression approach was used to examine the contributions of ultraviolet B (UVB) irradiance to age-standardized incidence rates of breast cancer in the 107 countries with data on these covariates - total column ozone thickness, per capita intake of alcohol and energy from animal and vegetable sources, cigarettes, proportion of female population overweight, and total fertility.

Age-standardized incidence rates were substantially higher at latitudes distant from the equator (R2 = 0.43, p < 0.0001). The dose–response gradient between modeled serum 25(OH)D levels and incidence rates of breast cancer followed a standard inverse dose–response curve Increasing increments in serum 25(OH)D in the range above 22 ng/mL were associated with incrementally lower incidence rates of breast cancer.

According to multiple regression, UVB irradiance adjusted for cloud cover was inversely associated with incidence rates (p = 0.04) after controlling for covariates.

Intake of energy from animal sources was also positively associated with incidence rates (p < 0.01). The overall coefficient of determination, R2, was 0.81 (p < 0.0001).

There was a protective effect of UVB irradiance on risk of breast cancer that was independent of fertility rate, proportion of the population overweight, alcohol intake, animal energy intake, and other covariates.

Source: The Breast Journal, May/June 2008, 14(3) pp. 255-260. PMID: 18422861 by Mohr SB, Garland CF, Gorham ED, Grant WB, Garland FC. Department of Family and Preventive Medicine, University of California San Diego, La Jolla; and Sunlight, Nutrition, and Health Research Center, San Francisco, California, USA [E-mail: cgarland@ucsd.edu]