Thousands of cases of cancer could be averted if people in colder climates increased their blood levels of vitamin D, according to authors of a paper in the August 2007 journal Nutrition Reviews. Numerous epidemiological and ecological studies have shown increased cancer rates in higher latitudes around the world, especially colon, breast, and prostate cancers. With higher latitude comes a lower exposure to ultraviolet light from the sun - a critical component of adequate vitamin D levels in the blood. Pre-vitamin D is activated in skin cells via UV light into active vitamin D3.

In this study, researchers at the University of California, San Diego, analyzed worldwide data on winter vitamin D levels and found breast and colon cancer incidence rose as blood vitamin D levels dropped. A blood level above 22 ng/mL was protective for colon cancer, while above 32 ng/mL was protective for breast cancer. These were not deemed optimal levels, but levels at which a protective effect was seen. The average late winter blood level of 25-hydroxy vitamin D in the United States is well below the protective levels, at 15-18 ng/mL.

The authors of the study suggested if individuals achieved a blood level of at least 55 ng/mL, approximately 60,000 cases of colon cancer and 85,000 cases of breast cancer would be prevented annually. They warned against getting too much sun exposure to achieve these levels; instead, they recommended moderate sun exposure without sunscreen in conjunction with supplementation of 2,000 IU of vitamin D per day.

Garland CF, Grant WB, Mohr SB, et al. What is the dose-response relationship between vitamin D and cancer risk? Nutrition Reviews 2007;65:S91-95.