I don't know how true this is but thought it interesting!
Lack of Sunshine Causes One Million Deaths a Year
If vitamin D3 levels among populations worldwide were increased, 600,000 cases of breast and colorectal cancers could be prevented each year, according to researchers from the Moores Cancer Center at the University of California, San Diego (UCSD).
This includes nearly 150,000 cases of cancer that could be prevented in the United States alone.
The researchers estimate that 250,000 cases of colorectal cancer and 350,000 cases of breast cancer could be prevented worldwide by increasing intake of vitamin D3, particularly in countries north of the equator.
The study examines the dose-response relationship between vitamin D and cancer, and is the first to use satellite measurements of sun and cloud cover in countries where blood serum levels of vitamin D3 were also taken.
Serum vitamin D levels during the winter from 15 countries were combined, then applied to 177 countries to estimate the average serum level of a vitamin D metabolite among the population.
An inverse association between serum vitamin D and the risk of colorectal and breast cancers was found.
Protective effects began when 25-hydroxyvitamin D levels (the main indicator of vitamin D status) ranged from 24 to 32 nanograms per milliliter (ng/ml). In the United States, late winter 25-hydroxyvitamin D levels ranged from 15 to 18 ng/ml.
Previous research has suggested that raising levels to 55 ng/ml was actually optimal to prevent cancer, the researchers said.
To increase your vitamin D3 levels, the researchers recommended a combination of dietary methods, supplements and sunlight exposure of about 10 to 15 minutes a day, with at least 40 percent of your skin exposed.
Nutrition Reviews August 2007 Volume 65, Supplement 1, pp. 91-95(5)
The Journal of Steroid Biochemistry and Molecular Biology March 2007; 103(3-5):708-11
Eurekalert August 21, 2007