The studies used meta-analysis to pool data from multiple previous studies, in order to increase precision and be able to draw conclusions that the original data did not allow. Researchers divided subjects into groups based on their blood levels of vitamin D, and compared the incidence of cancer between groups.
"The data were very clear, showing that individuals in the group with the lowest blood levels had the highest rates of breast cancer, and the breast cancer rates dropped as the blood levels ... increased," said Cedric Garland, co-author of one of the studies.
The blood level associated with a 50 percent lower risk of breast cancer could be reached by spending as little as 25 minutes in the sun for darker skinned people, or even 10 to 15 for lighter skinned individuals.
In the other study, researchers found that the same amount of sunlight corresponded with a two-thirds lower risk of contracting colorectal cancer.
"The evidence on the anti-cancer benefits of sunlight exposure and vitamin D are no longer deniable," said Mike Adams, author of "The Healing Power of Sunlight and Vitamin D."
"The risk reduction of breast cancer is absolutely astounding, far out-performing even the most aggressively hyped cancer drugs such as Herceptin," Adams said. "Sunlight should be the number one prescribed medicine in the United States, Canada, Europe and Asia. It costs nothing, is extremely safe, and prevents numerous diseases including breast cancer, prostate cancer, depression, osteoporosis, diabetes and even heart disease."
The breast cancer study was published online in the most recent issue of the "Journal of Steroid Biochemistry and Molecular Biology." The colorectal cancer study was published online in the February 6 issue of the "American Journal of Preventive Medicine."