By Marc Sorenson, EdD, Sunlight Institute…
An impressive research paper on colorectal cancer demonstrates that both vitamin D supplementation and ultraviolet-light (UV) exposure are effective in reducing that cancer. To begin the research, mice that are bred to be susceptible to intestinal tumors were put on a vitamin D-deficient diet, which reduced vitamin D levels significantly and spurred the development of non-cancerous tumors. Then, the diseased animals were either supplemented with vitamin D, or exposed to UV radiation. Remember that UV is the type of non-visible radiation produced by the sun.
In both cases (vitamin D supplementation and UV exposure), the area covered by tumors was significantly reduced. However, only UV exposure reduced the progression of the tumors to malignancy (full-fledged cancer).
The authors made the following statement regarding their somewhat surprising finding:
“Mortality from colorectal cancer decreases as ambient levels of UV radiation increase, suggesting that UV exposure is critical to the prevention of the disease. This study demonstrates the biological plausibility in mice of a causal relationship between moderate chronic UV irradiation or vitamin D supplementation and an impairment of outgrowth of primary intestinal cancer. While both UV irradiation and vitamin D supplementation decreased the overall area covered by tumors, only UV exposure inhibited malignant progression” [emphasis mine].
The Garland brothers in 1980 demonstrated that Sun exposure was inversely associated with colon cancer. They assumed that sun-stimulated vitamin D was the mechanism that caused the inverse association, but it now seems that sun exposure has cancer preventive mechanisms beyond vitamin D.
Remember: sunlight alone will always be better than vitamin D alone, because there are many healthful chemicals, beyond vitamin D, that are produced by sun exposure.
 Heggert Rebel, Celia Dingemanse-van der Spek, Daniela Salvatori, Johannes P.T.M. van Leeuwen, Els C. Robanus-Maandag and Frank R. de Gruijl. UV exposure inhibits intestinal tumor growth and progression to malignancy in intestine-specific Apc mutant mice kept on low vitamin D diet. Int J Cancer. 2015 Jan 15;136(2):271-7.
 Garland CF, Garland FC. Do sunlight and vitamin-D reduce the likelihood of colon cancer? Int J Epidemiol 1980;9:227–31.