By Marc Sorenson, EdD, Sunlight Institute
Research now indicates that sunlight has very positive affects on colon cancer, whereas vitamin D has no affect, and in high doses may be counterproductive, at least in rats. Dr. AA Irving and colleagues performed a 140-day investigation involving rats that had colon adenomas (a precursor to full-blown colon cancer) induced in their colons. The rats were given either vitamin D3, or the stored form, 25(OH)D3, in differing amounts. With low dose vitamin D in either form, no reduction in either existing adenomas or emerging tumors were seen. In higher doses, there was a dose-dependent increase in colon tumor numbers in both male and female rats.
The researchers said the following in their concluding statement: “Thus, the association between sunlight exposure and the incidence of colon cancer may involve factors other than vitamin D concentrations. Alternative hypotheses warrant investigation. Furthermore, this study provides preliminary evidence for the need for caution regarding vitamin D supplementation of humans at higher doses, especially in individuals with sufficient serum 25(OH)D3 concentrations.”
The takeaway from this research is that sunlight is protective against colon cancer in rats (and probably in humans), independent of vitamin D—another reason to embrace the sunlight (safely, of course).
 Irving AA, Plum LA, Blaser WJ, Ford MR, Weng C, Clipson L, DeLuca HF, Dove WF. Cholecalciferol or 25-hydroxycholecalciferol neither prevents nor treats adenomas in a rat model of familial colon cancer. J Nutr. 2015 Feb;145(2):291-8.