It is proposed that vitamin D is a protective factor against colon cancer. This hypothesis arose from the inspection of the geographic distribution of colon cancer deaths in the US, which revealed that colon cancer mortality rates were highest in places where populations were exposed to the least amounts of natural light—major cities, and rural areas at high latitudes.
The hypothesis is supported by a comparison of colon cancer mortality rates in areas that vary in mean daily solar radiation penetrating the atmosphere. A mechanism involving cholecalciferol (vitamin D3) is suggested. The possibility that an ecological fallacy or another indirect association explains the findings is explored.
The amount of sunlight reaching the earth's surface varies greatly from area to area in the US. Solar radiation (including ultraviolet and visible light) reaching the ground is measured by the US Weather Bureau at a number of weather stations which are distributed