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 throughout the country. 1 The mean daily solar radiation values for the contiguous United States, which vary between 300 and 500 gm-cal/cm 2, are shown in Figure 1. An estimate of the amount of sunlight reaching the earth in any particular state can be obtained by selecting the solar radiation contour that passes through that state or selecting the closest contours and taking their mean. This was done, and solar radiation values were calculated for each state. New Mexico and Arizona had the highest statewide mean solar radiation values (500 gm-cal/cm 2). These states experienced colon cancer rates for white males of 6.7 and 10.1 per 100000 population, respectively, over the period 1959–61. 2 New York, New Hampshire, and Vermont had the lowest statewide mean solar radiation values (300 gm-cal/cm 2) and experienced colon cancer rates for white males of 17.3, 15.3, and 11.3 per 100 000 populations, respectively, during the same period. 2 Since solar radiation contours provide an estimate of the amount of sunlight due to gross geographic and meteorological factors over large areas, they do not necessarily provide a completely accurate indication of the amount of solar radiation penetrating the atmosphere in cities. Even in areas where sunlight is intense, persons who live and work in cities may not receive much exposure to it. Behar has noted that rickets, a disease, which is believed to be related to inadequate solar exposure, and, consequently, vitamin D3 deficiency, occurs in large cities even in tropical and subtropical areas. 3 He attributes