Impediments to UVB and Vit D related to cancer
Anticancer Res. 2009 Sep;29(9):3597-604.
Current impediments to acceptance of the ultraviolet-B-vitamin D-cancer hypothesis.
Grant WB, Boucher BJ.
Sunlight, Nutrition, and Health Research Center (SUNARC), San Francisco, CA 94164-1603, USA. wbgrant@infionline.net
The ultraviolet-B (UVB)-vitamin D-cancer hypothesis was proposed in 1980. There have been numerous ecological, observational and other studies of the hypothesis. There are about 14 types of cancer for which it seems to apply: bladder, breast, colon, endometrial, esophageal, gallbladder, gastric, ovarian, pancreatic, rectal, renal and vulvar cancer and both Hodgkin's and non-Hodgkin's lymphoma. Nonetheless, the hypothesis has not yet been accepted by public health agencies. 
Some of the reasons for this include a 
  distrust of ecological studies, 
  some mistrust of observational studies, 
  and the existence of just one positive randomized controlled trial: 
     an analysis of a vitamin D and calcium supplementation study 
       involving post-menopausal women in Nebraska. 
Paradigm shifts such as this generally take time, in part due to opposition from those content with the status quo. In this paper, results of ecological studies in the United States using summertime solar UVB as the index of vitamin D production, which is highly asymmetrical with respect to latitude, and indices for other cancer risk-modifying factors (air pollution, alcohol consumption, dietary iron and zinc, ethnic background, socioeconomic status, smoking and urban/rural residence) are discussed in terms of supporting the hypothesis. These studies were not considered while other ecological studies were examined in recent critiques of the hypothesis. While additional randomized controlled trials would, of course, be helpful, the current evidence seems to satisfy the criteria for causality as outlined by A. Bradford Hill.