Does solar exposure, as indicated by the non-melanoma skin cancers, protect from solid cancers: vitamin D as a possible explanation

Hypothesis--ultraviolet-B irradiance and vitamin D reduce the risk of viral infections and thus their sequelae, including autoimmune diseases and some cancers

Many viral infections reach clinical significance in winter, when it is cold, relative humidity is lowest and vitamin D production from solar ultraviolet-B irradiation is at its nadir. Several autoimmune diseases, such as multiple sclerosis, type 1 diabetes mellitus and asthma, are linked to viral infections. Vitamin D, through induction of cathelicidin, which effectively combats both bacterial and viral infections, may reduce the risk of several autoimmune diseases and cancers by reducing the development of viral infections. Some types of cancer are also linked to viral infections. The cancers with seemingly important risk from viral infections important in winter, based on correlations with increasing latitude in the United States, an index of wintertime solar ultraviolet-B dose and vitamin D, are bladder, prostate, testicular and thyroid cancer, Hodgkin's and non-Hodgkin's lymphoma, and, perhaps, gastric cancer. The evidence examined includes the role of viruses in the etiology of these diseases, the geographic and seasonal variation of these diseases, and the time of life when vitamin D is effective in reducing the risk of disease. In general, the evidence supports the hypothesis. However, further work is required to evaluate this hypothesis.

The effect of solar UVB doses and vitamin D production, skin cancer action spectra, and smoking in explaining links between skin cancers and solid tumours

The report of differences between skin cancer rates and solid tumours in sunny versus less sunny countries [Tuohimaa P, Pukkala E, Scelo G, et al. Does solar exposure, as indicated by the non-melanoma skin cancers, protect from solid cancers: Vitamin D as a possible explanation. Eur J Cancer 2007; 43: 1701-12] raised some important questions regarding the roles of solar ultraviolet (UV) irradiance and cancer risk. The findings can likely be explained based on the effects of UVB dose on cancer risk, the action spectra of different skin cancers, the amount of skin exposed, and the differential effects of smoking on cancer risk. Solar UVB has been found inversely correlated with about 20 types of cancer in ecological and cohort studies in sunny countries. Vitamin D and calcium were recently found to greatly reduce cancer incidence in a prospective double-blind study. Epidemiological studies suggest that the action spectra for skin cancers vary, with solar UVB most important for squamous cell carcinoma, UVA most important for melanoma, and both important for basal cell carcinoma. These differences may explain the different standardised incidence ratios for solid tumours with respect to the different skin cancers in sunny countries. Smoking has been reported as a risk factor for non-melanoma skin cancers, but has been found inversely correlated with melanoma, which may explain some of the differences in standardised incidence ratios for solid tumours linked to smoking with respect to type of skin cancer. In Nordic countries, less skin is generally exposed (in head and neck regions, the most frequent sites of squamous cell carcinoma) resulting in reduced vitamin D production.