Another interpretation for the geographical gradient of cancer mortality rate

Introduction Many epidemiologic studies indicate that cancer mortality rate tends to increase at higher latitudes. One proposed hypothesis explaining this geographical gradient is that vitamin D, which is produced after exposure to solar radiation, has anti-carcinogenic effects. However, the association between solar radiation and cancer risk is not fully explained. The Hypothesis A number of recent studies have provided support for a link between photoperiodic environment and cancer. Accumulating epidemiological and genetic evidence indicates that the disruption of circadian rhythms might be directly linked to cancer. It is possible that part of the latitudinal gradient of cancer mortality rates is not due to the effects of vitamin D but to circadian rhythm amplitude reduction. Evaluation of Hypothesis In previous study, the author proposed effective day length (duration of photoperiod exceeding the threshold light intensity) as a climatic element indicating ambient daylight intensity and length. While day lengths are shorter in winter in the north than in the summer, on average, the possible sunshine duration is exactly the same at all points of the globe. However, effective day length is essentially a photoperiod that takes into account the intensity of light; it becomes shorter with decreases in solar radiation. The effective day length at more than 1000 lx increases in almost direct proportion to the intensity of solar radiation. This shows that the effects of solar radiation, which generates vitamin D, and effective day length, which reinforces amplitude of circadian rhythm, are essentially undistinguishable in ecological studies. Conclusion This result supports the hypothesis of circadian disruption from a meteorological standpoint. This indicates the possibility that part of the latitudinal gradient of cancer mortality rates is not due to the effects of vitamin D but to circadian rhythm amplitude reduction.