Journal of Photochemistry and Photobiology B: Biology
Solar radiation, vitamin D and survival rate of colon cancer in Norway
Introduction
Cell experiments, as well as animal experiments and epidemiological studies, indicate that vitamin D3 can protect against cancer induction and/or slow down tumor progression [1], [2], [3], [4], [5], [6], [7], [8]. In several countries on the northern hemisphere, the death and/or incidence rates of breast cancer, prostate cancer and colon cancer are larger in north than in south [9], [10], [11], [12], [13], [14], [15], [16]. Several investigators have related this to vitamin D3, since solar radiation plays a significant role for the status of this vitamin [17].
Norway covers a large distance from north to south and the Norwegian population is relatively homogeneous, both with respect to ethnical origin and clothing and sun exposure habits. Thus, the country is well suited for epidemiological investigations of the relationship between solar radiation and cancer, as we have demonstrated for skin cancer [18], [19], [20].
The annual exposure to ultraviolet radiation (UV) is about 50% larger in the south than in the north [18]. This is the main reason for the three times larger incidence rates of skin cancer in the south than in the north [18], [19], [20]. In any case, the gradient of skin cancer incidence rates clearly indicates that not only the measured UV exposures, but also the exposures obtained by the population, are larger in the south than in the north. It is commonly believed that skin cancer is mainly induced by UVB radiation (280–315 nm) [21], and so is vitamin D3 [22]. Some investigations suggest that UVA may also play a role in the induction of skin cancer [23], [24], although this issue is controversial.
In addition to the expected north–south gradient of vitamin D3 induction, there is a clear seasonal variation of the vitamin D3 metabolite calcidiol in Norway [25], [26].
In the present work we have attempted to investigate if the variation of vitamin D3 induction from north to south, as well as through the year, plays any role for the survival of colon cancer as determined 18 months after diagnosis.
Section snippets
Calculation of vitamin D3 induction in skin
Since only the overall annual variation of vitamin D3 in Norway, and not specifically the fraction related to solar radiation, has been measured, we attempted to calculate this fraction, using the action spectrum of production of previtamin D3 from 7-dehydrocholesterol [27]. These calculations were compared with the measured annual variation of calcidiol in Norway and Denmark [25], [26], [28], [29]. Calcidiol is present in the nanomolar range in human serum, and, since calcitriol, the active
Results
We found no significant annual variation of the incidence rates of colon cancer (Fig. 1(a) and (b)). In each season we found 25 ± 1.5% of the annual incidence rates. However, with respect to death rates, as measured 18 months after diagnosis, there was a clear seasonal variation, with the lowest death rates in the autumn (Fig. 1(a) and (b)). This was true for women (Fig. 1(a)) and for men (Fig. 1(b)). Thus, two parallel sets of data show the same trend. No significant north–south gradient was
Discussion
There is a clear annual variation in the serum level of calcidiol in the Nordic countries (Fig. 1(c)) as in practically all countries where such measurements have been performed. In most studies, as in those cited here [26], [28], [29], the maximal calcidiol level is found in July–August. However, it should be noted that the time points of maximal calcidiol levels are different from investigation to investigation (Fig. 1(c)). Nevertheless, there is a lag-time of approximately one month between
Abbreviations
- 1α,25-(OH)2D3
calcitriol
- 25-(OH)D3
calcidiol
- ID numbers
personal identification numbers
- RR
relative death risk
- UV
ultraviolet
- UVA
ultraviolet A (400–315 nm)
- UVB
ultraviolet B (315–280 nm)
Acknowledgments
We acknowledge the Research Council of Norway and Norwegian Cancer Society for providing financial support for Alina Carmen Porojnicu and Asta Juzeniene.
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