Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure

Abstract

Solar radiation is the main cause of skin cancers. However, it also is a main source of vitamin D for humans. Because the optimal status of vitamin D protects against internal cancers and a number of other diseases, a controversy exists: Will increased sun exposure lead to net health benefits or risks? We calculated the relative yield of vitamin D photosynthesis as a function of latitude with a radiative transfer model and cylinder geometry for the human skin surface. The annual yield of vitamin D is 3.4 and 4.8 times larger below the equator than in the U.K. and Scandinavia, respectively. In populations with similar skin types, there are clear latitude gradients of all major forms of skin cancer, indicating a north-south gradient in real sun exposure. Surprisingly, the incidence rates of major internal cancers also increase from north to south. However, the survival prognosis also improves significantly from north to south. Reasons for these findings are discussed in view of the role of vitamin D. In Norway, melanoma rates increased by a factor of 6 from 1960 to 1990, while the prognosis improved in the same period. After 1990, melanoma rates have remained constant or even decreased in age groups <50 years, whereas the prognosis has not improved further. These data, together with those for internal cancers and the beneficial effects of an optimal vitamin D status, indicate that increased sun exposure may lead to improved cancer prognosis and, possibly, give more positive than adverse health effects.

Fig. 1.

Vitamin D as a function of latitude. (A) The dependency of annual vitamin D photosynthesis on latitude, calculated by using the in vivo action spectrum of pre-vitamin D synthesis (43) and known fluence rates of solar radiation as earlier described (37). (B) Summer (filled circle) and winter (empty circle) values for 25(OH)D levels in different populations living at different latitudes. The numbers in the graph indicate the citation number in the reference list.

Fig. 2.

Cancer incidence and death rates as a function of latitude. (A) Incidence rates of four cancer forms in different countries as functions of the mean latitude of the country. Only countries populated predominantly by individuals with skin types I and II are included: Australia, New Zealand, Sweden, Norway, Denmark, and U.K. cancer data represent averages for the period 1987–1997. The data are from Cancer Incidence in Five Continents (ref. ; see www-dep.iarc.fr). (B) The ratios of death rates to incidence rates for the same countries as considered in A. Death rates are collected 2 years after incidence rates and represent averages for the period 1989–1999. Cancer mortality data are obtained from a WHO database (see www-dep.iarc.fr).

Fig. 3.

The incidence rates of prostate, breast, lung, and colon cancers as functions of the incidence rates of CMM. Data are from the same sources as those in Fig. 2.

Fig. 4.

Incidence rates of prostate and breast cancers in different countries as functions of the mean latitude of the country. Cancer data are obtained from Globocan 2002 (see www-dep.iarc.fr).

Fig. 5.

Incidence rates of CMM in Norway as a function of time. The rates are averaged over 2 years and shown for the period 1960–2003.

Fig. 6.

The ratios of death rates to incidence rates of CMM in Norway. The rates are averaged over 2 years for the period 1960–2003.