The "cancer-cold" hypothesis and possible extensions for the Nordic populations.

Cancer incidence is inexplicably high in cold countries. This has been revealed by recent genetic and epidemiological studies. These studies used data from the GLOBOCAN-2012 database, for 186 populations and for a variety of cancer types. Cancer incidence in Nordic people is particularly high for the frequent cancer forms, like breast, prostate and colon cancer. A relationship of cancer with cold is suspected since Inuit and Alaska Indians that live in even more extreme low temperatures have the higher cancer rates in the world. In this article, possible reasons for this phenomenon are discussed. These explanations are related with: evolutionary adaptation to extreme cold, the genetic background of Nordic people, the experimentally proven fast growth and metastasis of tumors at low temperatures, high concentration of certain air pollutants at cold environments, low levels of serum Vitamin D, overdiagnosis by the medical doctors and high quality of the health system in Nordic countries. Lifestyle parameters are not discussed in detail, although these may be equally crucial for cancer risk in cold countries. In conclusion, more studies are needed to elucidate the real causes of this epidemiological pattern.