A paper published in March 2011 by Michael F. Borisenkov reported that a number of cancer rates increase significantly with increasing latitude in Russia. The cancers that increase in both incidence and mortality are esophageal, kidney, pancreatic, stomach cancer and melanoma. The cancers that increase just in incidence are bladder, colon, ovarian and rectal cancer. The cancers that increase just in mortality are acute myeloid leukemia, immunoproliferative diseases and multiple myeloma.
The authors explained the findings on a “hypothesis of circadian disruption.” Melatonin levels rise when blue light levels are lower, and melatonin helps induce sleep. As the length of daylight increases, less melatonin is produced. There is evidence that melatonin reduces the risk of cancers such as breast, colon, endometrial, ovarian, prostate, and rectal. However, it does not appear that latitudinal studies can be used to evaluate this hypothesis.
Most of these cancers have been found inversely correlated with solar ultraviolet-B (UVB) doses for July or annual UVB in the United States. Many have also been found to have rates that increase with latitude in Australia, China, France, Japan, and Spain. Thus, the paper by Borisenkov provides further evidence for the UVB-vitamin D-cancer hypothesis.
For melanoma cancer, UVB and vitamin D seem to play a role, but skin pigmentation does, too. Those living in their ancestral homelands at higher latitude have paler skin than those living at lower latitudes. Melanoma rates increase with latitude in Europe, but not in the United States. In the U.S., pale-skinned people live at all latitudes.
A better way to check the melatonin-cancer hypothesis through ecological studies is through the seasonality of cancer. Breast cancer has been found to progress more rapidly in spring and fall [Borisenkov, 2005]. There are also two peaks of diagnoses, one in spring, one in fall [Oh, 2010]. The two peaks have been explained as being related to vitamin D production in summer and melatonin production in winter [Oh, 2010].