In explaining the biologic mechanism underlying their findings, both groups of researchers cite increasing vitamin D levels, which induces cutaneous conversion of 7-dehydrocholesterol to vitamin D3. While solar irradiation is the most important source of vitamin D for humans, fatty fish are a good dietary source and an especially important one for residents in the higher northern or southern latitudes in winter. High content of omega-3 fatty acids in fatty fish also carries benefit associated with its consumption.
The relationship between UVB exposure and kidney cancer incidence was examined in an ecological study published in the online edition of the International Journal of Cancer. The study was stimulated in part by previous research from one of the study's co-authors, William B. Grant, PhD (Cancer 2002; 94:1867-75), which identified inadequate exposure to solar UVB with a higher renal cancer mortality rate in the United States.
Additional study authors were Edward D. Gorham, PhD, Sharif B. Mohr, MPH, and Frank C. Garland, PhD, also of UC San Diego.
Shedding light on cancer risk
The results showed a statistically significant relationship between increased latitude and higher rates of renal cancer: Incidence was highest in countries most distant from the equator (north and south) and lowest near the equator. The association was maintained in subgroup analyses considering cancer incidence rates separately for men and women.
In addition, a highly statistically significant inverse association was found between UVB irradiance and renal cancer incidence, while cloud cover and caloric intake from animal sources were directly associated with kidney cancer rates.
"In 1980, our group introduced the theory that vitamin D prevents cancer of the colon. Since then, we have expanded our research to investigate its association with cancer of the breast and ovary, while Dr. Grant has been conducting ecological studies researching associations between vitamin D and many other cancers, including renal cancer. Findings from laboratory studies demonstrate vitamin D3 has potent anticarcinogenic effects, and kidney, breast, and ovary tissue all express vitamin D receptors and are sites of vitamin D3 biosynthesis," Dr. Garland explained.
"The present study found that the association between renal cancer and sunlight exposure was even stronger than that found for other malignancies that have long been accepted as having an association with vitamin D in serum and dietary intake," Dr. Gorham said. "Although our study should be considered as hypothesis-generating rather than definitive, I believe its findings support a recommendation to raise vitamin D levels in the serum."
Dr. Garland explained that latitude is always included as a measure of UVB irradiance in ecological studies of cancer incidence. Cloud cover was also examined as a predictive factor for kidney cancer incidence, recognizing that thick layers of clouds can block the amount of UVB reaching the earth's surface.