An introduction to cancerExposure to sunlight

Strong inverse correlations between solar ultraviolet-B (UVB) and many types of cancer have been found in numerous ecological studies in the United States1 2 3 4 5 and a case-control study based on death certificates6.

The United States is a particularly useful country to conduct ecological studies since dietary factors are largely shared and solar UVB doses in summer are asymmetrical, being much higher in the west than the east at the same latitude due to higher surface elevation and thinner stratospheric ozone layer, thus permitting use of an index other than latitude, which could have other bases such as temperature. Solar UVB in summer can increase serum 25(OH)D levels by 40 nmol/L according to a study of 45-year old residents of the UK7, and, likely, higher increases in the United States. Most of the recent ecological studies used indices for other risk-modifying factors such as alcohol consumption, smoking, Hispanic heritage, poverty and urban/rural residence3 4 5, limited dietary factors4, and air pollution from coal-fired power plants5. Thus, such studies largely overcome the concern regarding confounding factors. 

Latitude and annual sunlight doses have been used as indices of solar UVB doses and vitamin D production in a number of single country and multi-country ecological studies. Use of latitude in single-country studies other than the United States is generally useful since many confounding factors such as alcohol consumption, dietary factors, genetic background, pollution levels, and smoking prevalence tend to be fairly similar at all latitudes. In multi-country studies such factors should be included if the data are available. Representative single country studies include those in Australia8 9, China10, France11, Japan12, and Spain13. A representative multi-cancer, multi-country study is one for Europe14.

The Garland group at University of California San Diego has done a series of global studies finding that cancer incidence rates: bladder15, brain15, breast16, endometrial17, lung16, pancreatic15, ovarian2 and renal cancer18. In these studies, confounding factors such as selected dietary factors, alcohol consumption, and smoking were included as deemed appropriate. All of these papers feature the distinctive “smiley” parabolic curve since cancer incidence rates have minima near the equator and maxima at high latitudes.

No factor other than UVB production of vitamin D has been proposed to explain the latitudinal dependence of cancer rates in general. However, it has been proposed that a genetic factor explains much of the geographical variation for prostate cancer. In the United States, prostate cancer mortality rates19 are highly correlated with ancestry, with higher rates in northern states with more northern European descendents while the southern states have more white Americans with Hispanic heritage20. This finding, plus the fact that cholesterol is an important risk factor for prostate cancer21 led to the hypothesis that an important risk factor for prostate cancer was the apolipoprotein E4 allele, which increases cholesterol production in the liver. A multi-country ecological study was conducted in support of this hypothesis22. However, vitamin D does hinder the progression of prostate cancer23.

The relationship between sun exposure and non-Hodgkin’s lymphoma (NHL) was studied in a pooled analysis of 10 studies participating in the International Lymphoma Epidemiology Consortium (InterLymph) covering 8,243 cases and 9,697 controls in the USA, Europe and Australia. Risk of NHL fell significantly with the composite measure of increasing recreational sun exposure, pooled odds ratio (OR) = 0.76 (95% CI 0.63-0.91) for the highest exposure category (p for trend 0.01)24.

Another index of UVB irradiance at the personal or population level is incidence or death from non-melanoma skin cancer (NMSC), either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC). SCC is a bit problematic since smoking is also a risk factor for it25. NMSC mortality rates were found inversely correlated with 15 types of cancer in an ecological study of cancer mortality rates in Spain13. A cancer registry study found standardized incidence rates of 0.79 (95% CI, 0.68-0.91) for all solid tumors except lip and skin after diagnosis of SCC in sunny countries (Australia, Singapore and Spain) and 0.86 (95% CI, 0.80-0.92) after diagnosis of BCC26.

A recently found paper reporting a study of cancer incidence in regions of Rhineland-Palatinate, Germany with various amounts of land devoted to winegrowing found that while NMSC plus melanoma rates were higher in the regions with >5% of the land in winegrowing compared to regions with 27. This study was touted as strong support for the UVB-vitamin D-cancer hypothesis28.

Page last edited: 04 May 2011


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