The United States is a particularly useful country to conduct ecological studies since dietary factors are largely shared and solar ultraviolet-B (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 (16 ng/ml) according to a study of 45-year old residents of the UK6, and, thus, higher amounts 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 residence4.
Another ecological study in the United States found direct correlations with smoking and drinking surface water in addition to inverse correlation with solar UVB2; the likely reason for the link to surface water was production of chlorinated hydrocarbons when chlorinating the water7.
The study in China used UVB doses determined by the NASA satellite instrument, Total Ozone Mapping Spectrometer, and adjusted for urban/rural residence.
A 174-country ecological study found bladder cancer incidence inversely correlated with modeled UVB doses and per capita health care expenditures and directly correlated with cigarette smoking8. Thus, such studies largely overcome the concern regarding confounding factors.
Page last edited: 22 August 2011
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