Pancreatic cancerExposure to sunlight

Solar ultraviolet-B (UVB) irradiance is the most important source of vitamin D for most people on earth. Thus, studies investigating pancreatic cancer (PC) rates with respect to indices of solar UVB doses should be the best way to determine the effect of vitamin D in reducing the risk of PC. 

There are many ecological studies of PC mortality rate with respect to indices for solar UVB doses.

The first ecological study of PC was done in Japan in 1985, finding the simple correlation coefficient between PC mortality and the latitude was 0.72 for males and 0.73 for females1. Higher latitude in this case was an index of lower solar UVB doses. The correlation remained significant after controlling for food consumption.

Two subsequent studies in Japan confirmed the strong correlation between latitude and PC mortality rate23.There is about a 30% higher rate of PC mortality rates in northern Japan compared to southern Japan.

Japan lies between latitudes 31º and 45º N, which is nearly the same as continental United States, 29º and 47º N. This range is large enough that there is a large gradient in solar UVB dose.

In the United States, the UVB dose for July is highly asymmetrical, with UVB contours shifted about 600 miles (1000 km) to the north in the western states4 due to generally higher surface elevation and thinner stratospheric ozone layer due to the prevailing westerly winds pushing air masses up over the Rocky Mountains. Thus, latitude per se is not a good index of summertime UVB doses in the United States but satellite-determined values are.

There have been three ecological studies finding an inverse correlation between solar UVB and PC mortality rates in the United States567 and one for PC incidence rates5. The two studies from 2006 adjusted for confounding factors such as smoking.

There have been ecological studies in other countries as well. Latitude was found correlated with PC mortality rates in Australia8. In Spain, PC mortality rates were found correlated with latitude and inversely with non-melanoma skin cancer (NMSC) mortality rates9. The most important risk factor for NMSC death is integrated UVB irradiance.

Several multi-country ecological studies have also found PC incidence and/or mortality rate inversely correlated with UVB indices.

One for mortality rates for European countries controlled for diet, finding latitude and alcohol supply directly correlated and fish supply inversely correlated10.

In a study involving incidence rates for 104 countries, the 25(OH)D index was based on solar UVB from satellite measurements including effects of clouds and ozone, and ranged from 10 ng/mL to 55 ng/mL11. In addition, several other factors were included; for males, alcohol intake and cigarette consumption were significantly correlated, while for females, alcohol and sugar consumption were significantly correlated. The solar UVB index had the strongest regression coefficient but a p value (a measure of statistical significance) less than for alcohol intake for males and either alcohol or sugar for females. According to Bernoulli, P<0.05/n, where n is the number of factors in the regression analysis, is required for statistical significance). Thus, solar UVB doses are not statistically significant for females in this study.

Note that ecological studies integrate the effect of cancer risk-modifying factors throughout the entire lifetime while observational studies generally have a 0-15 year window of observation based on a single measurement of serum 25(OH)D level or oral vitamin D intake. Much of the risk of cancer appears to come from early life. For example: “The lag period between diagnosis of chronic pancreatitis and pancreatic cancer is usually one or two decades.”12.

Very likely, development of pancreatitis takes years as well as it is linked to such things as alcohol consumption and smoking13 and high fat diets and diabetes14. Thus, investigations of PC incidence with respect to serum 25(OH)D levels 5-15 years prior to diagnosis do not look at earlier times in the progression to PC that are likely more important.

Page last edited: 22 August 2011

References

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