New research published in the journal Cancer has found that UV affects prostate cancer incidence in the United States, with lower incidence in areas with greater amounts of UV.
Prostate cancer is the most common form of cancer among American men. Roughly 1 in 6 men will be diagnosed with prostate cancer during their lifetime. Prostate cancer usually occurs in older men with the average age of diagnosis being 67. In 2013, an estimated 238,000 men were newly diagnosed with prostate cancer. Thirty-thousand men died from prostate cancer this past year.
Researchers think vitamin D may play a role in prostate cancer. Like in other cancers, vitamin D works by helping cells function properly, telling them when to divide or die. When cells aren’t regulated properly, they can divide too much or not die when they need to, leading to cancer.
In the past two years, researchers have carried out experimental studies to see if vitamin D can alter markers in very early stage prostate cancer. In one open-label trial, for example, vitamin D supplementation decreased either number of positive cores or Gleason scores in 60% of enrollees.
If vitamin D does play a role in prostate cancer, then we may see a latitudinal relationship; the further from the equator, the higher the incidence of prostate cancer or the worst the prognosis once you have prostate cancer.
Those who live in northern latitudes have a difficult time producing vitamin D from the sun because there is less ultraviolet B radiation reaching the surface during a majority of the year. This is one explanation for the higher prevalence of vitamin D-related conditions, such as autoimmune diseases and depression, in areas farther away from the equator.
Not only does latitude affect vitamin D production, but so does skin type. Those with darker skin need to be exposed to the sun longer to produce adequate amounts of vitamin D. Therefore, at all latitudes, darker skin individuals will require more sun exposure to produce the same amount of vitamin D as those with lighter skin.
Because of these factors, researchers in the present study wanted to know, does average UV radiation in regions of the United States affect incidence of prostate cancer and racial differences in incidence of prostate cancer?
Researchers from New York University School of Medicine collected national data on incidence of prostate cancer and ultraviolet radiation (UV).
The researchers identified incidence of prostate cancer in the United States from 2000 to 2009 using the Surveillance, Epidemiology, and End Results program and found 773,964 white men and 132,417 black men who had been diagnosed with prostate cancer. They also identified 239,689 white men and 49,185 black men who died from prostate cancer during those same years.
The research team then looked at the average UV radiation in all regions of the United States from 1978 to 2005. They wanted to know, depending on how much UV certain regions got, if that affected prostate cancer and mortality rates from prostate cancer.
To analyze the changes in incidence based on region, the researchers divided the United States into ten regions, or deciles, from lowest UV areas to highest. For example, the first decile represented the lowest UV areas such as Washington State and Michigan while the tenth decile represented the highest UV areas, such as southern California, Hawaii, and Louisiana.
When the researchers looked at prostate cancer incidence in each decile, here is what they found:
- For white men, when compared with the 1st decile, incidence rates were 18.9% to 23.4% lower in the 7th to 10th decile (p<.005).
- For black men, when compared with the 1st decile, incidence rates were 34.2% lower in the 10th decile (p<.051).
- Incidence rates were higher for black men versus white men. Compared to 1st decile, the disparity in incidence rates was much lower in the fourth and fifth deciles. The implication here is that in these deciles, the amount of UV more equally affects black and white men.
When the researchers looked at prostate cancer mortality rates in each decile, here is what they found:
- For white men, when compared with the 1st decile, mortality rates were 7.8% to 15.9% lower in the 6th to 10th deciles (p<.001).
- Unexpectedly, for black men, mortality rates were higher in areas of high UV with 18.2% to 36.5% higher rates in the 3rd to 10th deciles compared with the 1st (p<.02). The researchers suggested the reason for this may be due to a small number of prostate cancer deaths in the 1st decile, possibly skewing the data.
The researchers concluded,
“We observed a decrease in prostate cancer incidence with increasing levels of the UV index. We also observed a reduction in the racial disparity in prostate cancer incidence in areas with moderately high UV indices compared with those with lower UV indices.”
The main limitation in this study was the measurement of UV index rather than vitamin D. Often, serum vitamin D levels differ from average UV radiation due to an individual’s lifestyle and supplementation. Additionally, the observational nature of their study means they can only identify association and not causality.
So, what do these results tell us? They further confirm a link between latitude of residence, vitamin D, and health. Not only did they find decreased incidence of prostate cancer with increased UV radiation in both skin types, but they also found that the disparity in incidence was less in areas of higher UV radiation.
Future research should work to determine if vitamin D can close the gap in incidence of prostate cancer between white and black men in the United States.