Research published in the journal In Vivo showed that tanning potential, but not sunlight and vitamin D, is a significant risk factor for prostate cancer.
Prostate cancer is the second leading cause of cancer-related deaths among men in the United States. African American men have the highest risk of developing prostate cancer out of all the ethnic groups and are more likely to be diagnosed with the disease at an advanced stage.
Researchers have hypothesized that the high incidence of prostate cancer in African Americans may be due to a deficiency of vitamin D caused by a reduction in the synthesis of vitamin D from the skin.
It is important to note that prostate cancer research has only looked at sunlight or vitamin D, and not both at the same time. Therefore, researchers conducted a study to determine the relationship between vitamin D, UV exposure and prostate cancer in African American men by looking at tanning potential and vitamin D status.
They recruited 91 African American men with prostate cancer and 92 African American men without prostate cancer.
They then looked at the difference between the two groups in terms of UV exposure, tanning potential, and vitamin D status.
UV exposure was assessed using a UV questionnaire designed to calculate the total amount of UV light exposure from birth until the age of diagnosis of prostate cancer, or age of data collection for controls.
Tanning potential was assessed by measuring the difference in the melanin concentration and color of unexposed areas of the skin and the melanin concentration and color that develops in skin that is exposed to sunlight. Tanning potential is a quantitative index of sun exposure that is related to cumulative lifetime sun exposure. An increased tanning potential indicates increased cumulative sun exposure.
A single vitamin D measurement was taken during the study to assess vitamin D status.
The researchers were interested in learning if vitamin D, UV exposure, and tanning potential all similarly affected risk of prostate cancer. This would indicate that sun exposure plays a role in prostate cancer, and this role is likely mediated in part by vitamin D production.
Here’s what they found:
- The average vitamin D level for the control group was 29.06 ng/ml and the average vitamin D level for the prostate cancer group was 26.75 ng/ml.
- Age and tanning potential were significantly negatively associated with prostate cancer risk (p=0.05, for both)
- UV exposure and vitamin D status were negatively associated with prostate cancer risk, however this association was not significant (p>0.05)
- After adjusting for both age and vitamin D status, tanning potential remained a significant predictor of prostate cancer risk (p=0.04).
The researchers stated,
“In the present study, we examined the association of serum vitamin D, skin tanning potential, and UV exposure with prostate cancer risk in African Americans. We found a significant association between skin tanning potential and prostate cancer. However, our results showed no significant association between prostate cancer and UV exposure or serum vitamin D.”
One large limitation of the study is selection bias. All of the participants were residents of the Washington, DC metropolitan area who all received similar levels of sunlight. Another limitation was the use of a single vitamin D measurement, which may not reflect long-term circulating vitamin D levels.
These results are puzzling, but the researchers offered a possible explanation: all of the participants were dark-skinned, meaning they likely had similar absorption due to melanin.
The researchers posited that the inverse association between tanning potential and prostate cancer is most likely due to a loss of melanin with age. The less melanin there is, the higher the potential for tanning. This explanation is supported by the fact that age also was strongly and significantly associated with prostate cancer risk.
The low vitamin D levels in both groups may have been the reason as to why a significant relationship was not found between vitamin D status and prostate cancer. There may be threshold beyond which vitamin D status becomes increasingly protective against prostate cancer.
Future research needs to utilize multiple vitamin D measurements in large populations over long periods of time and prospectively assess the risk of developing prostate cancer.