Lower UVB exposure correlated with higher risk of colorectal cancer

University of California San Diego School of Medicine researchers have identified a possible link between inadequate exposure to ultraviolet-B (UVB) light from the sun and an increased risk of colorectal cancer, especially as people age.

Reporting in the journal BMC Public Health, researchers investigated global associations between levels of UVB light -- one of several types of ultraviolet light that reach the Earth's surface -- in 2017 and rates of colorectal cancer across several age groups in 186 countries in 2018.

Lower UVB exposure was significantly correlated with higher rates of colorectal cancer across all age groups. After other factors, such as skin pigmentation, life expectancy and smoking were considered, the association between lower UVB and risk of colorectal cancer remained significant for people aged 45 and older.

"Differences in UVB light accounted for a large amount of the variation we saw in colorectal cancer rates, especially for people over age 45. Although this is still preliminary evidence, it may be that older individuals, in particular, may reduce their risk of colorectal cancer by correcting deficiencies in vitamin D," said Raphael Cuomo, PhD, assistant professor of anesthesiology at UC San Diego School of Medicine.

Lower UVB exposure may reduce levels of vitamin D, wrote the authors. Vitamin D deficiency has previously been associated with an increased risk of colorectal cancer. The authors suggested that future research could look directly at the potential benefits of correcting vitamin D deficiencies to reduce colorectal cancer risk, especially in older age groups.

With improvements in prevention, early detection and treatment, there are more than 1.5 million colorectal cancer survivors living in the United States. Still, an estimated 52,900 people will die this year from the disease, making it the second deadliest cancer among men and women in the U.S. An estimated 149,500 people will be diagnosed with colorectal cancer in 2021.

For the study, researchers used UVB estimates obtained by the NASA EOS Aura spacecraft in April 2017 and data on colorectal cancer rates in 2018 for 186 countries from the World Health Organization's Global Cancer (GLOBOCAN) database. They also collected data for 148 countries on skin pigmentation, life expectancy, smoking, stratospheric ozone (a naturally occurring gas that filters solar radiation) and other factors that might influence health and UVB exposure, derived from previous literature and databases. Countries with lower UVB included Norway, Denmark and Canada; countries with higher UVB included United Arab Emirates, Sudan, Nigeria and India.

The authors caution that other factors may affect UVB exposure and vitamin D levels, such as vitamin D supplements, clothing and air pollution, which were not included in the study. They also caution that the observational nature of the study does not allow for conclusions about cause and effect and that more work is needed to more fully understand the relationship between UVB, vitamin D and colorectal cancer.