Could the sun lower PBC risk? Study explores benefits of UV exposure.
Insufficient UV radiation or vitamin D may influence the development of disease
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People who are exposed to more sunlight over their lifetimes are less likely to develop primary biliary cholangitis (PBC), a study shows.
“This suggests that insufficient UV radiation or vitamin D may influence the development of PBC,” researchers wrote. Production of vitamin D, a nutrient involved in many body processes, including immune activity, depends on sunlight (UV) exposure.
The study, “Higher sun exposure is associated with a reduced risk of primary biliary cholangitis,” was published in the International Journal of Epidemiology by a team of researchers in Australia.
PBC frequency varies across different parts of the world, studies suggest
PBC is an autoimmune disorder in which the immune system mistakenly attacks healthy cells in the bile ducts, a series of tubes that carry the digestive fluid bile from the liver to the intestines.
It’s not clear what causes this self-targeting immune attack to develop, but it is thought to involve “a complex interaction between genetic and environmental factors,” the researchers wrote.
Previous studies have suggested that PBC frequency varies across different parts of the world. For example, research in Australia has indicated that PBC is most common in the south of the country, which is furthest from the equator. This has led some scientists to speculate that exposure to sunlight might affect PBC risk.
The idea that sunlight might affect disease risk isn’t a novel concept. Reduced sunlight exposure has previously been identified as a risk factor for multiple sclerosis, an autoimmune disorder in which the immune system wrongly attacks healthy cells in the brain.
One plausible explanation for these findings is that sunlight allows the body to produce vitamin D. UV light in sunshine may also modulate the activity of certain immune signaling molecules.
PBC cases more likely to have been born outside of Australia
In this study, a team of Australian researchers sought to directly assess the relationship between lifetime sunlight exposure and PBC risk. They recruited 200 people with definite or probable PBC (91% women, mean age 63.6 years) and 200 sex- and age-matched controls without a history of liver disease.
PBC cases were significantly more likely to have been born outside of Australia, to have a lower level of education, and to be smokers than controls.
All participants, who lived in the Australian state of Victoria at the time of recruitment, completed questionnaires evaluating their lifetime sunlight exposure. They were asked to fill in their place of residence and occupation for each year of their life from age 6, and to estimate how much time they spent in the sun in the summer and winter of each year.
In addition, “satellite data were used to estimate the average daily UV load according to the … location of residence,” the researchers wrote.
Participants also underwent tests to determine the amount of melanin in the skin. Melanin is the brownish pigment that gives human skin its color and helps protect the body’s cells from damage due to UV light.
Sun exposure appeared important across wide age span
The team then conducted statistical tests to assess differences in sunlight exposure between the two groups.
Results indicated that people who reported spending more time in the sun and those with higher UV load based on satellite data were significantly less likely to develop PBC for all 5-year age periods from 6 up to 35 years of age.
These associations remained statistically significant even after accounting for other factors including birthplace, education, smoking status, and skin melanin levels.
We have demonstrated for the first time that higher sun exposure, particularly in summer, is associated with a reduced risk of PBC.
For instance, people in the 6- to 10-year-old age group who reported more than four hours of sun exposure were 64% less likely to develop PBC. Also, higher cumulative UV load between ages 6 and 35 years was significantly associated with a 60% lower chance of developing PBC.
“Exposure seemed important across a wide age span because the cumulative UV load was significant when 5-year periods were added from ages 6-10 years up to age 35 years, with higher UV loads prior to PBC diagnosis (10, 5, and 3 years) also associated with lower PBC risk,” the team wrote.
The researchers also found that the association between sunlight exposure and PBC risk was consistently stronger for summer sun compared with winter sun.
In addition, people with higher levels of skin melanin were less likely to have PBC.
“In conclusion, we have demonstrated for the first time that higher sun exposure, particularly in summer, is associated with a reduced risk of PBC,” the researchers wrote, adding that this finding is “consistent with the hypothesis that exposure to UV may be protective for PBC.”
The scientists stressed a need for additional studies to replicate and validate these findings, noting that this type of research may help shed light on the underlying causes of PBC.
