Higher sun exposure is associated with a reduced risk of primary biliary cholangitis

Primary biliary cholangitis (PBC) is a chronic cholestatic disease with few identified environmental risk factors. There is both a large geographical variation and a latitudinal gradient of prevalence, which point towards an environmental factor being involved in disease pathogenesis. This study examined whether there was a relationship between lifetime sun exposure, ultraviolet (UV) load and the risk of developing PBC.

A case-control study of 200 prevalent cases and 200 age- and sex-matched controls was performed. Participants completed a lifetime calendar prior to the study interview and two validated time-in-sun questionnaires during the study interview. Satellite data were used to estimate the average daily UV load according to the latitude and longitude of the location of residence. Skin phenotype was assessed by using spectrophotometry, which estimated the cutaneous melanin density.

Increasing time in the sun and life calendar-derived UV load were consistently associated with lower odds of being a case, robust to adjustment for birthplace, education, smoking, and estimated cutaneous melanin density. The relationship was stronger for summer sun than for winter sun and was present for higher cumulative UV loads at ages 6-25 years [odds ratio (OR) 0.35, 95% confidence interval (CI): 0.19, 0.66] and 6-35 years (OR 0.4, 95% CI: 0.21, 0.76).

We have demonstrated for the first time that higher sun exposure and UV load, particularly in summer, were consistently associated with a reduced risk of developing PBC.