Association between bilirubin and nonalcoholic fatty liver disease in the non-obese Chinese population: a cross-sectional study

BACKGROUND: Serum bilirubin may play a role in preventing antioxidant and cytoprotective effects in physiological conditions. Serum bilirubin levels are inversely correlated with insulin resistance and the prevalence of cardiovascular diseases and diabetes mellitus. However, the correlation between serum bilirubin and nonalcoholic fatty liver disease (NAFLD) is unclear. NAFLD in non-obese participants may lead to serious health problems, calling for prompt recognition and early management. This study aimed at investigating the relationship between the serum bilirubin levels and NAFLD in non-obese Chinese adults. METHODS: We evaluated 4,900 non-obese subjects (body mass index <25 kg/m(2)) residing in Wuwei, China. The subjects received a baseline questionnaire, physical examination, abdominal ultrasonography, and laboratory check-ups. Fasting serum bilirubin was measured with an automated biochemical analyzer. NAFLD was diagnosed based on imaging findings of fatty liver disease on ultrasonography, without excessive alcohol intake and other known causes for chronic liver disease. A logistic regression model was applied to calculate the association between serum bilirubin level and NAFLD in non-obese subjects. RESULTS: NAFLD was diagnosed in 408 (203 men) of the subjects, and they had a mean age of 51-year-old. Non-obese NAFLD patients had lower serum direct bilirubin (DBIL) levels than control group did [2.50 (1.80–3.25) vs. 2.60 (1.90–3.50), P=0.004], but no significant differences in indirect bilirubin (IBIL) and total bilirubin (TBIL) levels of the two groups was seen (both P>0.05). After adjusting confounding factors such as age, gender, body mass index, blood glucose, and blood lipids, multivariate analysis showed serum DBIL (OR: 0.96, 95% CI: 0.83–1.11, P trend =0.6022), IBIL (OR: 1.02, 95% CI: 0.89–1.17, P trend =0.7756), and TBIL (OR: 1.00, 95% CI: 0.87–1.15, P trend =0.991) levels were not associated with NAFLD in the non-obese population. In addition, subgroup analyses (stratified according to age, gender, and medical histories of hypertension, and diabetes mellitus) suggested no independent association between NAFLD and DBIL, IBIL, or TBIL. CONCLUSIONS: Our data suggest that serum bilirubin levels are unlikely to be associated with NAFLD in non-obese subjects.