[Gilbert's syndrome: hyperbilirubinemia enemy or friend]

Gilbert's syndrome is a kind of benign inherited disease of bilirubin binding disorder, mainly due to the homozygous polymorphism A(TA)7TAA in the promoter of the gene for uridine diphosphate -glucuronosyltransferase 1A1 (UGT1A1), which is a TA insertion into the promoter, designated as UGT1A1*28, with UGT activity reduction to 30% of the normal value. Therefore, circulating fat-soluble unconjugated bilirubin cannot be converted into water-soluble conjugated bilirubin, leading to unconjugated hyperbilirubinemia. Bilirubin has a strong affinity for erythrocyte phospholipids, which interferes with membrane composition and dynamics, resulting in increased erythrocytes fragility, easy rupture, and gradual shortening of survival time. However, there are no obvious sign of hemolysis or abnormal iron metabolism, erythrocytes and bone marrow morphology. A small amount of chronic hemolysis stimulates extramedullary (normal bone marrow morphology) hematopoiesis, ensuing compensatory increase in circulating erythrocytes and hemoglobin. Hyperbilirubinemia may also weaken gastrointestinal motility, increase passive diffusion and absorption across the intestinal mucosal epithelium by 1.5 to 2 times, thereby aggravating or worsening hyperbilirubinemia mainly with unconjugated bilirubin circulation, which indicates that there is a causal relationship between the circulating bilirubin concentration and rapid erythrocytes turnover and hemolysis rate in patients with Gilbert's syndrome. Interestingly, bilirubin also has significant antioxidant and anti-mutagenic activities, and the potential health benefits of mild hyperbilirubinemia in Gilbert's syndrome include reduced prevalence of cardiovascular disease, type 2 diabetes mellitus (and related risk factors), certain cancers, and cardiovascular-related and all-cause mortality. Exogenous bilirubin and biliverdin supplements in intestinal epithelial cells can be absorbed and may increase circulating concentration of these antioxidant compounds. With this information, we hope to raise awareness of the potentially harmful and beneficial effects of benign hyperbilirubinemia, and explore and develop beneficial medical interventions.