Association Between Serum Bilirubin Levels And Type 2 Diabetes And Its Complications

Diabetes has become a serious public health problem worldwide. By 2015, there are 415 million diabetes in the world. China has 196 million diabetes, the most of many country, with ? 51 billion spent on diabetes. The epidemic of diabetes caused a heavy burden to Chinese medical. About 90%of all people with diabetes are type 2 diabetes.The pathogenesis of type 2 diabetes and it complications are complicated, but oxidative stress plays an important role. Oxidative stress induced by chronic hyperglycemia can directly damage bio-macromolecules, it also impairs ? cells, causes insulin resistance, injures macro-and micro-vascular by affecting various cell signaling pathways and inducing inflammation.Bilirubin, one of the major products of heme catabolism, has been recognized a potent endogenous antioxidant. Many experiment studies indicated that bilirubin could protect ? cells, increase insulin sensitivity, and reduce vascular reactivity. Some cross-sectional studies indicated that bilirubin levels were negatively related to type 2 diabetes and its complications, but cohort studies were few and the results were inconsistent. In addition, Bilirubin levels were related to gene polymorphisms. Our previous genome wide association studies (GWAS) about serum bilirubin levels had found the polymorphisms of uridine diphosphate glycosyltransferase 1A1 (UGTIAI) and solute carrier organic anion transporter family member 1B3 (SLCO1B3) gene were significantly related to the bilirubin levels among Chinese population. In addition, recent Mendelian randomization study reported that UGT1A1 gene polymorphism was causally related to type 2 diabetes. While genome wide association studies did not confirm UGT1A1 was a diabetic susceptibility gene. Moreover, the results derived from the white population could not directly generalize to Chinese Han population. Therefore, three questions remained to be elucidated in studies especially in Chinese population:1. Are serum bilirubin levels related to the risk of type 2 diabetes? 2. Does the gene polymorphisms influenced the bilirubin levels cause the incident of type 2 diabetes? 3. Are serum bilirubin levels related to the risk of diabetic complications?In order to answer these questions, based on the 2008 baseline data and 2013 follow up data of Dongfeng-Tongji (DFTJ) cohort, we explored the associations between serum bilirubin levels and risk of type 2 diabetes and diabetic complications. The main contents were shown as follows:Part 1 Association between serum bilirubin levels and type 2 diabetes risk Section 1 Association between serum bilirubin levels and the prevalence of type 2 diabetes-the cross-sectional studyObjective:To examine the association between serum bilirubin levels and the prevalence of type 2 diabetes in a middle-aged and older Chinese population.Methods:We selected participants with available data of serum bilirubin levels, and normal urobilirubin and urobilinogen from the DFTJ cohort at baseline. Those with self-reported history of liver diseases and cancer, abnormal levels of liver function, and missing data for fasting glucose were further excluded. Subjects with total bilirubin levels above 34.2 ?mol/L were also excluded. A total of 19,259 participants were included in the current study. Basic information about demographic data, disease history, family history of disease, and life style were collected by questionnaire. Serum bilirubin, plasma glucose, lipid profiles, liver function, and renal function were measured. Type 2 diabetes was defined as having fasting plasma glucose (FPG)?7.0 mmol/L or having self-reported doctor-diagnosed diabetes or taking antidiabetic medications. Multiple logistic regression models were used.Results:The prevalence rate of type 2 diabetes was 18.5%. The average levels of total bilirubin, direct bilirubin and indirect bilirubin were 13.3 (10.4-17.0),3.8 (3.1-4.6) and 9.5 (7.2-12.4)?mol/L, respectively. Compared with the lowest quartile, the odd ratios (OR) and 95%confidence intervals (CIs) for quartile 2-4 in total bilirubin were 0.88 (0.74-1.03),0.78 (0.67-0.92), and 0.82 (0.69-0.96), P trend= 0.01; in direct bilirubin, the OR (95%CI) were 0.86 (0.73-1.01),0.75 (0.63-0.88), and 0.74 (0.62-0.88), P trend< 0.001; while the counterpart in indirect bilirubin were 0.87 (0.74-1.02),0.77 (0.66-0.91),0.84 (0.71-0.99), P trend= 0.025 after full adjustment of age, sex, body mass index, central obesity, education, smoking, drinking, physical activity, family history of diabetes, liver function, disease history (hypertension, coronary heart disease, stroke). Moreover, serum bilirubin levels (total, direct and indirect) increased in new on-set diabetes and decreased with the diabetic duration.Conclusion:In a cross-sectional study design, we confirmed that three types of bilirubin were inversely associated with the prevalence of type 2 diabetes in a Chinese middle-aged and older population. In addition, the serum bilirubin (total, direct and indirect) levels were related to glucose metabolic status, and they increased in those with impaired fasting glucose and new-onset type 2 diabetes, but decreased with the prolonged duration of diabetes. Further prospective cohort studies are warranted to establish the role of serum bilirubin in the development of type 2 diabetes.Section 2 Association between serum bilirubin and the incidence of type 2 diabetes-the prospective studyObjective:To examine the association between serum bilirubin levels and the incidence of type 2 diabetes in a middle-aged and older Chinese population.Methods:In the Dongfeng-Tongji (DFTJ) cohort, based on the population of cross-sectional study, we excluded participants with diabetes, coronary heart disease, stroke at baseline. After further exclusion of participants who were loss to follow-up,12,530 participants remained in current study. We further validated the findings in a nested case-control study (509 cases and 509 controls) within the Singapore Chinese Health Study (SCHS). Cox proportional hazards model and conditional logistic regression model were used in the DFTJ and SCHS, respectively.Results:In the longitudinal analysis of DFTJ cohort (772 incident diabetes cases during 4.5 years of follow-up among 12,530 diabetes-free participants at baseline), positive association was found between direct bilirubin and T2D risk with a hazard ratio of 1.29 (95%CI,1.03-1.61;Ptrend=0.03) comparing extreme quartiles. Similar results were observed in the nested case-control study within SCHS with an odds ratio of 1.63 (95%CI, 1.03-2.58; Ptrend=0.02) comparing extreme tertiles. Total and indirect bilirubin levels were not significantly associated with type 2 diabetes in either cohort.Conclusion:Our findings do not support the negative association between serum bilirubin levels and incident type 2 diabetes, instead, direct bilirubin levels were associated with increased risk of type 2 diabetes. Larger prospective studies among other populations are warranted to confirm the associations of serum bilirubin levels and risk of type 2 diabetes.Section 3 Genetic polymorphisms related to serum bilirubin levels and their susceptibility to type 2 diabetesObjective:Some genetic polymorphisms have been proven to be associated with serum bilirubin levels. While the relation to the risk of type 2 diabetes remains unclear.Methods:We selected 9,502 participants with available data of serum bilirubin levels and genotype data based on the genome wide association study of the Dongfeng-Tongji cohort. We further selected two single nucleotide polymorphisms (SNPs):rs6742078 in UGT1A]gene, and rs2417940 in SLCO1B3 gene, and analyzed their associations to the bilirubin levels and the risk of type 2 diabetes. Multivariate unconditional logistic regression and generalized linear models were used.Results:In rs6742078, participants with AA genotype had the highest bilirubin levels, while in rs2417940, the counterpart were participants with TT genotype. However, we did not observe significant associations between the two SNPs and the risk of type 2 diabetes.Conclusion:Polymorphisms in the UGT1A1 and SLCO1B3 gene were significantly associated with the serum bilirubin levels, while they were not related to the risk of type 2 diabetes. More SNPs and larger population should be included in future studies.Part 2 Association between serum bilirubin levels and the incident coronary heart disease risk among type 2 diabetesObjective:Elevated serum bilirubin are associated lower coronary heart disease (CHD) risk in general population. We investigated the relation of serum bilirubin with incident CHD risk among type 2 diabetes patients.Methods:We selected 2,918 type 2 diabetes embedded in the Dongfeng-Tongji cohort. Serum total bilirubin, direct bilirubin, and indirect bilirubin were measured at baseline. CHD events during 4.4 years follow-up were diagnosed by WHO criteria. Cox proportional hazards models were used to examine the association of serum bilirubin levels with incident CHD risk among type 2 diabetes cases.Results:A total of 440 CHD cases were identified during 12,017 person-years of follow-up. Compared with those in the lowest quartile of serum total bilirubin, the hazard ratios (HRs) and 95%confidence intervals (CIs) of incident CHD were 0.95 (0.72-1.25), 0.89 (0.68-1.17) and 0.80 (0.60-1.06) from quartile 2 to 4 after adjusted relevant covariates (Ptrend= 0.10). The counterpart HRs (95%CIs) in indirect bilirubin were 0.78 (0.60-1.03), 0.84 (0.64-1.10) and 0.74 (0.56-0.99), Ptrend was 0.08; while in direct bilirubin were 0.65 (0.50-0.86),0.79 (0.60-1.04) and 0.77 (0.58-1.02), without statistical significance (P trend= 0.24). Moreover, serum total bilirubin and indirect bilirubin interacted with drinking status in the incident risk of CHD (P interaction were 0.021 and 0.037, respectively), and the association was evident in ever drinkers.Conclusion Our findings suggest that elevation of serum IBil levels are independent protective factors for incident CHD among type 2 diabetes, particularly in drinkers.Part 3 Association between serum bilirubin levels and the incident diabetic kidney disease riskObjective:Studies indicated that elevated serum total bilirubin levels are associated with lower risk of diabetic kidney disease (DKD). Few studies examined the associations of direct bilirubin and indirect bilirubin with the development of DKD.Methods:Type 2 diabetes patients (n= 2,958) with estimated glomerular filtration (eGFR)> 60 ml min-1 1.73 m-2 from the Dongfeng-Tongji cohort were selected and followed up for about 5 years. Development of DKD was defined as decline in eGFR> 30%during follow-up. Generalize linear model was used to assess the associations of bilirubin and DKD development.Results:Compared with those in the first tertile of serum total bilirubin, the relative risks (RRs) and 95%confidence intervals (CIs) of incident eGFR decline for tertile 2 to 3 were 0.83 (0.64-1.09) and 0.75 (0.56-0.99), P trend= 0.04. The counterpart RRs (95%CIs) in indirect bilirubin were 0.74 (0.57-0.97) and 0.75 (0.57-0.98), Ptrend= 0.04. No significant associations were observed in direct bilirubin. Moreover, total bilirubin and indirect bilirubin levels interacted with smoking, the bilirubin-DKD associations were evident in ever smokers.Conclusions:Our findings suggest that elevation of serum total bilirubin and indirect bilirubin levels are independent protective factors for development of DKD, particularly in smokers.