Plasma Homocysteine Level And Kidney Function Decline In A Chinese Hypertensive Population:a Community-Based Study

IntroductionHypertension is an important worldwide public health challenge because of its high prevalence and concomitant risks of cardiovascular and kidney disease. In China, hypertension has been estimated to affect266million adults accounting for nearly27%of the adult population. Hypertension is also one of the leading causes of end stage renal disease (ESRD). Several studies have shown that even modestly elevated blood pressure (BP) places individuals at increased risk of ESRD relative to those with normal BP. Therefore, early intervention for renal injury in a hypertensive population is of immense significance to prevent progression towards ESRD.The identification of factors that are responsible for the progression of renal disease in patients with hypertension is an ongoing area of interest. In addition to known risk factors, such as age, BP, diabetes, smoking and hyperlipidemia, several studies are emerging regarding the risk of elevated plasma homocysteine (Hcy) or otherwise termed homocysteinemia. Hcy, a sulfur containing amino acid involved in methionine metabolism, is a potential atherogenic molecule and is considered to be an independent risk factor for atherosclerotic vascular disease, particularly in patients with hypertension. Experimental studies have demonstrated that homocysteinemia induces glomerular injury and endothelial dysfunction through the redox signaling pathways. More importantly, recent studies have confirmed that homocysteinemia is associated with microalbuminuria in the general population and in patients with diabetes, suggesting that homocysteinemia may also lead to intrarenal arteriosclerotic lesions and promote renal function decline. However, it has not yet been answered whether plasma Hey level is associated with the progression of renal damage and renal function decline in hypertensive subjects, and whether this association acts as an independent risk factor for the progression of kidney disease in this population.Folic acid and vitamin B are essential for Hey metabolism. Deficiency in these nutrient components induces homocysteinemia, which can be corrected by supplementation of these vitamins. China is a country with a high prevalence of folic acid deficiency and homocysteinemia, particularly in the northern rural areas. Better and earlier identification of the hypertensive subjects at risk for the progression of renal disease may be warranted, given the success of interventions aimed at delaying ESRD among high risk groups.In this community-based, prospective cohort study including3,977hypertensive adults who had median Hey levels of13.5μmol/L and preserved renal function at baseline, we demonstrated that elevated plasma Hey level is associated with rapid decline in renal function and incident chronic kidney disease (CKD) during a mean follow-up of4.8years. Our findings show that homocysteinemia is an independent risk factor for the progression of kidney disease in patients with hypertension, at least in areas where homocysteinemia is prevalent.MethodsStudy populationThis is a community-based cohort study approved by the Ethics Committees of the Institute of Biomedicine, Anhui Medical University. All of the study subjects provided written informed consent. The study was conducted in two rural communities in Anqing and Lianyungang, China. Eligible subjects were those:(1) aged45to75years and (2) with hypertension (BP≥140/90mmHg or currently on antihypertensive therapy). Subjects were excluded from the study if they had (1) confirmed CKD (i.e., glomerular filtration rate [GFR]<60ml/min/1.73m2) at time of screening;(2) diagnosed secondary hypertension;(3) a history of stroke, myocardial infarction, heart failure, coronary revascularization, and congenital or acquired heart diseases;(4) cancer; and/or (5) liver disease and severe mental disorders.A total of6,616hypertensive adults were screened in2008,and4,135(62.5%) were assessed at follow-up and received a subsequent scheduled visit after5years. Among the4,135participants,158were excluded from the analysis because of missing follow-up data. The final analysis included3,977participants.Data collectionData collection at baseline and at the follow-up visit was conducted by trained research staff according to a standard operating procedure.Blood pressure measurement. Resting blood pressures were measured using a mercury sphygmomanometer in a seated position after>10minutes of rest. The mean of the three reads on the first baseline screening day was recorded and confirmed by three measurements on the second baseline screening day with an interval of at least one day. The mean of the three BP measurements on the second baseline screening day was used as baseline BP for analyses.Questionnaires and medical history. Questionnaires were administered to collect information on socio-demographic status, medical history and medications. Diabetes was defined as physician-diagnosed diabetes, or currently receiving hypoglycemic therapy, or fasting glucose level higher than7.0mmol/L.Anthropometry. Anthropometric measurements, including height and weight, were taken using a standard operating procedure.Phlebotomy. Venous blood was drawn after overnight fasting during the baseline visit and the follow-up visit. Both plasma and serum samples were collected and stored at-80℃. The samples were shipped by commercial cold chain transportation and measured in a central laboratory. Laboratory measurementsMeasurement of plasma Hcy. Plasma Hcy was measured by enzyme-cycling method using a commercial kit (Ausa Pharmed Company Limited, Shenzhen, China) according to the manufacturer’s protocol.Measurement of estimated glomerular filtration. Serum creatinine was measured by enzymatic method (sarcosine oxidase-PAP) using a commercial kit (Beckman coulter, Brea, CA, America) according to the manufacturer’s protocol. Estimated glomerular filtration (eGFR) was calculated using the two-level CKD-Epidemiology Collaboration (CKD-EPI) formula.Measurement of other parameters. Fasting glucose, total cholesterol, triglycerides and high density lipoprotein cholesterol (HDL-C) were measured using a Hitachi7020Automatic Analyzer (Hitachi, Tokyo, Japan).Renal outcome definitionsRapid decline of renal function was defined as eGFR loss of>3ml/min/1.73m2/year; prior studies have demonstrated that this rate is associated with increased cardiovascular disease morbidity and mortality, independent of baseline eGFR.Incident CKD was defined as follow-up eGFR<60ml/min/1.73m2. To avoid participants with minor fluctuations in eGFR that may be due to "noise", the definition also included a1ml/min/1.73m2/year decrease in eGFR.Combined renal outcomes was defined as one or more of the above outcomes.Statistical analysisAll of the analyses were conducted using the statistical package R software, version3.0.1. Mean and standard deviation and proportions were calculated for population characteristics. Because of gender differences in Hcy distribution, gender-specific quartiles of Hey were used to present baseline descriptive data. Trends in age-adjusted means across quartiles were compared by ANOVA.Multivariable logistic regression was used to assess the relations of Hcy with the following outcomes:rapid decline of renal function, incident CKD, and combined renal outcomes. In these analyses, Hcy was modeled both as a continuous variable (log-transformed) and a categorical variable (categorized into gender-specific quartiles). The model was adjusted for:age; age and eGFR; or multivariate analysis including age, gender, diabetes, SBP, body mass index (BMI), smoking, cholesterol/HDL-C ratio, triglycerides, eGFR, and previous use of angiotensin converting enzyme inhibitors (ACEIs)/angiotensin II receptor blockers (ARBs). Effect modification was tested using a multivariate logistic regression by stratifying the potential risk factors for combined renal outcomes. Adjusted splines of generalized additive models were created to evaluate the association between baseline Hcy and outcomes. All tests were two-tailed and P<0.05was considered significant.ResultsCohort descriptionData from3,977Chinese hypertensive subjects with a mean age of60years (range:45to77years) and mean eGFR of84.3ml/min/1.73m2at baseline were analyzed. The median of plasma Hcy at baseline was13.5μmol/L (Q1-Q3:11.3～16.5), with a significant difference between men (15.8μmol/L, Q1-Q3:13.3～19.0) and women (12.8μmol/L, Q1-Q3:10.6～15.4)(P<0.001).Participants with higher Hcy levels tended to be older with lower eGFR at baseline. The values for cholesterol/HDL-C and triglyceride levels were more likely to be increased across quartiles of plasma Hcy levels.Higher Hcy level is associated with increased risk for poor renal outcomesDuring18,504person-years (mean follow-up:4.8years),1425(35.8%) subjects reached combined renal events. Rapid decline in renal function developed in1246(31.3%) subjects and incident CKD was documented in550(13.8%) subjects. A higher Hcy level at baseline was associated with the renal events in the spline analysis.In an age-and eGFR-adjusted model, we observed a significant and progressive increase in the risk of rapid decline in renal function in subjects with plasma Hcy levels in the second quartile or more as compared with the bottom quartile. In a multivariate model adjusted for age, gender, diabetes, SBP, BMI, smoking, cholesterol/HDL-C ratio, triglycerides, eGFR, and previous use of ACEIs/ARBs, the odds ratio (OR) for rapid decline of renal function increased across quartiles of plasma Hey level, reaching a1.45(95%confidence interval [CI]1.15-1.81) for the top quartile compared to the lowest quartile (p for trend0.002).Higher plasma Hey was also associated with incident CKD in all models. After multivariate adjustment, higher plasma Hey remained an independent risk factor (adjusted OR1.68,95%CI1.16～2.43, p for trend0.012).In continuous analysis, higher plasma Hey was also associated with rapid decline of renal function (adjusted OR1.37,95%CI1.09-1.73, p=0.007), and incident CKD (adjusted OR1.41,95%CI0.99-2.00, p=0.054) in the multivariate adjusted models. The association between Hey level and renal outcomes remained unchanged after further adjustment for SBP at the end of the follow-up (rapid decline in renal function, adjusted OR1.38,95%CI1.10-1.74, P=0.005; incident CKD, adjusted OR1.45,95%CI1.02～2.05, P=0.039).Furthermore, we calculated eGFR using a modified Modification of Diet in Renal Disease (MDRD) equations based on the Chinese patients with CKD. Instead of the CKD-EPI equation and re-analyzed the adjusted OR for renal outcomes. The results for Hey and renal events were similar to those seen using the CKD-EPI equation models for both continuous and categorical analysis.Higher Hey level is associated with combined renal outcomes among strata of independent risk factorsIn the multivariate model, age≥65years, female gender, eGFR, SBP≥180mmHg, hypercholesterolemia and BMI>25, in addition to plasma Hey, persisted as independent risk factors for reaching the combined renal outcomes. Therefore, we further analyzed the association between plasma Hey levels and the combined renal outcomes among strata of these risk factors. In the multivariate models, an increased risk of reaching renal outcomes across increasing quartiles of Hey levels appeared to be similar among subgroups stratified according to age, SEX, eGFR, BMI, SBP and cholesterol level (p<0.05in all). Increased Hcy levels was more strongly associated with poor renal outcomes among subjects aged65or more than among those younger than age65(p for interaction<0.001).DiscussionIn this follow-up study of3,977hypertensive adults, we found an increased risk of rapid progression of renal damage and decline in renal function across quartiles of plasma Hcy levels; this increase was independent of other traditional risk factors for the progression of kidney disease. Our findings suggest that among hypertensive adults, elevated Hcy level may predict progressive renal damage and dysfunction, at least in populations with a high prevalence of homocysteinemia. The recognition of elevated plasma Hcy level as a risk factor for rapid progression of renal damage and dysfunction may help to identify hypertensive subjects for whom preventive interventions might be considered. Elevated Hcy might also be a modifiable risk factor. A number of strategies, including modification of dietary food intake components and supplementation with folic acid and vitamins B6and B12, have been reported to be efficient interventions to lower Hcy level and reduce cardiovascular risk. If such strategies are also found to be efficacious in preventing renal progression in subjects with hypertension, our findings may facilitate efforts to delay hypertensive end organ damage. Additional study is warranted to evaluate whether targeting Hcy levels can reduce the risk of renal progression in patients with hypertension.