Endogenous estrogen exposure and chronic kidney disease; a 15-year prospective cohort study

Subjects of the present study were recruited from among participants of the Tehran Lipid and Glucose Study (TLGS). This study is an ongoing prospective study, initiated in 1998, to determine the prevalence of non-communicable disease (NCD) risk factors among 15,000 participants aged > 3 years. This population was selected from the population of district 13 in Tehran using a multistage stratified cluster random sampling technique. This district was selected mainly due to the high rate of stability; moreover, its age distribution was representative of the overall population in Tehran. After recruitment, all study participants were followed in 3-year intervals. Demographic, habitus, reproductive history, family history, and cardio-metabolic risk factors were assessed through face to face interview at the initiation of the study and throughout the follow ups. A comprehensive physical exam including anthropometric measurements, systolic, and diastolic blood pressures was conducted at baseline and follow ups. Moreover, blood samples were drawn at each visit between 7:00 and 9:00 am after 12 h of overnight fasting for metabolic assessments. All blood analyses were performed at the TLGS research laboratory on the day of blood collection. The CKD status was identified at the initiation of the study and throughout the follow ups according to the Kidney Disease Outcome Quality Initiative guidelines (K/DOQI). The details of TLGS was published elsewhere [11].

There were 5226 women, regardless of menopausal status, aged 30–70 years selected for our study. After excluding those with CKD at baseline (n = 1425), those with missing data of CKD (n = 97), those with missing data on age at menarche (n = 1019), the number of remained women was 3141. We further excluded those with missing data for calculating EEE (n = 98), HRT users (n = 87), and those without at least one follow-up(n = 92). Finally, 3043 women remained for the present study.

The study flowchart is presented in Fig. 1.

Using a measuring tape with 0.5-cm accuracy, in a standing position against a wall, without shoes, and with shoulders in a normal position, height was measured. Using an electronic digital weighing scale with 0.1-kg accuracy, while women were minimally clothed, without shoes, weight was measured. Body mass index (BMI) was calculated using dividing weight in kilograms by height in meters squared.

Systolic and diastolic blood pressure (SBP and DBP) was measured twice after a 15 min rest in a sitting position on the right arm, and the mean was considered as the participants’ blood pressure. Weight was measured with individuals minimally clothed, using digital scales (Seca 707: range 0.1–150 kg) and recorded to the nearest 0.1 kg. Height was measured in a standing position, using a tape meter, while shoulders were in normal alignment.

Plasma glucose was measured using an enzymatic colorimetric method with glucose oxidase. Serum concentrations of creatinine (Cr) were tested by kinetic colorimetric Jaffe. The sensitivity of the assay was 0.2 mg/dL (range, 18–1330 μmol/L (0.2–15 mg/dL). Reference intervals based on the manufacturer’s recommendation was 53–97 μmol/L (0.6–1.1 mg/dL) in women. Intra-assay and inter-assay CVs were less than 3.1% at both baseline and follow-up phases. All biochemical assays were performed using commercial kits (Pars Azmoon Inc., Tehran, Iran) by a Selectra 2 autoanalyzer (Vital Scientific, Spankeren, The Netherlands) [12, 13].

Menopause was defined as the absence of menstruation during consecutive12 months according to the World Health Organization definition [14].

According to the Kidney Disease Outcome Quality Initiative guidelines (K/DOQI), CKD is defined as either kidney damage or Glomerular Filtration Rate (GFR) < 60 mL/min/1.73 m2 for > 3 months [15]. In the present study, we estimated GFR using the abbreviated prediction equation, provided by the Modification of Diet in Renal Disease (MDRD) study [16] as follows:

In this equation, estimated GFR (eGFR) is expressed as mL/min per 1.73 m2 and serum creatinine (SCr) is expressed as mg/dL; based on the guidelines, we considered CKD as an eGFR> 60 mL/min/1.73 m2, occurring at any time during the follow-up period.

Endogenous estrogen exposure (EEE) was defined as the time interval between age at menarche and menopausal age or age at CKD event or end of follow-up, whichever occurred earlier. To consider only E2 dominant of menstrual cycles, we omitted the cumulative durations of progesterone dominant phases of menstrual (only the first 2 weeks, follicular phase, for each menstrual cycle was considered) using as well as those of pregnancies and lactation (assumed 40 weeks for each birth or 20 weeks for each abortion); as a result, EEE is absolutely less than half of the time duration between menarche and menopause.

Hypertension was defined as systolic blood pressure (SBP)/ diastolic blood pressure (SBP) ≥ 140/90 mmHg or current treatment for diagnosed hypertension [17]. Diabetes was defined as fasting plasma glucose (FPG) ≥ 7.0 mmol/l or 2-h post 75 g glucose load ≥11.1 mmol/l or under current treatment for diagnosed diabetes [18]. Smoking status was categorized as ever smoker (current/past) or never smoker [19]. Physical activity was measured as the MET value multiplied by the duration of activity in minutes multiplied by the frequency of activity per week. Each activity was weighted via its relative power, referred to as a MET; one MET shows the energy spent for an individual at rest (1 MET = 3.5 ml/kg.min of oxygen consumption). Energy expending was estimated based on the metabolic equivalent, duration of the activity, and body weight. To get the total weekly leisure time energy expending the individual activities values were summed [20].

Results are reported as mean and standard deviation (SD) for continuous variables and number (percentage) for categorical measures. For continuous variables with skewed distribution, the median (inter-quartile range) was calculated. Using the Cubic Spline regression approach, we assessed the knot as 11 years for EEE duration. Furthermore, participants were categorized into two groups according to cut-offs of EEE duration, of< 11, and ≥ 11 years. Also, the analysis was repeated by age group < 45 and ≥ 45 years at baseline.

Time to event was specified as a time of censoring or date of incidence of CKD or age at menopause, whichever happened first. Participants were censored as a result of death, loss to follow-up, or the end of the observation duration. For censored subjects or for those moving out of the area and thus lost to follow-up, the most recent follow-up visit was used. “Time” was considered as the interval between the first and the last follow-up dates. The event date for CKD was defined as midtime between the dates of the follow-up visit at which the CKD was recognized for the first time, and the last follow-up visit before diagnosis. The incidence rate of CKD was calculated per 1000 person years of follow up between those with EEE < 11 and EEE ≥ 11 years. Cumulative incidence of CKD was measured via the Kaplan-Meier method and compared between these 2 groups, using the log-rank statistic.

Cox proportional hazards regression model was used to estimate the hazard ratio (HR) and 95% confidence interval (CI) for the group with EEE < 11 years compared to those with EEE ≥ 11 years. The Cox model was adjusted for age, BMI, smoking, hypertension, and diabetes. The proportional hazards assumption of the Cox models was assessed graphically and was satisfied. The Statistical Package for Social Sciences (SPSS version 20; SPSS Inc.) and STATA software (version 13; STATA Inc.) was used for data analysis.