Subjects
Subjects of the present study were recruited from among participants of the Tehran Lipid and Glucose Study(TLGS), , an ongoing prospective study, initiated in 1998, with the purpose of determining the prevalence of non-communicable disease (NCD) risk factors. There were 5226 women, 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) and those with missing data on age at menarche (n=1019), 3141women remained; 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); eventually 3043 women remained for the purpose of the present study.
The study flowchart is presented in Fig. 1.
Study procedure:
A standard questionnaire including information on demographics, smoking behavior, physical activity habits and medical history was completed via face-to-face interviews11.
Measurements:
Using systolic and diastolic blood pressure (SBP and DBP) was measured twice after a 15 minutes 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.
Blood samples were drawn between 7:00 and 9:00 am after 12 h of overnight fasting. All blood analyses were performed at the TLGS research laboratory on the day of blood collection. All sera were stored at –80°C until the time of testing. 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 men. 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).
Definitions:
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 months12. In the present study, we estimated GFR using the abbreviated prediction equation, provided by the Modification of Diet in Renal Disease (MDRD) study 13as follows:
GFR = 186 × (SCr) -1.154 × (Age) -0.203×0.742 ×(0.742 if female) × (1.210 if African-American)
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 as well as those of pregnancies and lactation (assumed 40 weeks for each birth or 20 weeks for each abortion).
Hypertension was defined as systolic blood pressure (SBP)/ diastolic blood pressure (SBP)≥140/90 mmHg or current treatment for diagnosed hypertension14. Diabetes was defined as fasting plasma glucose (FPG)≥7.0 mmol/l or 2-h post 75g glucose load ≥11.1 mmol/l or under current treatment for diagnosed diabetes15. Smoking status was categorized as ever smoker (current/past) or never smoker16. 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 activity, and body weight. To get the total weekly leisure time energy expending the individual activities values was summed17.
Statistical Analysis
Results are reported as mean and standard deviation (SD) for numerical variables and number (percentage) for categorical measures. For numerical variables with skewed distribution, median (inter-quartile range) was calculated. Using the Cubic Spline regression, participants were categorized into two groups according to cut-offs of EEE duration, of <11, and ≥11 years. Post hoc analysis was conducted using the cut off of 45 years for age 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, a negative event, the most recent follow-up visit was considered, as leaving the residence area; “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 follow-up visit at which the CKD was recognized for the first time, and the last follow-up visit prior to diagnosis. Incidence rate of CVD 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.
Multivariable Cox proportional hazards regression model was used to estimate the hazard ratio (HR) and 95% confidence interval (CI) between groups (EEE< 11 and EEE≥ 11 years ) which was adjusted for age , BMI, smoking, hypertension and diabetes. The proportional hazards assumption of the Cox models was assessed graphically. The Statistical Package for Social Sciences (SPSS version 20; SPSS Inc.) and STATA software (version 13; STATA Inc.) was used for data analysis.