Study population
This longitudinal study was performed within the framework of the ongoing community-based Tehran Lipid and Glucose Study (TLGS). The prospective TLGS was originally conducted to determine and prevent the risk factors for non-communicable diseases (NCDs) (15). In this respect, TLGS consists of 5 phases, the first of which was a cross-sectional study and began in 1999. Four other subsequent phases (phases II (2002–2005), III (2006–2008), IV (2009–2011), and V (2012–2015)) were performed as prospective follow-up surveys. The current study is based on the data of the third phase. Briefly, using multistage cluster random sampling methods, 15005 people ⩾ 3 years of age were selected from three medical health centers in district 13 of Tehran.
Ultimately, a representative sample of 3021 individuals with 20- 79 years of age and complete data, were recruited and followed to the fifth phase. Based on the exclusion criteria, participants on pregnancy or lactating states, on energy consumption out of the predefined limits (800 > x > 4200 kcal/d) or specific diets or with any history of myocardial infarction, cerebral vascular accident, cancers and CKD at baseline were excluded. Hence, a total of 2054 individuals were enrolled for the final follow- up analysis of 11058.464 person-years.
All participants were initially asked to provide written informed consent. The study protocol was also approved by the ethics committee research council of the Research Institute for Endocrine Science (RIES), Shahid Beheshti University of Medical Science, Tehran, Iran.
Dietary assessment
The habitual food intake was evaluated by a validated and reliable semi-quantitative food-frequency questionnaire (FFQ) at baseline (16, 17). The individual consumption frequency of each food item was designated by trained and experienced dietitians on daily, weekly or monthly basis. The portion sizes were collected in household measures and converted to grams. The USDA Food Composition Table (FCT) was used to calculate and interpret the energy and nutrient content of each food item. The estimation of total polyphenol and subclasses’ intake was based on the Phenol-Explorer database (www.phenol-explorer.eu/contents) (18).
Measurement of covariates
The physical activity level of each participant was assessed by the Modifiable Activity Questionnaire which has previously been validated for the Iranian population (19). A metabolic equivalent (MET-h per week) was calculated according to a list of most common and daily routine activities.
Anthropometric measurements including weight and height were collected to the nearest 0·1 kg and 0·1 cm, respectively. The weight was recorded in light clothing via a SECA digital weighing scale (Seca 707; Seca Corporation; range 0·1–150 kg), and height was taken without shoes on. BMI was calculated as weight (kg) divided by square of height (m2). Arterial blood pressure was measured manually, using a mercury sphygmomanometer with a suitable cuff size for each participant after a 15-min rest in the supine position.
Systolic (SBP) and Diastolic blood pressures were determined by the initial tapping and disappearance of Korotkoff sound, respectively. Blood pressure was measured twice and the average was considered as participant’s final blood pressure. Blood samples were taken from all participants at the TLGS research laboratory after 12 to 14 hours of overnight fasting.
Fasting plasma glucose (FPG) and 2-h plasma glucose (equivalent to 75 g anhydrous glucose; Cerestar EP) were measured by enzymatic colorimetric method using glucose oxidase, with both inter- and intra-assay CV being < 2%. Serum creatinine was measured under the standard colorimetric Jaffe_Kinetic reaction method at baseline (2006–2008) and after 6 years of follow-up (2012–2015). Both Intra- and inter-assay CVs were below 3·1%; all analyses were performed using commercial kits (Pars Azmoon Inc.).
Definition
In this study, the estimated GFR (eGFR) was expressed as ml/min/1.73m2 of body surface area, using the Modification of Diet in Renal Disease (MDRD) equation (20). The MDRD equation is as follows:
eGFR = 186 × (Serum creatinine)-1.154×(Age)-0.203×(0.742 if female)×(1.210 if African-American)
Patients were classified based on individual eGFR levels pertain to the national kidney foundation guidelines (21); given this, eGFR ≥ 60 ml/min/1.73m2 are considered as non- CKD patients and eGFR < 60 ml/min/1.73m2 are those diagnosed with CKD. Hypertension was primarily defined as SBP/DBP ≥ 140/90 mm-Hg or current therapy for a definite diagnosis of hypertension (22). Furthermore, diabetes was defined in accordance with the criteria of the American Diabetes Association (ADA) as fasting plasma glucose ≥ 126 mg/dl, 2-h post 75-g glucose load ≥ 200 mg/dl or current therapy for a definite diagnosis of diabetes (23).
Statistical analysis
In this study, the normality distribution of variables was assessed by the Kolmogorov–Smirnov test and Histogram chart. The participants’ characteristics and nutritional states across quartiles of total polyphenols were reported as mean ± SD for quantitative variables and percentage for categorical variables. Linear regression and Chi-square tests were used to investigate the trend of continuous and categorical variables in proportion to total polyphenols quartiles, respectively. Hazard ratio (HR) and 95% confidence intervals (CI) of incident CKD were assessed by quartiles of total polyphenols by Cox regression analysis; the lowest quartile of dietary exposures considered as a reference group. Moreover, three models were specified for the first being the crude, the second with adjustments for sex, age, physical activity and total calorie intake, and third additionally adjusted for BMI, diabetes and hypertension. The proportionality assumption underlying the Cox model was examined, and no evidence of violation was observed. All analyses were done using IBM SPSS version 16 (SPSS, Chicago, IL, USA) with the significance level set as P < 0.05 (two-tailed).