Subjects
This study was performed within the framework of the Tehran Lipid and Glucose Study (TLGS), a prospective cohort of 15005urban participants aged ≥ 3 years with the aim of preventing non communicable diseases (NCD) 15. The baseline survey was a cross-sectional study conducted from 1999 to 2001, followed by prospective surveys2 (2002–2005), 3 (2006–2008), 4 (2009–2011) and 5 (2012–2015).
From among 12519 participants who showed up in survey 3, 3656 were randomly selected for dietary assessment. For the current study, we selected men and women ≥ 20 years, who accounted for 3029 participants, of whom 2636 were CKD free. We excluded individuals who reported daily energy intakes outside the range of 800-4200 kcal/day (n=148). Furthermore, participants with a history of myocardial infarction (n=16), cerebro-vascular accident (n=4) or cancer (n=6), those having special diets for diabetes (n=75) or hypertension (n=87), and pregnant women (n=19) were excluded. Finally, 2076 participants were followed until survey 5 (response rate: 91%), with a median duration of 5.98years (Figure 1).
Measurements
Dietary assessment
A valid and reliable semi-quantitative168-item food frequency questionnaire (FFQ) was used to assess dietary intakes during the year preceding enrollment 16, 17. During a face-to-face interview, participants’ intake frequency for each food item during the previous year on a daily, weekly, or monthly basis was collected by trained and experienced dieticians. The FFQ contained usual foods with standard portion sizes commonly consumed by Iranians and their frequency of concumption on a daily, weekly or monthly basis. Portion sizes of consumed foods were then convertedto grams using house-hold measures.As the Iranian Food CompositionTable (FCT) is incomplete, the USDA FCT was referred to measure nutrients. For national foods not listed in the USDA FCT, the Iranian FCT was alternatively used.
Dietary diabetes risk reduction score was calculated according to the study by Rhee et al. 8 using eight components. For the components assumed to be beneficial e.g. cereal fiber, nuts, coffee, and PUFA to SFA ratio, we assigned a score of 1 to 4 based on the participant's quartile of intake in ascending order. On the contrary, for the detrimental components, including GI, TFA, SSBs, and red and processed meats, a score of 1 to 4 was assigned according to quartile of intake in descending order. The DDRRS was calculated as the sum of these values and ranged between 8 and 32, with higher scores indicating a healthier overall diet.
Covariates assessments
Participants were interviewed by qualified interviewers using pretested questionnaires, to collect data on socio-demographics, medical history, medication use, and smoking habits in the third survey of the TLGS.
Physical activity during the preceding year was determined using modifiable activity questionnaire (MAQ) and calculating metabolic equivalent task (MET) hours per week. The reliability and validity for the Persian translated MAQ has been confirmed previously 18. The MET value of the activity was multiplied by each of the activities duration and all MET-hour products were summed to reach an estimate of daily physical activity, indicating energy expenditure per kilogram of body weight during an average day.
Weight was measured in light clothing with the precision of 0.1 kg on a SECA digital weighing scale (Seca 707; Seca Corporation; range0·1–150 kg). Height was also recorded without shoes with 0.1 cm precision. Body mass index (BMI) was then calculated by dividing weight (kg) by square of height (m2). Blood pressure was also measured by means of a standardized mercury sphygmomanometer on the right arm while sitting, after a 15-min rest in the supine position. The onset of tapping Korotkoff sound marked the systolic blood pressure (SBP), while the disappearance of Korotkoff sound marked the diastolic blood pressure (DBP). It was measured twice and the mean of the two measurements was considered as the participant's blood pressure.
A 12-14 h overnight fasting blood sample was drawn from each subject for biochemical measurements. 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 technique utilizing glucose kits (Pars Azmoon, Tehran, Iran). Both inter- and intra-assaycoefficients of variation were 2.2% for FPG. Serum creatinine was assessed by standard colorimetric Jaffe_Kinetic reaction method both at baseline and after 5.98 years of follow-up. Both intra- and inter-assay coefficients of variation were <3.1%.
Definitions
Hypertention was defined as SBP/DBP ≥140/90 mmHg in participants younger than 60 years and SBP/DBP ≥ 150/90mmHg in thoseaged 60 years or above,or current therapy for a definite diagnosis of hypertension in participants60 years or older, according to JNC 8 hypertension guidelines 19. Diabetes was determined according to the American Diabetes Association criteria as fasting plasma glucose ≥126 mg/dl or 2-h post 75 g glucose load ≥200 mg/dl or current therapy for a definite diagnosis of diabetes 20. We used the Modification of Diet in Renal Disease (MDRD) equation formula to express GFR in mL/min/1.73 m2of body surface area 21.
eGFR = 186 × (Serum creatinine)-1.154 × (Age)-0.203 × (0.742 if female) × (1.210 if African-American).
Participantswere then categorized based on their eGFR levels according to the national kidney foundation guidelines 22: eGFR ≥60mL/min/1.73 m2 as not having CKD and eGFR<60 mL/min/1.73 m2as having CKD.
Statistical analysis
Data were analyzed using the Statistical Package for Social Sciences program (SPSS) (version 15.0; SPSS Inc., Chicago, IL, USA) and STATA software package. P-values <0.05 were assumed statistically significant. The Kolmogorov–Smirnov test of normality and histogram chart were used to assess normality. DDRRS was categorized into quartile cutoff points of <18, 18-20, 21-22 and >22. Continuous variables were reported as mean ± SD and categorical variables as percentages. For the continuous variables, age adjusted mean values were calculated using analysis of covariance (ANCOVA) while generalized linear models were used for the age adjusted percentages of categorical variables. Tests of trend across quartiles of DDRRS (as median values in each quartile) were conducted using linear regression test. Median (25-75 interquartile range) follow-up time was 5.98 years (25–75 interquartile range: 5.5-6.5; Fig.1). Cox proportional hazard regression models were used to assess the hazard ratios (HRs) and 95% confidence interval (CI) of CKD across quartiles of DDRRS. Age, sex, smoking status, total energy intake, BMI, hypertension, diabetes, eGFR, and physical activity were regarded as confounders. To calculate the trend of HR across increasing quartiles of DDRRS, we considered the quartile categories as continuous variables.