by Study participants
The TLGS study was initiated in 1999 in the city of Tehran and its data are collected prospectively at 3-year intervals. Detailed information is provided elsewhere22. The current study was conducted under the TLGS framework.
In the third TLGS survey (2006-2008), out of 12523 participants, 3568 were randomly selected for dietary assessment. We excluded individuals based on at least one of the following criteria: lack of age and gender data (n = 40), age ≤18 years (n = 513), history of myocardial infarction, stroke, or cancer (n = 27 men and 14 women), daily energy intake less than 800 or more than 4200 kcal per day in men and outside the range of 500-3500 kcal per day in women (n = 297), and pregnant or lactating women (n = 53) . Interestingly, some people fall into more than one category. Finally, 2717 (45.6) participants in the third survey (baseline) remained to enter the study for follow-up until the fourth survey (end of follow-up, 2009-2011) (Fig. 1). It must be said that only 1244 participants had data on insulin.
Flowchart of the study and follow-up population.
After excluding diabetic patients at baseline (n = 183) and people with missing diabetes data at baseline (n = 222) and follow-up (n = 425), 1884 participants remained for the final diabetes analysis . For analyzes on HTN, after excluding participants with baseline HTN (n = 351) and those with missing baseline (n = 26) and follow-up (n = 381) BP data, 1959 participants remained.
For MetS analyses, individuals with MetS at baseline (n = 724) and those with missing baseline (n = 18) and follow-up (n = 328) MetS data were excluded, and ultimately 1647 individuals remained for final analysis. Additionally, from 1244 participants with insulin data, after excluding individuals with IR (n = 187) at baseline, 1057 individuals were followed up through the fourth survey (follow-up rate = 100%) and all remained for the final analysis on the RE.
Demographic and anthropometric evaluations
Demographic information was assessed using a pretested baseline questionnaire, in which subjects were asked to answer questions about age, gender, smoking status, education level, medical history, medication use, etc.
Anthropometric assessments were performed by a trained and experienced dietitian at baseline. Weight was measured using a SECA digital scale (Seca 707; Seca Corporation, Hanover, Maryland) to an accuracy of up to 100 g on light clothing. Height was measured using a stadiometer with a minimum of 1 cm in a standing position with no shoes and shoulders in normal alignment. BMI was calculated as weight (kg) divided by the square of height (m2). Waist circumference (WC) was measured with an accuracy of 0.1 cm using a non-elastic tape measure, between the lower ribs of the thorax and the iliac crest at the navel, over light clothing and without any pressure.
Biochemical and clinical measurements
Biochemical and clinical measurements were performed at baseline or at the end of study follow-up. Blood samples from all participants were collected after 12-14 hours of overnight fasting in a stationary sitting position between 7:00 and 9:00 and centrifuged within 30-45 minutes of collection. All samples were analyzed at the TLGS Research Laboratory at the time of collection using the Selectra 2 Automated Analyzer (Vital Scientific, Spankeren, The Netherlands). Fasting blood glucose (FBS) was measured using an enzymatic glucose oxidase colorimetric method. The inter/intra-assay coefficient changes for FBS were both 2.2% for FBS. The 2-hour oral glucose tolerance test was performed using 82.5 g of glucose monohydrate solution (equivalent to 75 g of glucose anhydrous), which was administered orally to all individuals > 20 years of age, with the exception of diabetic patients on antidiabetic drug therapy based on the prescription of the endocrinologist. Fasting serum insulin (FSI) was measured by electrochemiluminescence immunoassay (ECLIA), using Roche Diagnostics kits and the Roche/Hitachi Cobas e-411 analyzer (Gmbh, manhim, Germany). The inter- and intra-dose coefficient changes for insulin were 1.2 and 3.5, respectively. Triglyceride (TG) levels were measured using the enzymatic colorimetric method with glycerol phosphate oxidase. The inter- and intra-assay CVs for TG were 0.6 and 1.6%, respectively. Serum high-density lipoprotein cholesterol (HDL-C) was measured after precipitation of apolipoprotein B-containing lipoproteins with phosphotungstic acid (PTA).
Blood pressure (BP) was measured after resting for at least 15 minutes sitting in a chair, twice on the right arm, with a minimum interval of 30 seconds, using a mercury sphygmomanometer and the Korotkoff sound technique, with a accuracy of 2 mmHg. The mean of the two measurements was considered the BP of the subject; systolic blood pressure (SBP) was recorded with the first sound audible and diastolic blood pressure (SBP) with the sound disappearing.
Definitions
IR
The Homeostatic Model of Insulin Resistance Assessment (HOMA-IR) was used to assess IR (HOMA-IR = FBS (mmol/L) × Insulin (μU/mL)/22.5). HOMA-IR ≥ 3.2 was determined as a criterion for IR23.
Diabetes
Diabetes was defined according to American Diabetes Association (ADA) criteria as FPG ≥ 126 mg/dL or 2-hour glucose load after 75 g ≥ 200 mg/dL or intake of oral glucose-lowering drugs24.
HT extension
SBP ≥ 140, DBP ≥ 90 or taking antihypertensive drugs were determined as criteria for HTN25.
MetS
MetS was defined according to the Joint Interim Statement as the presence of 3 of the following 5 factors26: (1) Abdominal obesity as WC ≥ 95cm for both sexes, according to new Iranian adult WC cut-off points27; (2) FPG ≥ 100 mg/dl or drug treatment; (3) fasting triglycerides ≥ 150 mg/dl or drug treatment; (4) fasting HDL-C < 50 mg/dL for women and < 40 mg/dL for men or drug treatment; and (5) high blood pressure was defined as SBP ≥ 130 mmHg, DBP ≥ 85 mmHg, or antihypertensive drug treatment.
Visceral adiposity index (VAI)
This index was calculated for men and women as follows:
Males: VAI=(WC (cm)/(39.68+(1.88×BMI (kg/m2)))×(TG (mmol/L)/1.03)×(1.31/HDL (mmol /L)).
Females: VAI = (WC (cm)/(39.58+(1.89×BMI (kg/m2)))×(TG (mmol/L)/0.81)×(1.52/HDL (mmol /L)).
residual life BMI
We regressed WC on BMI to obtain BMI-independent WC values, calculated as the differences between each individual’s WC and the BMI-predicted WC.
Assessment of physical activity
The modified and validated version of the modified Activity Questionnaire (MAQ) for the Iranian population was used to assess the participants’ baseline BP status28. Individuals were asked to report frequency and time spent on BP during the past year as light, moderate, heavy, and very intense intensity. BP levels were converted into weekly equivalent metabolic hours (MET.h/Wk.). Detailed information is available elsewhere16.
Assessment of dietary intake
Dietary intakes were assessed using a valid and reliable 168-item semi-quantitative food frequency questionnaire (FFQ). FFQ validity was previously satisfied by comparing questionnaire-derived food group values with estimated values from twelve 24-h dietary recall surveys29.30. The frequency of consumption of each food during the last year on a daily, weekly or monthly basis was collected during a face-to-face interview by trained and qualified dietitians. The portion sizes of foods consumed reported in household measurements were then transformed into a gram scale using the United States Department of Agriculture (USDA) Food Composition Table (FCT). Additionally, USDA FCT is used to calculate energy and nutrient content. Iranian FCT has also been used for some local food products not available in the USDA FCT. Dietary intakes in the third survey (2008-2011) of TLGS were considered as baseline exposure.
Index calculation
The DIR and LIR scores were calculated using the method explained previously21. The DIR index includes 12 dietary items, including pickles, refined grains, doogh, lemon juice, sugary drinks, fish (as items directly related to IR), starchy vegetables, snack foods, low-fat dairy, broth, red meat and high-fat foods. full-fat dairy products (as items inversely related to IR). Additionally, the LIR index contained seven dietary and lifestyle items, including BMI, refined grains, doogh (as items directly related to IR), low-fat dairy, physical activity, starchy vegetables, and high-fat dairy. of fat (as elements inversely related to RE). The dietary intakes of each food group were converted into portions per 1000 Kcal of energy intake. Then each component, including dietary items, body mass index, and physical activity (MET.h/week), multiplied by their respective weights, which were reported in the development study21and the values for all components of each DIR or LIR were added together to obtain the final score. A higher score of the DIR and LIR indices means a greater potential for diet and lifestyle factors to increase the risk of insulin resistance and vice versa.
statistic analysis
Data were analyzed using the Social Sciences Statistics Package (version 20.0; SPSS Inc, Chicago, IL). Histogram plots and Kolmogorov-Smirnov analysis were used to evaluate the normality of the variables. Participants were classified according to DIR and LIR tertiles. Baseline characteristics of individuals were expressed for continuous and categorical variables such as mean ± standard deviation (SD) or median (25-75) interquartile range (IQR) and percentage, respectively. Trends of qualitative and quantitative variables in tertiles of DIR (such as the median value in each tertile) were tested using chi-square and linear regression. Multivariate logistic regression was used to estimate the risk of IR, T2D, HTN and MetS as dependent variables and DIR and LIR scores as independent variables; odds ratios (OR) and 95% confidence intervals (CI) were reported. Regression models were adjusted for age, sex, energy intake, smoking, education level, and occupational status, as well as BP and VAI (for DIR only), and lifetime TG/HDL-C ratio and residual BMI (for LIR only). ). P-values <0.05 were considered statistically significant.
Ethics approval and consent to participate
Written informed consent was obtained from the participants. All procedures performed in studies involving human participants adhered to the ethical standards of the institutional and/or national research committee and the 1964 Declaration of Helsinki and its subsequent amendments or comparable ethical standards. The study protocol was approved by the research council of the Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences
