2.1 Study design and population
The present cross-sectional study was carried out on the data collected from two cohort studies (Shahedieh Cohort Study and Yazd Health Study -YaHS). Dietary foods and supplements have been investigated in a YaHS sub-study called Yazd Nutrition Survey (YNS), locally known as TAMYZ in Persian. Detailed information about the protocol of the YaHS-TAMYZ study conducted in urban area is published previously (41; 42). In Shahedieh study the investigated individuals included a total of 8966 from the suburb region within the age range of 35–70 years. The YaHS study recruited 10000 people from the urban and rural areas within the age range of 20–70 years. Participants were being on a weight loss or specific diet and having a total daily energy intake of less than 800 or higher than 6500 kcal or history of diseases such as diabetes, cardiovascular diseases, stroke, fatty liver, hypertension, cancer, and thyroid, were excluded from the study because such diseases may change the participants' diet.
2.2 Dietary assessment
The validated semi-quantitative FFQ with 178 items and 551 questions was used as an interview by trained interviewers to evaluate the dietary foods and supplements (43). In this regard, participants were supposed to report their usual consumption frequency of food items (number of times per month, week, or day) in the past 12 months. Moreover, food photo book were provided to find out exact explanations about the portion sizes of food by the interviewer. We combined single food items into 40 groups based on their similarity and converted all food items to gram per day using household portion size of food intakes (44) finally, the nutrient intakes were calculated.
2.5 Calculation of HEI diet score
The latest version of HEI-2015 were designed in 2015, in accordance to the 2015–2020 dietary guidelines provided for Americans (45). In this method, the HEI-2015 diet score was calculated based on 13 food groups with a total maximum score of 100. Nine components (adequate intake) including total fruits, whole fruits, total vegetables, greens and beans, whole grains, dairy, total protein foods, seafood and plant protein and fatty acids ((Polyunsaturated fatty acid + Monounsaturated fatty acids)/ Saturated fatty acid). Four components (moderate intake) contain refined grains, sodium, added sugars and saturated fats. In this method similar to HEI-2005 and HEI-2010, we scored each of these components on a density basis out of 1,000 calories with the exception of fatty acids, which is a ratio of unsaturated to saturated fatty acids. Adequacy components were assigned 0 to 5 points in order of minimum and maximum consumption. However, the maximum scores that can be assigned to dairy, whole grains and fatty acids, is 10. For the moderation components, the minimum and maximum consumption can range from 0 to 10, respectively. Scores of HEI-2015 was calculated for each participant regarding the 13 food items were summed. Later, the participants were categorized based on the dietary pattern scores' quintiles (quintile 1: low consumption, quintile 5: high consumption of a given food pattern). Next, the contributors' characteristics were measured across quintiles of each dietary pattern and the data were calculated by mean ± standard deviation for continuous variables and percentage for categorical variables.
2.3 Anthropometric assessment
Trained investigator measured Height and body weight. All anthropometric indicators gauged with three repetitions; before the interview, after completing one-third of the questionnaire, and after completing two-thirds of the questions. We gauged Height by using a wall-fixed tape measure without bumps with a precision of 0.1 cm with barefoot while contributors' heads, shoulder blades, buttocks, and heels were rested against the wall. Moreover, participants' weights were measured using a portable, digital scale (Omron BF511 Inc. Nagoya, Japan) with a precision of 0.1 kg, while they were in light clothing and without shoes. We computed the body mass index (BMI) according the following formula: weight (kg) by height (meters) squared. Waist circumference recorded to the nearest 0.5 cm using non-stretch tape placed midway between iliac crest and lowest rib while participants were in the standing position (46).
2.4 Assessment of covariates
Additional variables of participants including age, gender, marital status, smoking status (non-smoker, ex-smoker, current smoker), socioeconomic status (SES) (weak, moderate, high) and diseases were obtained using the demographic and medical history questionnaires. Interviewers assessed the SES score according to predefined questionnaire about the infrastructure facilities (source of drinking water and sanitation facility), housing condition (e.g., the number of rooms, type of home ownership), durable assets' ownership (e.g., dishwasher, car, television), and education level (47). Then, the total SES score, ranging from 0 to 3, was measured by adding up the assigned scores; a score of 3 showed high SES. Moreover, the Iranian version of International Physical Activity Questionnaire (IPAQ) was applied to calculated the contributors' physical activity (never, < 1 h/week, > 1 h/week), and participants with more than 1 h of activity per week were supposed as physically active (48).
2.6 Statistical analysis
The scores of the HEI-2015 diet were classified into quintiles. The first quintiles considered as the reference and the quintile categories were also considered as ordinal variables in the analyses to compute the overall trend of odds ratios (OR) across increasing quintiles of dietary pattern scores. Analysis of variance was performed for describing the mean differences of the continuous variables and we applied chi-squared test to assess the difference between categorical variables. Multivariable logistic regression analysis was also fitted in different models to determine the association of quintiles of HEI-2015 score and obesity. In first model, we adjusted age, energy intake (kcal/d), gender. Besides, in the second model we adjusted additional confounder including smoking status, SES, marital status (married, single, widowed, divorced), physical activity level, and diseases. The IBM SPSS version 20.0 was run to analyze the data and the significant P value was set at < 0.05.
Finally, the relationship between HEI-2015 dietary pattern with odds of obesity as well as central obesity in the general population of both studies (Shahedieh + YaHS), was examined with meta-analysis (fix method) by comprehensive meta-analysis software.