Study design and population
The present case–control study was conducted in Tehran's gastroenterology and diabetes clinic. Patients who were referred to evaluate for their probability of NAFLD by an Ultrasonography (USG) test because of having an abnormal or slight elevation in liver enzymes or being at risk of metabolic syndrome or having metabolic syndrome, etc., were assessed for eligibility criteria of the present study. Individuals who were diagnosed with NAFLD by USG guidance and physician's confirmation were included in the case group and other people who were not identified as having the disease were included in the control group.
We included 20–60 years old participants with willing to cooperate in the study without a history of renal and hepatic diseases (Wilson’s disease, autoimmune liver disease, hemochromatosis, virus infection, and alcoholic fatty liver(, cardiovascular disease (CVD), diabetes, malignancy, thyroid disorder, and autoimmune, not following a specific diet (due to a particular disease or weight loss) and not using potentially hepatotoxic or steatogenic drugs.
The sample size calculation was conducted using the G power software version 3.1. Considering the odds ratio (OR) of NAFLD by 1.35 for the highest versus lowest tertile of EDIH using two previous studies [11, 12], type I error of 5%, and study power of 80% (β = 0.20), and the ratio of controls to cases as 2, we needed a sample of 95 NAFLD patients and 190 controls. However, we recruited 120 patients with NAFLD and 240 controls to keep track of any possible drop-outs. Individuals in case and control groups matched on age ± 2 years.
Dietary assessment
Dietary intakes were collected using a validated 168-item semi-quantitative FFQ among Iranian adults [16]. Participants were asked to report their average dietary intake during the previous year by choosing one of the following choices: never or less than once a month, 3–4 times per month, once a week, 2–4 times per week, 5–6 times per week, once daily, 2–3 times per day, 4–5 times per day, and six or more times a day. Portion sizes of each food item were converted into grams by using standard Iranian household measures [17]. Daily energy and nutrients intakes for each participant were computed using the United States Department of Agriculture’s (USDA) Food Composition Table (FCT) [18]. The Iranian FCT was used for some traditional foods that are not listed in USDA FCT.
The calculation of indices
Indices were calculated using the method introduced by Tabung et al. [6]. For EDIH 15 food items were categorized into two groups according to their potential to induce or reduce hyperinsulinemia. Those with positive associations were processed meat (sausage), red meat (beef or lamb), fish (canned tuna or fish), margarine, poultry (chicken or turkey with or without skin), French fries, high-energy beverages (cola with sugar, carbonated beverages with sugar, fruit punch drinks), tomatoes, low-fat dairy products (skimmed or low-fat milk and yogurt) and eggs. Furthermore, food items with inverse association were coffee, green leafy vegetables (cabbage, spinach, or lettuce), whole fruits, and high-fat dairy products (whole milk, cream, cream cheese, and other cheese). The EDIH score was calculated as follows*:
EDIH = (Red meat * 0.250 + processed meat * 0.199 + margarine * 0.054 + poultry * 0.183 + butter * 0.094 + French fries * 0.581 + other fish * 0.172 + high-energy beverages * 0.104 + tomatoes * 0.095 + low-fat dairy * 0.025 + eggs * 0.124 + coffee * -0.035 + whole fruits * -0.029 + high-fat dairy products * -0.046 + green leafy vegetables * -0.055) / 1000. *All food groups were included as serving per day.
The ELIH encompasses 11 dietary and lifestyle factors, including BMI, margarine, butter, red meat, and fruit juice (apple juice, cantaloupe juice, orange juice, or other fruit juice) with a positive association and coffee, whole fruit, physical activity, high-fat dairy products, snacks (potato chips, corn chips or popcorn, crackers, and Cheetos) and salad dressing with the inverse association. The ELIH score was calculated as follows*:
ELIH: (Body mass index (kg/m2) * 0.051 + margarine * 0.041 + butter * 0.058 + red meat * 0.089 + fruit juice * 0.042 + coffee * -0.020 + whole fruits * -0.029 + physical activity (MET-h/week) * -0.001 + high-fat dairy products * -0.054 + snacks * -0.024 + salad dressing * -0.059) / 1000. *All food groups were included as serving per day.
Physical activity and anthropometric measurements
Physical activity was recorded using the International Physical Activity Questionnaire (IPAQ) [19] during a face-to-face interview, and it was expressed as Metabolic Equivalents per week (METs/week) [20, 21]. Participants’ weight was recorded by trained dieticians using a standard digital Seca scale (made in Germany), wearing minimum clothes and without shoes and the nearest 100 g. Height was measured in a standing relaxed shoulder position with no shoes using a tape meter mounted on the wall to the nearest 0.5 cm. BMI was calculated as weight (kg) divided by height in square meters (\({\mathrm{m}}^{2}\)).