In the current cross-sectional study, adult HD patients from 50 HD centers in Tehran, Iran, were evaluated sequentially from August 2019 to June 2020. First, the list of all the HD centers in Tehran was obtained from the Iran Dialysis Center, and then by referring to each of the 50 HD centers in Tehran, the names of all the HD patients were taken, and then the names of the patients who met the eligibility criteria to be enrolled in this study were recorded (n = 2,302). Second, the names of HD centers in Tehran were sorted alphabetically, and then the names of the patients in these centers were listed. Finally, 291 out of 2,302 subjects were selected using the systematic sampling method. Adult subjects (age ≥ 18 years) on HD for at least 6 months prior to enrollment were included. A history of HIV infection, malignancies, chronic or acute pancreatitis, liver disease, and inflammatory diseases were considered as the exclusion criteria. All patients were on HD three times a week (4 h per session) via bicarbonate dialysate and polysulfone capillary dialyzers. All of the enrolled HD patients provided written informed consent forms. Upon approval by the Ethics Committee of the National Nutrition and Food Technology Research Institute of Iran (IR.SBMU.NNFTRI.REC.1387.319), the study was initiated under the Declaration of Helsinki.

Dietary assessment
The usual dietary intakes of the HD individuals were examined through 4 non-consecutive days including 2 dialysis days and 2 non-dialysis days using a 24-h recall approach. Since dietary intakes of patients may be different on dialysis vs. non-dialysis days, both days were selected to capture day-to-day variation in diet (23). Through a face-to-face interview with a trained dietitian, participants were asked to recall all the drinks and food items consumed within 24 h. Portion sizes models were used to help people in estimating portion size and improve accuracy. Using Nutritionist IV software (First Databank® Inc., Hearst Corp., San Bruno, CA, United States) and the USDA food and nutrient database (24), dietary intakes were analyzed to determine the daily intakes of energy, macronutrients, and micronutrients of HD patients.

DII calculation
We used an approach suggested by Shivappa et al. to calculate energy-adjusted DII (E-DII). Before the E-DII calculation, the energy-adjusted amount of each food item was calculated using the residual technique (25). Of 45 dietary items suggested by Shivappa et al., 28 food items were available for E-DII calculation including vitamins A, D, E, B1, B2, B3, B6, B9, B12, C, β-carotene, n-3 fatty acids, n-6 fatty acids, cholesterol, saturated fatty acids (SFA), trans fatty acids (TFA), polyunsaturated fatty acids (PUFA), mono-unsaturated fatty acids (MUFA), magnesium, zinc, iron, selenium, caffeine, dietary fiber, carbohydrate, fat, protein, and energy (11). Initially, subjects’ dietary consumption was subtracted from the “standard world mean” and then it was divided by the “global standard deviation” to calculate the Z score of each dietary parameter. Subsequently, the Z score of each food item was transformed to the centered percentile to minimize knowledge skewness and then multiplied by the score of inflammatory properties of each food item. Lastly, the overall E-DII for each participant was calculated by summing up the inflammatory scores of 28 food items calculated previously. Shivappa et al. suggested a DII score range of −8.87 to +7.98 with higher DII values representing a diet with pro-inflammatory properties, while lower values indicate a diet with anti-inflammatory features