Study population
The present cross-sectional study was conducted in 2020 on 200 members of the Navy military forces in Mazandaran Province. Included subjects were aged from 18 to 45 years old who was willing to collaborate on the study. People with diagnosed chronic diseases such as diabetes, hypertension, cardiovascular disease, kidney disease, injury and inability to perform physical assessments were not included in the study.
Demographic characteristics and physical activity
General characteristics including date of birth, educational status, use of medications and supplements, history of previous illnesses, history of chronic illness in first-degree relatives, history of surgery, and adherence to a specific diet were collected using a demographic questionnaire designed by authors.
Dietary intake assessment
In this study, a semi-quantitative food frequency questionnaire was used to estimate food intake. The validity and reliability of this questionnaire has already been confirmed in Iran. The questionnaire includes 147 food items that assess food consumption over the past year.
The dietary inflammatory index (DII)
The DII calculation was performed according to previous studies (20). First, the intake of each of the food parameters that could be estimated based on the food frequency questionnaire was subtracted from the global intake of that food parameter and divided by its global standard deviation, which obtained a Z score. The resulting number is converted to a percentile (a number between 0 and 1) and multiplied by 2 and subtracted from 1 to remove the skewness to the right. This number multiplied by each food parameter's inflammatory score and all numbers were summed to obtained overall dietary inflammatory score for each person. In general, 30 dietary parameters (including vitamin B12, vitamin B6, beta-carotene, caffeine, carbohydrate, cholesterol, energy, total fat, folic acid, garlic, iron, magnesium, mono-unsaturated fatty acids, niacin, omega-3 fatty acids, omega-6 fatty acids, onion, protein, poly-unsaturated fatty acids, riboflavin, saturated fatty acids, selenium, thiamin, trans fatty acids, vitamin A, vitamin D, vitamin E, zinc, and tea) were used in the present study to estimate the dietary inflammatory index. Finally, the subjects were divided into two groups based on median into low or high inflammation in diet.
Anthropometric measurements
Weight was assessed using a Secca scale with minimum possible cloth and an accuracy of 100 g. Height was evaluated without shoes with a Secca stadiometer. Also, waist and hip circumference were measured using a tape measure with an accuracy of 0.1 cm. Body fat percentage was estimated using the skin fold and caliper method. Body mass index (BMI) and waist to hip ratio (WHR) were calculated by dividing weight to squares of height and waist circumference to hip circumference, respectively.
Physical performance evaluation
Physical function was evaluated in two components of aerobic performance and physical strength. The 12-minute cooper test was used to measure aerobic capacity and estimate maximum oxygen uptake (VO2max). In this regard, subjects were asked to run/walk as maximum distance as possible for 12 minutes. Two weeks before main test, a trial test was conducted for all subjects to familiarize them with the process of assessment. The covered distance for each participant was recorded and following equation was used to estimate VO2max:
VO2 max (ml/kg/min) = (22.351 × distance covered in kilometers) -11.288
To assess physical strength, the number of repetitions for sit up, pull up, and push up in 1 minutes was recorded. They were asked to have a short warm-up and then complete their tests with a 30 minutes interval between tests.
Statistical analyses
Quantitative and qualitative variables are reported as mean ± SD or frequency (%), respectively. To assess the relationship between DII and anthropometric measures and physical performance, participants were classified into 2 groups according to DII median (high DII and low DII). The normality of quantitative variables was inspected using the Kolmogrov-Smirinov test. The independent samples t-test was used to investigate the association between DII and quantitative variables. The relationship between qualitative variables and DII was assessed by the chi-square test. The association between DII and dependent variables was evaluated by multivariate linear regression. In the crude model weight, BMI, waist circumference, hip circumference, WHR, body fat percent, VO2max and physical strength were entered to model as response variables, and DII groups were entered as independent variables. In the adjusted model, the effect of energy intake was controlled. Data were analyzed using SPSS software version 25 (IBM Corp. IBM SPSS Statistics for Windows, Armonk, NY). P-values <0.05 considered statistically significant.