The Combined Association of Dietary Inflammatory Index and Resting Metabolic Rate on Cardiorespiratory Fitness in Iranian Adults


 Background: It has been shown that inflammation may be related to obesity and cardiovascular diseases. No study has examined the combined association of dietary inflammatory index (DII) of the diet and resting metabolic rate (RMR) on cardiorespiratory fitness (CRF). Therefore, we investigated the combined association between DII and RMR on CRF.Methods: This cross-sectional study was conducted on 270 adult subjects.The DII was calculated using a validated semi-quantified food frequency questionnaire. RMR was measured using an indirect calorimetric method. Socio-economic status, anthropometric measures, body composition and blood pressure were documented by a trained interviewer. CRF was assessed by using a graded exercise treadmill test. Results: Those who were classified into low DII/high RMR compared with high DII/low RMR had significant higher VO2Max (mL/kg/min) (p=0.005). Subjects in the low DII/low RMR category had significant lower VO2Max (mL/kg/min) in comparison with low DII/high RMR (p=0.001) and high DII/high RMR (p<0.001) respectively. Participants with a high DII score and low RMR had lower VO2Max (mL/kg/min) compared with those with a high DII score and high RMR (p=0.002). Moreover, we revealed that participants in the high DII/ high RMR group had lower odds of VO2max (L.min) compared with the low DII/ low RMR group which was significant. (OR: 0.93, 95% CI: 0.30, 0.38, p=0.009).Conclusions: Overall, consumption of a pro-inﬂammatory diet, is associated with 7% lower odds of VO2max among Iranian healthy adults. This study suggests that researchers should focus on combined relationships rather than single pair-wise associations for having a better judgment.


Introduction
Resting metabolic rate (RMR) is the least energy needed to keep up essential body function during a stable resting state and fasting status [1]. It is estimated that lean body mass is accounts for 60-85% of RMR. Previous studies have shown a signi cant association between RMR and body fat and weight in which obese people have a lower RMR. Additionally, in ammation may have a role in weight gain with increased fat mass that is described by the equilibrium between energy intake and energy expenditure [2,3]. Indeed, increased body mass in obese individuals results in increasing C-reactive protein (CRP) and in ammatory cytokines [4]. Therefore, obesity is considered an in ammatory state [5]. Besides, this chronic in ammation in adipose tissue accelerates the complications and diseases caused by obesity [6]. The results of the study showed a positive association between the general indicator of C-reactive protein (CRP) and the risk of coronary heart disease with mortality from cardiovascular disease [7,8]. Accumulating evidence also suggests that obesity reduces cardiorespiratory tness (CRF) [9]. CRF is a modi able and independent risk factor for mortality from cardiovascular disease (CVD) [10]. Previous studies have shown that high CRF, which is evaluated by the peak of oxygen uptake (VO 2Max ), is associated with a reduced risk of cardiovascular disease and related mortality [11]. Therefore, in ammation and VO 2Max are signi cantly associated with other major cardiovascular risk factors [11]. Several important factors like diet are involved in reducing or increasing in ammation. Recently, increasing awareness of in ammation and health, as well as understanding the effective role of nutrition in modifying the in ammation process, led to the development of the dietary in ammatory index (DII), which is an essential tool to estimate the in ammatory potential of people's diet. The purpose of making this index is to classify people's diet from maximally antiin ammatory to maximally pro-in ammatory [12]. Accumulating evidence con rms that a high DII diet is associated with an increased risk of many diseases like metabolic syndrome, diabetes, hypertension, and cancer [4,9,10,13]. In previous studies association between the DII and anthropometric measures, such as BMI, waist circumference, and waist to height ratio (WHtR) was assessed [14]. As there is an association between less RMR and obesity and given the fact that CRF and obesity are two important risk factors for CVD mediating in ammation, this study aimed to investigate the relationship between the DII and RMR with CRF in a sample of Iranian adults. We hypothesized that the higher in ammatory index of the diet in our participants is associated with low RMR and CRF in Iranian adults.

Study design
In the current study, 276 adults (118 men and 152 women) have participated. Participants were recruited through a recruitment message placed in the social network. Subjects were chosen by convenience sampling. The research criteria included apparently healthy adults with age range of 18-50, having a desire to take part in the study, and being rsident in Tehran. We excluded those who had extreme values of dietary intake (less than 800 kcal/d or more than 4200 kcal/d, respectively), suffering from kidney, liver, and lung diseases and other conditions affecting the body composition status or infectious and active in ammatory diseases, pregnancy, lactation, routine supplement or drug use, such as weight loss, hormonal, sedative drugs, thermogenic supplements like caffeine and green tea, conjugated linoleic acid (CLA), etc. After removing 3 subjects due to the above-mentioned reasons, only 270 participants remained for statistical analysis. All necessary explanations about project were given to the participants. All procedures were in accord with the ethical standards of the Tehran University of Medical Sciences (Ethics Number: IR.TUMS.VCR.REC.1396.4085), which approved the protocol and informed consent form. All participants signed a written informed consent prior to the start of the study.

Anthropometric measures
The height of participants was measured without shoes by a wall stadiometer with precision close to of millimeter (Seca, Germany). We determined waist circumference (WC) by a non-elastic tape xed in the middle of the iliac crest and the lowest rib on the exhale. Body composition including weight, fat mass (FM), fat free mass (FFM), lean body mass (LBM) and body mass index (BMI) was measured by InBody (InBody720, Biospace, Tokyo, Japan) with following protocol: avoid food ingestion for at least 4 hours, minimum intake of 2 liters of water the day before, no coffee or alcoholic beverage consumption during at least 12 hours. Subjects were asked to empty their bladder immediately before the test [15].

Assessment of other variables
Subjects completed a self-administered questionnaire to assess the participants' demographic including age, gender (male/female), smoking (not smoking/quit smoking/smoking) and education (under diploma/diploma/educated). Physical activity was assessed using the international physical activity questionnaire (IPAQ) [16]. Subjects were quanti ed into three categories including very low (<600 MET-minute/week), low (600-3000 MET-minute/week), moderate and high (>3000 MET-minute/week) calculated based on Metabolic Equivalents (METs) [17].

Dietary intakes
A validated 147-item semi-quantitative food frequency questionnaire was utilized to evaluate habitual food intake [18]. Nutritional data was gathered by experienced and trained nutritionists through duly interviews. Participants reported their intake frequency for each food item during the past year on a daily, weekly, monthly, or yearly basis. Portion sizes of consumed foods that were reported in household measures were converted to grams. The food items were analyzed for their energy content using the Nutritionist 4 software (version 7.0; N-Squared Computing, Salem, OR, USA), modi ed for Iranian foods.

Resting metabolic rate
An indirect calorimetric method (Cortex Metalyser 3B, Leipzig, Germany) was used to estimate resting metabolic rate (RMR). It is based on calculating the amount of oxygen consumed by the body. First, a ventilated hood was given to individuals, to inhale the respiratory air into the lungs, then the device determines the amount of oxygen consumed by the body according to the amount of metabolism using the volume of oxygen concentration. The calculation of RMR was under the following conditions:1) fasting over the past 12 hours 2) abstention from alcohol, caffeine, for at least 4 hours although. It is considered an ideal period of 12 hours to ensure that the body is resting and after digestion and absorption. 3) Subjects trained that rested in a supine position for 15 min also 5 minutes added to time [19].
Cardiorespiratory tness testing VO 2max by the treadmill and the respiratory gas analyzer (Cortex Metabolizer 3B) was measured according to the Bruce protocol [20]. This protocol is divided into successive 3-minute stages, that starts at a speed of 2.7 km·h −1 and an incline of 10% gradient for 3 minutes and becomes faster based on the participant's tolerance.
Indications for terminating the test include if the patient request to stop due to chest pain, shortness of breath, or fatigue, when participants had more than 90% maximum heart rate predicted for age, a respiratory exchange ratio ≥ 1.10, and when a plateau is identi ed (<150 ml x min −1 increase) in VȮ 2max contrary to an increment in speed.
Two of the three criteria should meet. After that individuals cool down with 3 minutes-4 km/h walk and stretching exercises. DII development DII score was determined by multiplying the dietary in ammatory weights [21] of 29-item nutrients or food. Afterward, these values were summed. First, the daily intake of macro-and micronutrients (carbohydrate, protein, total fat, cholesterol, saturated fatty acids, monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), n-3 fatty acids, n-6 fatty acids, β-carotene, vitamin A, vitamin C, vitamin D, vitamin E, vitaminB6, vitaminB12, ber, folic acid, niacin, ribo avin, thiamin, iron, zinc, selenium, magnesium, onion, caffeine) were computed to lessen the between-person variation in dietary intake; due to lack of some nutrients in our documents (trans FAs, avan-3-ol, avones, avonols, avanones, anthocyanidins, iso avones, pepper, thyme/oregano, rosemary, garlic, ginger, saffron, and turmeric and tea), we excluded these items. Adjusted intake of food parameters for each individual was standardized to its corresponding global mean and standard deviation. The derived Z score values were converted to percentile and centered, by doubling the values and subtracting one, to normalize the scoring system and to avoid skewness. The centered percentile value for each food parameter is then multiplied by its respective overall food parameter score to obtain the food parameterspeci c DII score. Finally, the DII score was determined by summing all of the food parameter-speci c DII score.
The greater the DII score, the more pro-in ammatory diet, and more negative scores demonstrate a more antiin ammatory diet.

Statistical analysis
The normality of distributions was checked using Kolmogorov-Smirnov and Shapiro-Wilk statistical test. All variables had normal distributions. Then subjects were categorized based on median values of DII score and RMR both separately. In the next step, we merged these dichotomized groups of DII and RMR to compute four independent groups (low DII/ low RMR, low DII/ high RMR, high DII/ low RMR and high DII/ high RMR). To compare general characteristics across the four groups, we used one-way analysis of variance (ANOVA) and chisquare tests for quantitative and qualitative variables, respectively. To compare participants' dietary intakes within four groups, analysis of covariance (ANCOVA) to adjust for energy intake. We used ANOVA to examine signi cant differences across the four above mention groups. Post hoc Tukey test was used to compare pairwise mean differences. Analysis of covariance test was performed to compare the mean of CRF among DII/RMR groups after adjusting for potential confounders such as age, sex, smoking status, energy intake, physical activity, and BMI. CRF values were then transformed into binary variables according to their median values.
Binary logistic regression was performed to nd the association of CRF with DII/RMR categories in various models. First, we adjusted age and sex. Then we additionally controlled for smoking and physical activity status. In the nal model, we moreover adjusted BMI. To obtain the overall trend of odds ratios across the combined effect of DII and RMR, we considered these classi cations as an ordinal variable in the logistic regression models and the rst tertiles regarded as the reference group. All statistical analysis was performed with the SPSS (Statistical Package for Social Sciences) for Windows 25.0 software package (SPSS, Chicago, IL). The level of statistical signi cance was pre-set at p< 0.05.

Results
The general characteristics of participants are shown in Table 1. This research included a total of 270 participants (118 men and 152 women) with an age range of 20-59 years old. The mean of age, height, weight, BMI, WC, FFM and systolic blood pressure had signi cant differences across study groups. For other variables, we did not see any signi cant difference. The distribution of sex among the four groups was signi cantly different.    Multivariate adjusted odds ratios and 95% con dence intervals for CRF by the combined effect of DII and RMR are presented in Table 4. In the crude model, those who were in the high DII/ high RMR group, compared to the other classi cations were more likely to have higher VO 2max (ml/kg/min) (OR=3.21; CI95%:1.55-665, p=0.001), there was no association after adjusting for confounding variables. Moreover, we found that participants in high DII/ high RMR group, had lower odds of VO 2max (L.min) which was signi cant (OR: 0.78, 95% CI: 0.37-1.65, p=0.045). When potential confounders were taken into account, such association remained signi cant (OR: 0.93, 95% CI: 0.38-0.38, p=0.009).

Discussion
The results of our study demonstrated that the mean of VO 2Max (mL/kg/min) was higher in participants that were classi ed as high DII/ high RMR. Also, we found that those who were classi ed into low DII/high RMR compared with high DII/low RMR had signi cant higher VO 2Max (mL/kg/min). subjects in the low DII/low RMR category had signi cant lower VO 2Max (mL/kg/min) in comparison with low DII/high RMR and high DII/high RMR respectively.
Participants with a high DII score and low RMR had lower VO 2Max (mL/kg/min) compared with those with a high DII score and high RMR. Moreover, we revealed that participants in the high DII/ high RMR group had lower odds of VO 2max (L.min) which was signi cant.
In line with our results, a study by Potteiqer et al [22]. Showed that participants lost 5 kg of body weight and about 4% of their adipose tissue during a 16-month exercise program. Also, after nine months, it was associated with a signi cant increase in VO 2max and a signi cant increase in RMR in both sexes. Eventually, the results showed that following a moderate-intensity aerobic exercise program along with reduced caloric intake from foods lead to an increased RMR and weight loss and body fat in obese people [22]. Another study by Poehlman et al. indicated a high correlation between aerobic capacity and RMR in men. They also showed that RMR was higher in strong men with high physical tness [23]. On the other hand, positive stepwise gradient in RMR according to tertiles of CRF in a cross-sectional study by shook et al. indicate the key role of aerobic capacity on resting metabolic rate.
In this study, participants with moderate to high CRF had higher RMR than those with low CRF [24]. Previous results by Kim and colleges have also shown that a difference in measured RMR and predicted RMR in obese men and also shown that there is a signi cant difference between measured RMR and predicted RMR in Korean obese men. This study also reported a positive association between their aerobic capacity and RMR [25]. The reasons for these con icting ndings may be related to the various sample sizes of studies or also various studies design, even though lack of adjustment for different confounders such as individuals medical and family history.
Two possible mechanisms mentioned in studies regarding the effect of physical activity on RMR are as follows: physical activity can affect RMR by accelerating muscle growth and affecting physiological processes.
Cardiorespiratory tness also appears to be a key predictor of RMR, although it operates independently of skeletal muscle mass [29]. This difference in RMR according to CRF groups is probably due to physiological processes [24]. Other mechanisms for explaining how CRF and physical activity affect RMR levels may be related to sympathetic nervous system regulations [30][31][32], the function of neuroendocrine system [33,34], structure changing of myocytes [35], and various immune responses [36].
Several limitations are better to be considered in the explanation of our ndings. The main limitation of our study is its cross-sectional design which does not accurately state the cause-and-effect relationship. Another limitation is the low sample size of our study. Also, our estimates of the dietary in ammatory index were limited to the items in the standardized food frequency questionnaire in Iran and some food items are not consumed by the Iranian population.
However, some strengths of our study should be noted that the present study is the rst study from Iran to examine the combined association of dietary in ammatory index and RMR on cardiorespiratory tness. As well, we have used the standardized 168 items FFQ that has been collected for the Iranian eating habits assessment.
Moreover, we adjusted several important confounders which could affect our main results. Therefore, the results of the present study can be a positive step in the direction of anti-in ammatory diet recommendations by physicians.

Conclusion
In conclusion, consumption of a pro-in ammatory diet, as indicated by High DII/ High RMR, is associated with 7% lessen odds of VO 2max among Iranian healthy men and women. However, more studies on this area are needed to con rm the veracity of our results. This study suggests that researchers should focus on dietary indexes rather than single antioxidant nutrients for having a better judgment.