Strong inverse associated was found between inflammatory lifestyle and CRF. No association was found between dietary inflammatory index and CRF. No association was found between CRF and DLIS.

Cardiorespiratory fitness (CRF) is applied to evaluate a person's ability to perform physical work, and its definition is maximum capacity of the cardiovascular and respiratory system to supply oxygen to the skeletal muscles through exercise (1). CRF represents the ability to transport inhaled oxygen to the mitochondria (2). A maximal cardiorespiratory exercise test is performed to measure CRF and accordingly, it is defined as the level of oxygen consumption at peak exercise performance (VO₂ peak), or the maximal oxygen consumption (VO₂ max), so VO₂ peak or VO₂ max is the main variable of CRF (2). CRF is an important health indicator because of its strong relationship with all-cause mortality (3). It is thought to be more important than other traditional risk factors such as smoking, hypertension, high serum cholesterol, and type 2 diabetes (2). CRF can be useful in predicting the risk of cardiovascular disease (CVD), systematic arterial hypertension, diabetes, metabolic syndrome, and cancer (4–9). It has been shown that CVD death rates in individuals with moderate to high levels of CRF were 71% lower than those who are unfit (10). CRF was inversely associated with circulating pro-inflammatory factors including C-reactive protein (CRP), Interleukin (IL)-6, and IL-18, and positively associated with the anti-inflammatory cytokine IL-10 (11). Recently, many studies have reported that high CRF is associated with lower levels of CRP (12, 13).

Although nearly 50% of the individual difference in CRF is thought to be determined by genetic factors, it can also be influenced by lifestyle-related factors such as physical activity, smoking, and body mass index (BMI) (14–16). A lifestyle that includes physical activity can improve the level of CRF especially in those who have a low level of CRF (16). The World Health Organization (WHO) has reported that the recommended physical activity (PA), defined as at least 30 min/d moderate PA, is not met by more than 60% of the world's population (17).

Meanwhile, cigarette smoking has unfavorable effects on CRF, in a way that, aerobic capacity and, thus, the ability of oxygen supply during exercise, declines substantially in smokers (8). Non-smokers had a higher level of VO₂ max in comparison with current smokers and quit smokers (14). Increased BMI was also related to lower CRF (15). Poor dietary choices and a massive decrease in work-related PA in both gender and household women lead to the global prevalence of obesity during the past decades (18–20).

The relationship between CRF and dietary intake in young and older adults have been examined by some investigations. It has been documented that older adults who have higher CRF are more likely to follow the dietary recommendations than those who are unfit (21, 22). There is evidence that the diet-disease association may be mediated partly by the potential impact of dietary habits on CRF (23, 24).

Low CRF is an unfavorable health condition that is highly accompanied by low-grade systemic inflammation (25). Sedentary lifestyle (26), cigarette smoking (27), adiposity (28), and poor diet quality (29) are also important drivers of systemic inflammation in the human body. Therefore, the effects of dietary and lifestyle-related factors on CRF may be mediated, in part, by their unfavorable impacts on low-grade systemic inflammation. However, the combined effects of these factors on CRF has not been investigated.

To estimate the inflammatory potential of dietary and lifestyle behaviors, a new index has been developed to take important anti- and pro-inflammatory dietary and non-dietary components into account. The dietary and lifestyle inflammation index (DLIS) is a new-developed index (30) that combined the effects of anti- and pro-inflammatory dietary components, as well as four lifestyle-related components including PA, alcohol drinking, cigarette smoking, and BMI to present a broad picture of the effect of human lifestyle on inflammation. Thus, considering the vital role of CRF in health, the objective of this cross-sectional study was to determine the association of DLIS with CRF in adults.

To the best of our knowledge, this is the first study that assessed the association of the inflammatory potential of the diet and lifestyle combined with CRF in young adults. The main finding of our study was that higher adherence to a pro-inflammatory diet and lifestyle, reflected by high DLIS, had a strong inverse association with odds of having CRF. There was also a strong inverse association between LIS and CRF. However, our results indicated no association between the inflammatory potential of the diet, as assessed by DIS, with CRF.

In this study, participants were more likely to have a lower level of CRF with a pro-inflammatory lifestyle. The association between single components of the DLIS and levels of CRF has been investigated in previous research. Recent investigations from the Aerobics Center Longitudinal Study in the US presented that adopting a more anti-inflammatory lifestyle including having higher PA, being at a normal weight, and not smoking were related to higher levels of CRF in both men and women (36). Existing evidence suggests an inverse association between cigarette smoking and levels of CRF (37). De Borba et al. reported that non-smokers showed a higher level of VO₂ max compared to active smokers and passive smokers. However, the VO₂ max of passive smokers did not differ from active smokers (14). An experimental investigation indicated that even a short-time passive smoker exposure in non-smoking adults can exert substantial adverse effects on the cardiorespiratory and immune response to PA (38).

A randomized controlled trial demonstrated evidence for a dose-response association between PA and progress in CRF. The study mentioned that promoting either quality or quantity of PA seems to have extra influence on CRF (39). Chatrath et al. showed that overweight children are more likely to perform poorly physical tests than their non-obese peers (40); however, such a result was not found in boys (15).

There is also evidence for an association between dietary factors and CRF. Carbone et al. reported that carbohydrate intake, especially in the form of sugars, was inversely, and higher consumption of mono- and polyunsaturated fats were positive, associated with CRF (41). A cross-sectional evaluation within the CARDIA study presented that there is a positive association between diet quality, as assessed by a pre-defined diet quality index, and CRF. The study also showed a negative association between a data-driven meat pattern, rich in red and processed meat, and CRF, and in contrast, a positive association for fruit and vegetable patterns (42). Borney et al. demonstrated that lower consumption of carbohydrates, fiber, folate, calcium, vitamin A, vitamin B-6, and vitamin C; and higher consumption of total, saturated and monounsaturated fats and cholesterol were associated with low physical fitness in both genders (21). However, similar to our findings, some investigations have reported no association between dietary habits and CRF (43–46). The present study is the first try to examine the association of inflammatory potential of the diet with CRF and this highlights the need for further research to fully investigate the association between DIS and CRF.

Recent investigations presented that there is an association between diet and lifestyle with inflammatory factors (47). Montonen et al. documented that there was a relationship between high consumption of whole-grain bread and lower levels of high-sensitive C-reactive protein, whereas a high intake of red meat was related to higher levels (48). Dietary patterns characterized by high intakes of refined starches, sugar, and saturated and trans-fatty acids and low intakes of natural antioxidants, nuts, fiber (from fruits and vegetables), and whole grains may lead to a stimulation of the innate immune system, most likely by an extreme creation of pro-inflammatory cytokines related to a declined production of anti-inflammatory cytokines. Choosing healthy sources of carbohydrate, fat, and protein, in association with regular PA and not smoking have a vital role against inflammatory biomarkers (49–51). Lifestyle factors such as smoking cessation, augmenting PA, and weight loss are associated with a reduction in CRP concentration (52, 53). Therefore, CRF may be associated with inflammatory factors (54, 55), and as a result, the strong inverse association between the DLIS and levels of CRF found in the present study may be mediated partly by inflammatory pathways.

Strengths of this study that are worth to be mentioned are the careful measurements of dietary intakes of each participant which were recorded by skilled nutritionists and valid and reliable questionnaires. Also, we applied a valid tool to calculate the DLIS. Potential limitations should also be considered. The results of our investigation ought to be taken with caution due to its cross-sectional design. Hence, prospective studies are needed to confirm the long-term influence of the DLIS on CRF. Moreover, in this study two items were not included for calculating the DLIS. Missing items include supplementation score and alcohol intake. Although we adjusted several confounding variables in our study, the results may have been affected by residual confounding. Finally, we used FFQ for dietary assessment that has been shown to have some limitations in evaluating dietary information (56). Also, we have to mentioned some of variables were skewed.

In conclusion, the present study examined the joint association of inflammation-related lifestyle behaviors including cigarette smoking, sedentary lifestyle, and being overweight/obese with CRF and found a strong inverse association between a pro-inflammatory lifestyle with CRF. We did not find any association between dietary inflammatory properties with CRF. Future studies should address the relationship between the inflammatory potential of the diet and CRF.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the Ethical Committee of the Tehran University of Medical Sciences approved the study protocol (Ethical Approval ID: IR.TUMS.VCR.REC.1397.472). The patients/participants provided their written informed consent to participate in this study.

SS-B and KD contributed to the conception/design of the research. ZN and NJ contributed to the acquisition of data. MF and ZN participated in the analysis and interpretation of the data. MF drafted the manuscript. AJ critically revised the manuscript. SS-B agrees to be fully accountable for ensuring the integrity and accuracy of the work. All authors read and approved the final manuscript.

This manuscript has been granted by Tehran University of Medical Sciences (Grant No: 33887). The funder had no role in the design, analysis or writing of this article.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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