Chronic obstructive pulmonary disease (COPD) is an inflammatory disease that is characterized by irreversible and progressive airflow limitation and has become a major public health problem worldwide due to its high morbidity and mortality (1). A report estimated that the global prevalence of COPD (stage II or above) is ~10% (12% for men and 9% for women) (2). The prevalence of COPD in people aged ≥40 years (9.7%) was significantly higher than that in people younger than 40 years old (2.7%) (3). COPD is one of the most burdensome diseases according to the World Health Organization's burden of disease.

The risk factors for COPD are multifactorial, including smoking, diet, genetics and environment, and alterations in dietary patterns may play a role in COPD progression (4–6). Additionally, the role of diet in inflammatory lung diseases has been indicated in previous studies (7, 8). A western diet increases the risk of COPD (9), while a diet high in vegetables, fruit, and fish is associated with a lower risk (10). Dietary Inflammation Index (DII) is an effective tool for measuring the inflammatory potential of an individual's diet (11). A higher DII score indicates that the diet is more prone to promoting inflammation, while a lower DII score indicates that the diet is more resistant to promoting inflammation (12). But there are other health conditions which do not correlate with DII, recently demonstrated in osteoporosis (13).

Thus, exploring the relationships between COPD, diet and inflammation will provide some clues for future prevention or treatment of COPD disease. However, to the best of our knowledge, the correlation between DII and COPD has not yet been studied. In the present study, 9,929 participants were obtained from the 2013 to 2018 National Health and Nutrition Examination Survey (NHANES) to explore the relationship between dietary inflammation and COPD.

Using the 2013–2018 NHANES data, we explored the association between COPD and the DII in US adults above 40 years old. We found that COPD participants had significantly higher levels of WBC, hs-CRP, and DII than those without a COPD diagnosis history. Thus, COPD was more prevalent in individuals with higher DII scores, and higher DII scores were related to a higher BMI, lower education, lower family income, higher smoking rate, and certain races, such as non-Hispanic white.

The close relationship between COPD and inflammation has been demonstrated for many years (18). Previous studies have suggested that inflammation increases the risk of acute exacerbation of COPD (AECOPD) (19), while anti-inflammation by glucocorticoids is a common strategy in alleviating COPD (20). Emerging evidence indicates that dietary patterns such as the Mediterranean diet significantly affect levels of inflammation (21). Another investigation indicated that vitamin D, a nutrient partly derived from food supplement, is associated lower risk of obstructive lung disease (22). Therefore, it is plausible that dietary modifications would exert profound impacts on the inflammatory response of COPD. Malnutrition or cachexia is accompanied by advanced COPD (23). Suitable supplementation of nutrients along with a low-inflammation diet might be essential. However, no studies have explored the association between DII and COPD in adults over 40 years old. This investigation provides some information about the relationship between COPD and diet. The DII score was positively correlated with COPD diagnosis. One potential mechanism is that the nutrients that are digested and absorbed into circulation directly affect pulmonary disease. Another interesting finding of this study is that participants with higher age seem to have lower DII scores. These results might be due to the appetites of individuals at different ages (24). Additionally, older individuals have a different gut microbial composition (25, 26), which may affect the metabolism of the host body under stimulation with different diets.

Common pro-inflammatory foods are red meat, refined carbohydrates, sweetened beverages, sweets, fried food, margarine, etc. (Supplementary Table 1) (27, 28). High consumption of processed red meat increases the risk of chronic obstructive pulmonary disease (29, 30). Previous studies have suggested that pro-inflammatory diet is associated with white blood cell counts increase (31), C-reactive protein (CRP) is a biomarker for assessing acute exacerbations of COPD (32). The effect of pro-inflammatory diets on COPD may be through the increase in WBC and hs-CRP levels, but in the current study, we cannot test specific hypotheses. Therefore, future longitudinal studies can consider the potential mechanism of diet-driven inflammation in COPD. Similarly, future research should determine the use of anti-inflammatory diets (for example, increase green leafy vegetables, herbs, spices, and certain fruits and reduce the consumption of sweets, red meat) to reduce WBC and hs-CRP levels, thereby reducing COPD occurrence. Age, race, education, annual family income, current smoking status, and hs-CRP are significantly related with COPD statues in our study. Smoking and age are two most important risk factors of COPD progress (33–35). Besides, education, annual family income and race are strong social determinants of health status in COPD patients (36–38).

The major limitations of our study is our DII is not compared with the energy-adjusted DII (E-DII), which construct a referent database of energy-adjusted nutrient scores on the basis of data from the same 11 countries used to compute the DII, with 16 publications so far (39). It would be very import to compare different types of DII score across studies conducted virtually anywhere in the world, meanwhile, the E-DII cannot be computed without access to the unique comparative databases, so we failed to perform the comparison in our study (39). Hebert J et al. reported that flavonoids as systemically important regulators of combustion should be included in the DII score, but flavonoids were not included in this study (39). It's also worth noting that we did not have enough people aged 40–50 in this study. However, the proportion of COPD patients in the 40–50 year old group was lower than that in the 50–60 and ≥60 year old groups, which is consistent with the previous studies, for that COPD is a kind of aging disease, and the incidence of disease gradually increases with age (35). Besides, age is not the most important observation indicator in this study, we think our results might still worthy of attention.

There is a positive association between the DII and COPD in adults over 40 years old, and the DII might be clinically used as a predictor for the risk of COPD. However, the potential mechanism by which food drives inflammation in COPD needs further investigation.

Publicly available datasets were analyzed in this study. This data can be found here: https://wwwn.cdc.gov/nchs/nhanes/Default.aspx.

HL designed the research. ZL analyzed the data. XT wrote the paper. XM, YZ, BZ, GZ, and YH revised and improved the grammar and data interpretation. LF and GH revised the paper. All authors read and approved the final manuscript.

We sincerely acknowledge the financial support of the National Natural Science Foundation of China (81772287, 81371902), the Natural Science Foundation of Fujian Province (2020J011241, 2021J05283, 2021J05287), and the Guangdong Basic and Applied Basic Research Foundation (2019A1515111063).

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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