Asthma is a complex chronic respiratory disease whose primary symptoms include wheezing, coughing, and breathlessness. Nevertheless, the precise pathogenesis of asthma remains unclear (1, 2). According to the Global Initiative for Asthma (GINA) 2024 guidelines (3), approximately 300 million people worldwide are suffering from asthma, and it kills about 1,000 people every day. The World Health Organization (WHO) also predicts that the number of people with asthma could increase by 100 million by 2025 (4). Asthma not only adds to the global disease burden but also contributes to rising healthcare costs, loss of labor, and reduced ability to perform daily activities. Asthma affects adults in particular, potentially making chronic diseases like cardiovascular disease and diabetes more difficult to manage, and is especially common in older populations (5). While childhood asthma is usually associated with allergies (6), adult asthma can be caused by a variety of factors and often responds poorly to standard treatment regimens (7). Therefore, identifying the risk factors for adult asthma and taking preventive measures are crucial to improving patients’ quality of life and reducing healthcare expenditure.

The prevalence of obesity has increased rapidly in recent years. It is estimated that over one billion people around the world are affected by obesity (8). Body Mass Index (BMI) is commonly used to assess the degree of obesity in adults, with a BMI of 30 kg/m² or more defined as obese. Numerous studies have confirmed that the risk of developing asthma increases with increasing obesity (9–12). BMI is a commonly used measure of obesity in studies exploring the relationship between obesity and asthma (13, 14). However, BMI does not reflect body fat distribution (15, 16). Therefore, more precise methods are needed to explore the relationship between obesity and asthma risk. Measures of body fat distribution have shown better predictive value than BMI in predicting diabetes and cardiovascular disease (17, 18). In order to reflect fat distribution more accurately, Thomas et al. (19) proposed a new anthropometric index, the Body Roundness Index (BRI). This index calculates body roundness based on an elliptical model of body shape. It uses eccentricity to estimate the percentage of visceral and total body fat. Unlike BMI, BRI considers waist circumference and provides a more complete picture of visceral fat distribution. Studies have shown that visceral fat has a greater effect on asthma development (20), so we hypothesized that BRI may be a better predictor of asthma incidence than BMI. BRI has a wide range of clinical applications. Previous studies have shown that the BRI has demonstrated superior predictive ability to traditional anthropometric methods in assessing the risk of diseases such as cardiometabolic diseases (21–23), kidney disease (24), and cancer (25), providing a more accurate tool for predicting and preventing obesity-related health problems in clinical practice. Until now, no studies have explicitly examined the association between the Body Roundness Index (BRI) and asthma.

To fill a knowledge gap about whether BRI is associated with prevalence in asthmatics, this study aims to analyze the data from the National Health and Nutrition Examination Survey (NHANES) in the United States from 2001 to 2018 and explore the relationship between BRI and asthma in U.S. adults, with the expectation of providing a new approach to the prevention and treatment of asthma.

This study is the first to quantitatively analyze the relationship between Body Roundness Index (BRI) and asthma in U.S. adults. After adjusting for relevant covariates and based on cross-sectional data from the NHANES from 2001 to 2018, we used weighted logistic regression analysis to find that higher BRI was associated with an increased risk of asthma in adults. The findings suggest a J-shaped association between BRI and asthma risk, where changes in BRI have little effect on asthma risk below a certain threshold. However, once that threshold is exceeded, asthma risk rises as BRI increases. We found that the critical tipping point was 4.34. In addition, we found that BRI was more effective in predicting asthma risk than traditional obesity indicators such as BMI and waist circumference. Subgroup analyses suggest that management strategies for obese populations identified based on BRI should focus specifically on patients 60–79 years of age, women, nonwhite race, divorced/widowed/separated, diabetes, cardiovascular disease, and hypertension. These findings may lead to the development of new clinical guidelines.

Primary asthma prevention has been a challenge in research and clinical practice compared to timely and effective treatment options (38). The link between asthma and obesity is particularly complex, with obese asthmatics more likely to have persistent symptoms, increased absenteeism, increased medication use, and other related problems. Moreover, obese asthmatics are less likely to experience symptom relief and are more likely to develop severe persistent asthma (39). However, accurately assessing the actual extent to which obesity affects asthma is also a challenge. Although Body Mass Index (BMI) is a commonly used measure of obesity and is easy to measure, it has limitations. For example, a study of more than 40,000 U.S. adults by Tomiyama et al. found that half of the overweight population and nearly one-third of the obese population were metabolically healthy. In contrast, over 30% of the normal-weight population had metabolic abnormalities (40). The prevailing view is that the distribution of body fat and body composition cannot be described by BMI alone and that significant differences exist even among individuals with the same BMI (41). Prillaman suggests that the definition of obesity needs to be updated (42). He argues that BMI ignores essential factors such as visceral fat, which may significantly impact health. Therefore, in this study, we used the Body Roundness Index (BRI) to assess the association between obesity and asthma in adults more accurately.

Obesity can be categorized as visceral obesity, subcutaneous fat accumulation in the trunk and extremities, and fat ectopic in skeletal muscle (43). Visceral adipose tissue, in particular, being metabolically active, produces a variety of cytokines that are negatively correlated with one-second forceful expiratory volume (FEV1), FEV1% prediction, and forced lung capacity (FVC) (43, 44). Studies have noted an association between visceral adipose tissue (VAT) and the development of asthma and decreased lung function in adolescents and adults (20), whereas peripheral obesity has not been significantly associated with these health problems. A large-scale study conducted in Vienna found that a significantly higher proportion of asthmatics were obese compared to controls. There was a significant increase in visceral adipose tissue (VAT), especially in adult and elderly asthmatics (45). Indicators of abdominal obesity, such as waist circumference and waist-to-height ratio, are positively correlated with the prevalence of asthma. However, these indicators do not accurately reflect visceral adiposity (9, 46–49). Although magnetic resonance imaging (MRI) and abdominal computed tomography (C.T.) can assess visceral fat, both methods are costly and not easily accessible (44, 50). In contrast, the Body Roundness Index (BRI) is less costly and more accessible to patients.

The mechanisms linking asthma and visceral adipose tissue are not fully understood. Possible pathways of association include: (1) Adipose tissue may secrete adipokines such as leptin and adiponectin, as well as cytokines such as IL-6, IL-8, and TNFα, which triggers an inflammatory response, which in turn may lead to airway inflammation and asthma (51). (2) Airway hyperreactivity may be induced by the release of adipokines into serum during adipose tissue inflammation (52). (3) The link between obesity and asthma may also include chronic low-grade systemic inflammation, insulin resistance, and dyslipidemia (51). Studies have shown that visceral adipose tissue is more likely to be associated with adverse inflammation, blood glucose, and lipid levels than subcutaneous adipose tissue (53). Visceral fat, not subcutaneous fat, is associated with asthma (54). Further studies have also confirmed that abnormalities in visceral fat are associated with decreased lung function and increased inflammation in patients with obesity-related asthma (55). Increased visceral fat is associated with elevated levels of IL-6 (56, 57), which can disrupt the balance of fatty acid metabolism and trigger an active inflammatory state in the body (58, 59). Researchers have realized the link between IL-6 and the development of asthma (60). These findings support the general view that visceral fat is associated with a more active inflammatory state in the body. This view provides a plausible explanation for our findings: adults with a higher Body Roundness Index (BRI) are more likely to have more visceral fat, which triggers a robust inflammatory response that increases their risk of developing asthma.

This study has several strengths. First, the NHANES survey conducted between 2001 and 2018 used scientific sampling methods based on a representative sample of the U.S. general population and followed a rigorous study design with comprehensive quality control measures to ensure the reliability and scientific validity of the data. Second, the study meticulously adjusted for socioeconomic status, co-morbidities, and other possible confounders based on clinical knowledge and historical research, further ensuring the robustness of the findings. Finally, to compare the correlation between BRI and other obesity indicators, the corresponding Z-scores were calculated and included in the linear model analysis, thus ensuring the accuracy of the results. The study results showed that the correlation between BRI and asthma exceeded that of traditional obesity indicators such as waist circumference and BMI. Therefore, from a public health perspective, BRI, as a non-invasive and easy-to-administer screening tool, effectively assesses asthma risk in adults.

However, this study has its limitations. Firstly, given the study’s cross-sectional nature, we were unable to establish a causal association between BRI and asthma in adults, and future studies will need to validate this relationship with the help of cohort studies. Second, the study’s results may apply to specific countries and ethnic groups, and their generalizability has yet to be verified. Differences in fat distribution in different populations due to genetic and environmental factors may lead to differences in fat distribution, which in turn may affect the accuracy of BRI in predicting asthma prevalence. Therefore, future studies should include multinational and multiethnic samples to validate our findings further. Finally, the lack of detailed clinical variables in the database, such as medication history or specific type of asthma, is missing information that limits the depth of the study and needs to be supplemented by further research. Despite its limitations, this study demonstrated its value by revealing an association between BRI and asthma in adults. It was particularly innovative in exploring this relationship using National Health and Nutrition Examination Survey (NHANES) data.

In sum, our findings support an association between Body Roundness Index (BRI) and adult asthma risk, where higher BRI values may predict an increased risk of asthma in adults. This finding is essential for deepening our understanding of the relationship between obesity and asthma. It suggests that BRI has the potential to serve as an indicator of asthma prevention. Future studies need to further validate the efficacy of the BRI as a predictive tool for assessing adult asthma risk in different countries and ethnicities to expand its applicability to the population.