Asthma is a common chronic respiratory disease. Exacerbations are inevitable for asthma patients,even following medical guidance, resulting in further decline in lung function (1). In the past few decades, asthma prevalence has steadily increased.According to the 2008-2010 Global Burden of Disease(GBD)study, there are 334 million people worldwide suffer from asthma (2). Asthma led to 1.6 million hospitalizations and 183,000 emergency department visits in 2017 (3). In 2009, deaths due to asthma per 10,000 people with asthma were 1.9 in adults and 0.3 in children (4). A large portion of the direct medical costs of asthma are related to hospitalization for severe or poorly controlled asthma (5). As a result,asthma has become one of the most common diseases worldwide,resulting in a significant burden on society (6). By identifying its risk factors,including smoking, alcohol consumption, air pollution, and occupational exposures, asthma can be prevented (6–8).

There is an increase in asthma prevalence due to unhealthy dietary patterns becoming more prevalent. According to research, obese people in the United States are responsible for 250,000 asthma cases a year (9). Adolescents with obesity and overweight subjects to increased risk of asthma (10). The metabolic complications associated with obesity are not the same for everyone. Traditionally, body mass index(BMI)is used to assess obesity, but it cannot differentiate between lean body mass and fat mass (11). The presence of visceral adiposity in conjunction with central obesity can be more relevant to poor metabolic characteristics and is increasingly valued by researchers (12). Furthermore, numerous studies indicate that visceral adipose tissue is more closely associated with diabetes, hypertension, cardiovascular disease and cardiometabolic risk factors than subcutaneous adipose tissue (13–15). In order to assess body fat amount and distribution, a variety of methods are used, including densitometry(dual-energy X-ray absorptiometry, DXA), magnetic resonance imaging(MRI), computed tomography(CT), and mechanical methods. These methods are characterized with high accuracy in assessing body fat,and the first three provide fat imaging and location (16). Since these processes are technically complex, time-consuming and high cost, they cannot be routinely used in clinical settings.The weight-adjusted waist index(WWI) was proposed (17) in 2018. Comparing to BMI, the WWI is a better indicator of fat and muscle mass composition, and it primarily reflects central obesity, independent of body weight (18). Among adults with increased WWI, there was an increased prevalence of hypertension (19), proteinuria (11), cardiovascular mortality (20), and hyperuricemia (21). Nevertheless, no studies have been conducted to determine, if WWI is associated with asthma prevalence. Our objective of this research is to determine the value of the WWI in estimating asthma prevalence in United States(US)adults.

This study demonstrated a positive association between the WWI index and asthma prevalence among US adults. The WWI index increased by 1 unit was associated with a 15% increase in asthma prevalence. By using a generalized additive model and a smoothed curve fit, we were able to visualize the association between WWI and asthma prevalence clearly. Asthma prevalence and the WWI index did not have a linear correlation ( Figure 2 ). The threshold and saturation effects showed that 10.53 was the most crucial inflection point.

Chronic diseases like asthma have placed increasingly huge burdens on health care costs, quality of life, and prevalence around the world (23). To prevent asthma, it is critical to practice primary prevention, especially targeting populations that are adapted to the WWI index may be the most effective. Accordingly, we conducted a sensitivity subgroup analysis and found out that almost all populations showed a positive association with asthma prevalence. This was excluding Mexican Americans and groups with unknown blood relatives with asthma. The WWI index has a significant positive association with asthma prevalence, indicating that the WWI index is widely used among asthmatic patients. In light of previous studies, we believe that our findings are accurate. First of all, when grouped by age, we found that older people had a higher prevalence of asthma. There was an increased prevalence of asthma with age, with a greater trend among men than among women (24). Asthma prevalence has also been found to be higher in older adults than in middle-aged adults, especially in men (25). We also found that high WWI prevalence among men was associated with higher asthma prevalence among women. In contrast, hypertension (26), diabetes mellitus (27, 28) had significantly higher asthma prevalence.

In epidemiological studies on asthma, phenotypes can be categorized according to the age at onset, the duration of the disease,and the clinical features of the disease (29). Asthma mortality and morbidity in older patients with asthma are higher than those in younger people (30). Asthma that develops late in life is susceptible to misdiagnosis and improper treatment, which can have profound negative consequences for the health of the patient (31, 32). It has also been shown in previous studies that abdominal obesity can lead to delayed-onset asthma, rather than early-onset asthma (33). A significant finding of this study is the correlation between the WWI index and the age of the first asthma attack. As a result of our findings, 3.89 years will be added to the age of asthma onset for each unit increase in the WWI index. WWI and age of first asthma onset were positively correlated even after smoothing curve fitting. It also implies that there is an increase in late-onset asthma. There is no report on this promising finding yet. This result needs to be confirmed by a large multicentre prospective study to further confirm its accuracy.

Asthma and obesity have associated mechanisms that have yet to be fully elucidated. It is possible to suggest several plausible relationships:

Furthermore, our study has several advantages. The NHANES 2001-2018 survey was conducted on a representative sample of the general U.S.population following a well-designed study protocol with extensive quality assurance and quality control procedures. Our results are reliable and can be applied to a broader range of individuals since they were adjusted for confounding covariates based on clinical understanding and previous studies. There are also limitations to our study. As a cross-sectional study, we were unable to establish a deterministic relation between the WWI index and asthma. Besides, the diagnosis of asthma was based on a questionnaire. Although previous studies have confirmed the acceptable accuracy of questionnaires (38, 39), recall bias remains. Furthermore, the database did not disclose detailed clinical variables such as medication history or asthma type classification, so further investigation is necessary. This study has many limitations, but its strength lies in its ability to reveal the relationship between the WWI index and asthma onset.

This study showed an association between the modifiable risk factor WWI index and the prevalence of asthma and age at first asthma onset. A higher WWI index was associated with an increased prevalence of asthma and an earlier age of first asthma onset. Our findings suggest that weight control and a healthy lifestyle can reduce the occurrence of asthma, although the deterministic relation between the two cannot be clearly established, but is still of interest.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

The studies involving human participants were reviewed and approved by approved and reviewed by the National Center for Health Statistics Institutional Review Board(NCHS). The patients/participants provided their written informed consent to participate in this study.

LY and YC: Conceptualization, methodology, software. MX, RL, JZ and ZH: Data curation, writing original draft. MC and GW: Writing-review & editing. All authors contributed to the article and approved the submitted version.