Spirometry plays a crucial role in the prognostication, diagnosis, and monitoring of chronic obstructive respiratory diseases, particularly chronic obstructive pulmonary disease (COPD) and asthma (Liou and Kanner, 2009; Halpin et al., 2021). International guidelines recommend the use of spirometry indices, specifically the pre- and post-bronchodilator test (BDT) forced expiratory volume in the first second (FEV1), forced vital capacity (FVC), and FEV1/FVC values, for the diagnosis and evaluation of COPD and asthma (Pellegrino et al., 2005; Halpin et al., 2021; Stanojevic et al., 2021; Reddel et al., 2022). It is important to note that these spirometry indices can be influenced not only by age, sex, and race, but also by factors such as height, weight, and body mass index (Talaminos Barroso et al., 2018; Mozun et al., 2022; Shah et al., 2022).

The impact of COPD and asthma on the global population is significant both financially and medically (GBD Chronic Respiratory Disease Collaborators, 2020). Lung function tests, particularly spirometry indices, should be correctly employed for the detection and treatment of these illnesses, as well as for understanding the factors that may affect them (Dharmage et al., 2023). BMI is a crucial factor associated with the nature and severity of COPD and asthma. Higher BMI has been linked to lower all-cause mortality and a decreased risk of COPD, but conversely, lower BMI has been linked to an increased risk of COPD and its exacerbations (Guo et al., 2016; Zhang et al., 2021; Shin et al., 2022). Furthermore, patients with asthma who have higher BMI also have higher respiratory burden scores (Klepaker et al., 2022). The effect of BMI categories on spirometry indices in COPD and asthma, however, has not been thoroughly researched. Therefore, investigations examining their connections are necessary.

In order to better manage, evaluate, and diagnose COPD and asthma, it may be valuable to recognize BMI differences in spirometry indices. Therefore, the aim of the present study was to explore the association of BMI with spirometry indices in COPD and asthma.

In the present study, the association of BMI categories of spirometry indices in patients with COPD or asthma were analyzed, respectively. In COPD, the prevalence of underweight was much higher than in obese patients. Underweight could worsen the FEV₁%, FVC%, as well as the FEV₁/FVC (absolute value, %), conversely overweight and obesity could make FEV₁% and FEV₁/FVC (absolute value, %) much better in COPD patients. On contrary, patients with asthma showed higher prevalence of overweight and obesity than underweight. Within the cohort of asthma, underweight also decreased FEV₁%, FVC%, and FEV₁/FVC (absolute value, %), but overweight and obesity did not affect spirometry indices except FEV₁/FVC.

In patients with COPD, the proportion of the underweight category is near 20%, moreover, this category was older than the normal weight category. According to a report from World Health Organization, undernutrition is among the key driver of other respiratory diseases, like tuberculosis (World Health Organization, 2022). It serves as a reminder that undernutrition in older COPD patients should be more closely monitored by patients and physicians. Conversely, patients with asthma had a higher prevalence of overweight and obesity. Obesity is regarded as a health epidemic worldwide (Afshin et al., 2017). In asthma, more attention should be paid to the effect of higher BMI categories on pulmonary function.

The association between BMI and lung function in respiratory diseases has been previously examined. Schaeffer et al. (Schaeffer et al., 2022) used linear regression to assess the association of BMI with lung function in interstitial lung disease. They found that total lung capacity and residual volume% were worse in patients with overweight and obese BMI, and the obesity was associated with a lower FVC%. What’s more, the underweight BMI category was also associated with a higher residual volume%. Lee et al. (Lee et al., 2022) explored the risk of impaired lung function among obese individuals without metabolic abnormalities, subjects with metabolically-healthy obesity had the highest FEV₁% and FVC% but the lowest FEV₁/FVC ratio compared with non-obese and metabolically unhealthy obese subjects. Another study included 72 men with COPD also showed that BMI was positively correlated with FEV₁ (r = 0.275, p = 0.020) (Sami et al., 2018). Although there are some similarities to the results of previous studies, our findings have differences. We assessed both pre-BDTs and post-BDTs indices, especially the post-FEV₁/FVC ratio< 0.7 is the most crucial parameter for diagnosing COPD, so it is important to assess the association of BMI with this parameter. In addition, our research found no significant difference in FEV₁ and FVC but a higher FEV₁/FVC ratio in obese asthma patients, which is inconsistent with Lee et al. (Lee et al., 2022). The lower trend of FVC and subtle change of FEV₁ may be the reason for the higher FEV₁/FVC.

The present study investigated the association of BMI categories with spirometric indices based on patients with moderate to severe ventilatory dysfunction in COPD and asthma. It demonstrates the harm of the underweight BMI category for pulmonary function in both COPD and asthma, which may be explained by nutrient deficiencies. Our findings on gained residual volume in underweight individuals are consistent with a previous study in interstitial lung disease (Schaeffer et al., 2022). In overweight and obesity groups, the higher spirometry indices in COPD were observed. It may due to restriction by the loading of obesity reduced hyperinflation from the balance of obstructive respiratory disease (Ora et al., 2009; Choe et al., 2018). However, excess fat not only promotes the secretion of a range of inflammatory factors, but fat accumulation can also cause internal organs to be squeezed, which affects the breathing of the lung function (Choe et al., 2018). Hence, the relationship between obesity and lung function in COPD patients remains to be further clarified. In contrast, the worse trend of FVC% were found in higher BMI categories in asthma. It is possible due to excess weight exacerbates the restrictive defect of asthma. A previous study also demonstrated reduced lung volumes in obese subjects without lung disease (Jones and Nzekwu, 2006).

Despite the study having explored the association of BMI with spirometry indices, it had some limitations. Firstly, the data we used was all acquired from BDTs, since most of the time BDT was completed when FEV₁% < 70%, subjects with normal or mild severity ventilatory dysfunction would be mostly excluded. As a result, we could not know the effect of BMI on these populations. In the future, we should include these subjects to generalize the scope of the study. Secondly, the BDTs records we used were all from the Chinese population. Considering that race is one of the critical impactors of normal spirometry values, it is possible that BMI categories have different effects on other ethnicities. Additionally, we only evaluated COPD and asthma, other common chronic obstructive respiratory diseases, such as bronchiectasis, cystic fibrosis, etc., were not studied.

To conclude, the lowest BMI has a negative impact on spirometry indices in both COPD and asthma. Increasing BMI may positively affect pulmonary function in COPD. Pulmonologists and physicians should pay attention to these effects to better diagnose and manage patients. Maintaining a suitable BMI is important for patients with asthma or COPD. Weight maintenance should be embedded in the management of these illnesses.