This study investigated the association between obesity parameters and lung function of 8,284 adults from the typical rural areas in central China. Our results show that the prevalence of obesity in this study is in a high level, which is higher than the reported obesity rates in China. Although the prevalence of general obesity defined by BMI was higher in men than that in women, the prevalence of abdominal obesity defined by WL, WHR and BFP were lower in men than those in women. Lung function levels was lower in obesity group when compared with non-obesity group. Spearman correlation analyses showed that these obesity parameters were significantly correlated with the measured lung function indicators. After adjustment for potential confounders, the defined obesity and obesity-related parameters were negatively associated with lung function parameters such as FVC and FEV1.
Some previous studies have shown that obesity can alter chest wall mechanics, reduces lung volumes and increases airway resistance [4, 11, 13]. Nonetheless, there was inconclusive or inconsistent result about the association between obesity parameters such as BMI and BFP and lung function indicators including FVC and FEV1 according to previous studies, which may due to their small sample sizes, scattered obesity parameters, and specific groups [19, 21, 44, 45]. In this study, we found several parameters such as BMI, WC, WHR, WHtR, BFP and VFI, representing the incremental degree of obesity, are negatively associated with lung function indicators, which is consistent with results of most previous studies. As we envision according to previous reports, height, HC and BM are positively associated with lung function levels. Our results suggested that obesity group have lower lung function levels compared with non-obesity group. In other words, individual with obesity status trend to have smaller lung capacity and lower lung function.
Recently, some studies have proposed that WC related parameters such as WHR and WHtR might be better indicators than BMI to assess the role of obesity in predicting lung function in general Caucasian population [44–46]. In this study, we investigated the association between several obesity parameters and series of lung function indicators in the general Han Chinese population with a large sample. The results of Spearman correlation analyses and linear regression analyses both showed that elevated BMI, WHR, WHtR, BFP, and individuals with obesity defined by BMI, WHR, WHtR and BFP have lower lung function levels such as FVC and FEV1. However, increased height, BM, WC, HC, and individuals with obesity defined by WC have a higher FVC and FEV1.
It seems that there is some conflict between several obesity related parameters mentioned above when linked to lung function in this study. Careful comparison of them provided a reasonable explanation of the differences, that is, both height, weight, WC, and HC are important factors related to lung function. Height related obesity parameters such as BMI, WHtR and obesity defined by BMI and WHtR were negatively associated with lung function because that height is in the denominator position in the formula of BMI and WHtR and a positive relationship between height and lung function. Similarly, HC related WHR and obesity defined by WHR were inversely linked lung function due to that HC is in the denominator position in the formula of WHR and a positive relationship between HC and lung function. WC and obesity defined by WC were positively associated with lung function since there is a positive relationship between WC and lung function.
According to the results of Spearman correlation analyses and linear regression analyses, the order of closeness of obesity parameters to lung function is as follows: (1). Height > Weight > HC > WC; (2). WHtR > BFP > BMI > WHR. It is well known that height of adult individuals remains constant over time. Therefore, weight, HC and WC are the obvious and crucial parameters for obesity condition of individuals in their adulthood. Previous studies shown that BMI and BFP (for general obesity) and WHR and WHtR (for central obesity) are recommended to assess the relationship between obesity and lung function. WC and obesity defined by WC and WHR were positively associated with FVC and FEV1, which may indicate that lung function improved with the increase of WC to some degree, which is consistent with the results of previous studies [21, 47–49]. However, compared with WC, height, weight and HC have more influence on the lung function, which is manifested by tightly relationship between obesity parameters including WHtR, WHR, BMI and lung function indicators such as FVC and FEV1.
Our findings indicate that several obesity parameters and obesity defined by them are significantly associated with lung function levels such as FVC and FEV1. The direction (inverse) of effect for different obesity parameters such as WHtR, WHR, BFP and BMI on lung function is similar from each other, which suggest that obesity condition is significantly associated with lung damage. Systemic/limb/peripheral obesity defined by BMI, WHtR and BFP were negatively associated with FVC and FEV1, while abdominal/belly/central obesity defined by WC and WHR were positively associated with FVC and FEV1. These results may suggest that take only WC into account is inappropriate to investigate the relationship between obesity and lung function. In other words, obesity defined by BMI, WHtR and BFP are better than obesity defined by WC and WHR to estimate their relationship with lung function.
There were several strengths in our study. First, we recruited a large population sample of adults across a broad age range from a typical rural area in central China, which let us understand the prevalence of obesity and lung function levels in the rural area. Second, several obesity parameters make it possible to comprehensively investigate their association with lung function to obtain more accurate and detailed information. Third, the large sample size, assessment of potential confounders, and several effect models facilitate investigation of the independent effect of peripheral and central obesity on lung function. However, several limitations should be acknowledged for this study. First, the cross-sectional nature of this study makes it difficult to draw a causal conclusion between obesity and lung function. However, we will re-recruit subject to determine the causal relationship between obesity and lung volume and function in the same adult population. Second, although several important factors have been taken into account in the analyses, residual confounding caused by other unmeasured variables remain possible such as muscle mass and strength, metabolic factors, markers of inflammation, nutritional status, and history or indicators of allergy. Third, it is needed to determine whether these results are generalizable to other ethnic populations in further investigation.