In the current study, we recruited 2101 college students to study the effects of abdominal fat distribution on lung function in young adults. We found that VCI was negatively correlated with FM, VAT, SAT, and VFA through partial correlation analysis after controlling muscle mass. Among these negative correlations, VCI was more inversely correlated with VFA for men but with SAT for women, respectively. Further correlation analysis for BMI-, BFP-, and WHR-subgroups suggested VCI was more negatively correlated to VFA in each of these male subgroups, in comparison with other indices of body fat distribution. However, for women, strong negative correlations between VCI and SAT were observed only in BMI-underweight, BMI- normal, BFP-low fat, and BFP-normal fat subgroups. On the contrary, for BMI- and BFP-overweight+obese subgroups, VCI was more negatively correlated with VAF or VAT. We concluded that fat distribution is highly associated with lung function in young adults, and in general, VCI was more negatively correlated with VFA for men but with SAT for women, respectively, in comparison with other tested indices.
In addition to cardiovascular disease, type 2 diabetes, and inflammatory bowel disease, abdominal fat distribution has been associated with impaired lung functions. Similar to other diseases [26, 27], different abdominal fat compartments and distributions may be differently associated with lung functions. However, only a few studies have assessed the independent associations of VAT and SAT with lung functions. Visceral fat, but not the SAT, WC, and BMI, has been inversely associated with impaired lung function of men aged 50–70 years with the metabolic syndrome . In addition, visceral adiposity is associated with the decrease in lung function in female asthma patients with a mean age of 55.39 years . These observations were obtained from the participants with the ages over 50 years and subjected to limitations of health conditions (metabolic syndrome or asthma). It is unclear whether these associations are universal and applicable to healthy people. To address this question, we recruited college students in this study and take their advantages to investigate the association between fat accumulation and lung function. College student subjects are believed to be developmentally mature both physically and psychologically. In addition, they tend to be homogeneous on dimensions such as age, education, dwelling, and food source, as well as exercise behavior. Therefore, college student subjects might enhance research validity and minimize the possibility of undue influence because of their apparent homogeneity, especially for the research purpose targeting on young adults.
Dual energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis are two common methods to determine body composition. DXA method is featured with high accuracy and has been recognized as a gold standard technique to measure human body composition [28, 29]. DXA can be used to determine either whole or regional body composition. However, this standard method is hardly feasible in routine clinical practice, especially in our current study with over 2000 participants. In addition, the radiation exposure from DXA may cause potential participants’ psychological concerns and subsequently unwillingness to be involved in this project, though the method is safe. Therefore, in this study, we chose BIA method instead which has been considered inexpensive and rapid . Some studies have shown good concordance between the two methods while others have not [31,32,33,34,35,36,37,38,39]. These conflicting results may probably be due to the differences in used equations, population size, age, ethnicity, gender, and body weights in the sample studied. A recent study that directly compares the measurement of FM and FFM by DXA and BIA methods in a large cohort of patients suggested that BIA and DXA methods are interchangeable at a population level and FM obtained by BIA and DXA were strongly correlated . In particular, BMI between 16 and 18.5, body composition values measured by DXA and BIA were very closed. However, BIA method tends to overestimate or underestimate FM and FFM values beyond the above BMI range.
Sex differences in fat distribution are well documented. Women are generally characterized by having more SAT, whereas men are more prone to high amounts of VAT [40,41,42], which is consistent with our finding although the VAT difference between men and women was not statistically significant. Of note, we also showed men had higher VFA than women. Gender is considered as an essential factor regulating the body composition, in particular the obesity, mostly upon genetic and epigenetic regulation . Other biological differences between male and female also contribute to the different composition , including hormone expression and lifestyle. For example, estrogen could increase the decomposition of SAT but decrease VAT .
Jianhui et al. have found that the effects of different fat distribution on thoracic activity varied; central obesity tended to reduce diaphragm and chest activity significantly, while peripheral obesity had relatively slight effects on respiratory movement . Since the fat distribution varies in men and women, it is reasonable that gender difference in fat distribution contributes differently to lung functions for men and women, in addition to the gender difference in lung morphology (lung size, airway diameter, and diffusion surface) and hormones [47,48,49,50]. In this study, VCI was negatively correlated with FM, VFA, VAT, and SAT, respectively, for both men and women. However, women’s VCI was more negatively correlated with SAT, while male VCI was more inversely correlated with VFA, suggesting a gender difference of effects of fat distribution on lung functions, confirming our hypothesis. Such observation is probably due to the fact that compared with subcutaneous fat, the accumulation is more visceral for men , and the increase of visceral fat is faster, resulting in excessive accumulation of fat in the internal organs of the abdominal cavity and affecting lung ventilation function for male . Therefore, visceral fat had a predominant impact on lung function than subcutaneous fat for young men. For women, a previous study by Park et al. showed that VAT rather than SAT is more important for pulmonary function in female with an average age of 53.4 years . The difference may come from different employed indicators for lung function, VCI in our study vs FVC and FEV1 in the previous report. Another possibility is the age difference. Considering the fact that postmenopausal women tend to distribute fat in their viscera , it is plausible that the age difference contributed to the contradiction with the previous report.
To better understand these observed negative correlations, we performed further analysis by dividing participants into different subgroups based on the BMI, BFP, and WHR criteria. For men, VCI was more negatively correlated to VFA regardless of subgroups, in comparison with other indices of body fat distribution, suggesting VCI-VFA negative correlation is universal for the whole population. This is probably due to the fact that visceral fat deposition is predominant in young men regardless of obesity classification as discussed above . On the contrary, female VCI was more negatively correlated with SAT in BMI-underweight, BMI-normal, BFP-low fat, BFP-normal fat, and WHR-normal subgroups, which is consistent with the finding as a whole female population. VFA and VAT were more inversely correlated with VCI in BMI-overweight, BMI-overweight+obese, and WHR-obese subgroups. Taken together, for women, the fat distribution indices of different subgroups have different effects on the lung functions. It is plausible that female’s fat is mainly deposited under the skin but slowly in viscera before the body turning into overweighted or obese, which contributes to a negative correlation between SAT and VCI. However, during the development of obesity, fat tends to accumulate in visceral region since the amount of subcutaneous fat has plateaued. A previous study showed that obese women with high visceral fat have worse lung function compared to those with high subcutaneous fat under the same fat content . Further studies or animal models are needed to illustrate the potential mechanism.
In addition to VAT, VFA, and SAT, we also showed that the BMI, FM, BFP, and WHR were negatively correlated with the vital capacity index. The increase of BMI, body fat rate, and WHR has been related to the development of obesity, whose increase is indicative of excessive fat accumulation in the abdominal cavity and on the chest wall. These changes will impair pulmonary function by affecting vital capacity and breathing regulation, as well as increasing the work of breathing, reducing lung volumes, rendering respiratory muscles dysfunctional, and impairing gas exchange [56,57,58]. In the present study, we also noticed gender differences in BMI, BFP, and WHR and the vital capacity index. Women’s FM and BFP were higher than men, while men’s BMI, WHR, and vital capacity index were higher than women.
The present study had several limitations. Firstly, lung function was only evaluated by vital capacity index due to a large sample size. FVC, FEV1, and FEV1/FVC are considered for later studies to establish the relationship between fat accumulation with other lung function indices. Secondly, because this study is cross-sectional, our study lacks the analysis about the causality underlying the relationship between fat accumulation and impaired lung function. Future experiment is expected to explain the effects of SAT and VAT on lung functions.