Adipose tissue makes up around 15–20% of the body weight of an average person, which actively participates in the functioning of the body. However, unusually high levels of this type of tissue have been clearly linked with health problems including non-communicable disorders as well as cardiovascular disorders [21, 22].
In recent years, the associations of obesity with reduced pulmonary function and chronic airway disease have received considerable attention [23–25]. In particular, a close relationship between obesity and VCI has been suggested . However, evidence is limited with regard to whether body composition is associated with VCI among the Chinese medical students.
In this study, a cross-sectional study was conducted to evaluate the relationships between body composition and VCI of medical students in China. By comparing the body composition, the results showed that FFM and VCI of male students were higher than those of females, while FM was lower than those of female students. This seems to be related to the physiological condition of both sexes, due to the role of androgen, the body FFM of men increases significantly, while for women estrogen plays an important role in the increase of FM .
After Pearson correlation and further stepwise regression analysis, we found that among the subjects of different genders, only FM was negatively correlated with VCI, and the negative correlation still existed after adjusting for confounding factors. Other studies, when taken together, also support this result of the correlation [26, 28, 29]. Although the mechanism underlying the association has not been elucidated, considering comprehensively, and increased FM can potentially cause decreased VCI in below ways: first of all, the increase of FM leads to weight gain, and the body overcomes its own load, which will cause an increase in vital capacity after a period of time . Secondly, the adipose tissue of the chest and abdomen may lead to a decrease in the compliance of chest and lung tissue, at the same time, the structure of respiratory muscle and trachea is abnormal, and the ventilatory capacity of lung tissue is limited. Previous studies have shown decreased chest wall compliance in obese patients who are awake and breathing spontaneously, which supports our speculation [31–33]. The third possible explanation is that adipose tissue can be involved in the secretion of a variety of inflammatory factors, since it is considered one of the largest endocrine organs in the body [34, 35]. In the study of body composition, adipose tissue is the site of the early link between inflammation and obesity. Hotamisligil et al  found that TNF- α was over-expressed in adipose tissue of obese mice, providing the first link between obesity, diabetes and chronic inflammation. Later studies confirmed that there are over-expressed pro-inflammatory and inflammatory mediators in adipose tissue of obese patients, such as TNF-α, IL-6, monocyte chemoattractant protein-1, and macrophages infiltrate adipose tissue . It is worth noting that a variety of cytokines produced by fat-induced inflammation are also detected many times in lung diseases, so adipose tissue may regulate the endocrine system, lead to chronic inflammation, and then cause or aggravate the occurrence and development of abnormal lung function [38–40].
After determining that FM is the main influencing factor of VCI, we further divided our subject population into four groups to explore the relationship between FM and VCI of participants from different BMI levels. Of note, we also showed that there was a positive correlation between FM and VCI in male students with BMI < 18.5. This result may provide a new point of view that FM has a protective effect on pulmonary ventilatory function in low-weight male students. An interesting physiological explanation may be that a protective increase in lung recoil has been found in patients with higher BMI , while the loss of lung recoil is often associated with decreased lung function and COPD progression . Since the FM varies in men and women, it is reasonable that gender difference in FM contributes differently to lung functions for two sexes with low weight, in addition to the gender difference in hormones.
This study has several limitations. First, this study is a cross-sectional study which can only explore the correlation between body composition and lung function rather than causality. Also, this study clarified the relationship between FM and VCI, but neglected the effects of adipose tissue in different parts of the body on lung function; thus, we cannot confidently extrapolate findings to how body fat distribution affects VCI in Chinese medical students. Finally, the subjects of this study are only Han and Manchu medical students, which may lead to the results of the study can not be extrapolated to other ethnic groups. Here, we also suggest to strengthen the research on the correlation between body composition and pulmonary ventilation function of thin and weak groups, in order to better provide effective guidance for such groups.
Conclusion: Generally speaking, there was a negative correlation between FM and VCI in Chinese medical students with both sexes. However, FM has a potential role to improve the VCI among low-weight male students.