In this study, we investigated the association between BIA variables and pulmonary function indices, assuming that body composition analysis can be related to pulmonary function indices. Body composition analysis is associated with many known diseases such as cardiovascular diseases, diabetes, cancer, osteoporosis, osteoarthritis as well as body composition with obstructive pulmonary disease (24).
In previous studies, the association between anthropometric indices and dimensions with pulmonary function indices had been investigated (13). In the present study, it was observed that the correlation coefficient between BIA measures and pulmonary function indices was relatively higher than anthropometric indices and dimension ratio correlation coefficient with pulmonary function indices. The only anthropometric index that correlates to or better than BIA indices with the pulmonary function was height. The best BIA indicators in correlation with pulmonary function are Bone mass, FFM, TBW, ECW, and ICW.
On the other hand, one of the variables that traditionally predicts lung function is the age variable. Pulmonary function decreases with age (30). One of the common predictors of pulmonary function in various studies is the age, which was also used as a predictor of pulmonary function in this study.
In previous studies, it was observed that pulmonary function indices such as FVC, FEV1, and FEF25-75 were significantly different in the three groups of underweight, normal, and overweight. Body fat percentage has a higher correlation coefficient with lung function than BMI. In a study, pulmonary function and body composition indices in male and female athletes were found to have a significant correlation between pulmonary function and body fat percentage, muscle mass, FFM, trunk muscle mass, trunk fat mass, and wrist and hip circumference. In another study, age, sex, body weight, BMI, fat percentage, FFM, WHR were significantly associated with lung function, but their R2 was not high enough to interpret it (31).
In the present study, no correlation was observed between BMI and pulmonary function. This may be due to the essential role of the weight component in BMI. Body mass index does not differentiate between FM and FFM (13).
In the present study, the FFM index was suggested as a strong correlation coefficient with pulmonary function. In a study that looked at the incidence of adolescents with cystic fibrosis, the Lean Body Mass Index, or Fat-Free Mass, was found to be strongly correlated with lung function relative to BMI (32). Similar results were observed in the present study.
One of the most important factors in reducing pulmonary function is obesity, especially since the increase in adipose tissue in the abdominal and chest areas can lead to limited chest dilation (13). Obese people experience a vicious cycle due to reduced mobility and a sedentary lifestyle, resulting in increased mortality rate (33). Abdominal obesity as a major component in the development of metabolic syndrome causes mechanical impairment of pulmonary function in middle-aged adults (34). Accumulation of visceral fat with insulin resistance and impaired glucose tolerance, high blood pressure, impaired lipid metabolism such as metabolic syndrome, and arteriosclerosis are involved (35).
Also, one of the findings of the present study was the inverse and significant relationship between visceral fat and pulmonary function. Previous studies have shown that airway inflammation is more likely to occur in people with a higher body fat percentage (36).
In previous studies, one of the indicators that had an association with the pulmonary function was waist circumference., In the present study in the presence of other anthropometric indicators, demographic information and BIA, and in the presence of all mentioned indicators, age, sex, height, visceral fat, and FFM, waist circumference had a statistical association with FEV1 that, in the FVC index in addition to the mentioned factors, ICW is also considered as a predictor.
In different spirometry devices and based on several studies that have been done to predict the normal range of spirometry, four components are used: age, sex, race, and height. It is expected that weight is also a predictor of the normal spirometry range. However, various studies show that weight is not a predictor of the normal range of pulmonary function in spirometry.
Based on the regression results of the present study, we observed an interesting result. The total body weight is composed of two components: Fat weight and Fat-Free Mass (weight of other lean components). In regression analysis, it was observed that FFM has a direct association with FEV1 and FVC. The visceral fat mass has an inverse association with both indicators of pulmonary function.
Since Visceral fat can be considered as a representative of a large part of body fat, it can be concluded that the result of FFM and body fat, which are considered as total body weight, counteract the effect of each other and therefore weight. It is not included in various studies as a predictor of normal lung function.
On the other hand, obtaining FFM, Visceral fat by direct weight measurement method is fairly expensive and not available to everyone, so it is possible to estimate FFM or visceral fat by measuring some dimensions and enter this predictor into the lung function normal range estimates formula.