The main results of our study showed that there were no significant differences between the obese and non-obese subjects in FEV1, FVC, FEV1/FVC ratio and FEF 25-75; however, there was a significant difference between the two groups in regard to PEF. The obese subjects had lower PEF values than the non-obese subjects. Low PEF in obese subjects can be explained by an increase in total respiratory resistance and airway resistance with obesity. The higher airway resistance, the higher BMI and subsequently, the lower PEF [14
To our knowledge, this is the first study to address the relationship between obesity and spirometry tests among the Middle East population. Many studies conducted in the rest of the world and addressed such relationship showed heterogeneous results. The effects of obesity on spirometric values are not consistent across all studies with some studies shown no effects and some other studies shown positive effects. This discrepancy between studies could be explained by the wide variations in ethnicity of different population in PFT values or this may be a result of methodological differences in these studies.
However all studies that addressed other pulmonary function tests values (e.g. lung volumes and capacities) showed that the obesity directly correlated with these values. Our study was limited to the spirometric values (FEV1, FVC, FEV1/FVC, FEF 25-75 and PEF) and did not include the other pulmonary function tests (e.g. lung volumes and capacities).
In contrast to other studies, a study conducted by Costa D et al. addressed the relationship between spirometric tests and obesity produced results similar to those of our study. The author recruited 20 obese young women with a BMI of 35-49.99 kg/m2 who were sedentary, non-smokers and had no lung disease and 20 non-obese control young women who were also sedentary non-smokers and had no lung diseases with body mass indices between 18.5 and 24.99 kg/m2. There were no significant differences between the obese group and the non-obese group with respect to the age, vital capacity (VC), tidal volume (TV), FVC, and FEV1. However, the obese group had a greater inspiratory reserve volume (IRV), a lower expiratory reserve volume (ERV), and lower maximal voluntary ventilation (MVV) than the non-obese group [15
Jones RL et al. studied pulmonary function test results from 373 patients with wide range of BMIs. He found significant inverse relationships between BMI and the values of VC and total lung capacity (TLC). Moreover, the functional residual capacities (FRC) and ERV decreased exponentially with increasing BMI, to the extent that morbid obesity resulted in the patients breathing near their residual volumes (RV) [16
]. According to Koenig, this fact is attributable to pressing the diaphragm upwards due to the expanded abdominal volume of obese individuals [11
In mild obesity, the spirometry results may be normal or may suggest a restrictive process, with a symmetric reduction in FEV1 and FVC [17
] on other hand, The VC and TLC are well preserved in mild obesity because there is a compensatory increase in inspiratory capacity (IC). It is therefore important to be able to separate the changes that may be explained by an increased BMI from those that may be related to another process. At present, there is no way to differentiate these factors other than a conducting a careful history and physical examination combined with other clinical data [18
The most important change in pulmonary functions in obesity is a decrease in lung compliance due to the increased the weight of chest wall and the higher position of diaphragm in the thoracic cavity resulting in a decrease in the lung functions which subsequently leads to increase in work of breathing [16
]. In addition, the deposition of fat on the chest wall may impede the expansion and excursion of the rib cage, through a direct loading effect or by altering the inter-costal muscle function [19
]. Furthermore, obesity has been shown to be associated with markers of systemic and vascular inflammation such as the hormone leptin [20
]. These inflammatory factors may exert local effects on lung tissue, leading to subtle reductions in airway diameter.
The Strength of our study is the recruitment of subjects who were healthy without co-morbidity and the selection of subjects for the study who had been seen by a physician prior to being tested, and there were no indications that they had any co-morbidity. Anther strength of this study is that it is the first study in Saudi Arabia and among Middle East population to address the relationship between obesity and spirometry tests values.
Our study had the limitation of using BMI as an indictor of obesity. BMI is a global measure of body mass that includes both fat and lean mass and takes no account of differences in fat distribution. If the reduction of lung volumes in obesity is due to a direct mechanical effect on lung volumes, then the distribution of body fat should modify the relationship between BMI and lung volumes. Abdominal and thoracic fat are likely to have direct effects on the downward movement of the diaphragm and on chest wall properties, while fat on the hips and thighs would be less likely to have any direct mechanical effect on the lungs. However, the standard classification of obesity uses BMI as a reflection of obesity and this classification is used globally by WHO and other related health organizations. It is considered to be the gold standard at the present time.
Another limitation in our study is that we did not measure all the pulmonary function tests variables; our study was limited to the spirometry tests values and did not include lung volumes measurements. We elected to limit our study to the spirometry tests values because these values had not been previously investigated in relation to obesity in the Saudi population, and will be very difficult to apply data from other parts of the world to the Saudi population due to the differences in ethnicity. Our study addressed an important gap in the literature regarding the effects of obesity on spirometry tests values among Saudi population.
In conclusion, obesity does not have direct effect on the spirometry tests results (except PEF) among health non-smoking adults Saudis and if there is any effect, it should be explained by alternative diagnosis or underlying respiratory diseases. We strongly recommend conducting larger study including all pulmonary functions tests variables (spirometry tests and other lung volumes) among Saudi population, moreover, it will interesting to use other indices of obesity like abdominal girth, sub-scapular skin fold thickness, and the ratio of abdominal girth to hip breadth as reflection of obesity instead of BMI and find if such results will differ in comparison to the results of using BMI.
Based on our result, we highly recommend to physicians who use spirometry test in their practice to search for alternative diagnosis in case of findings abnormal spirometry tests results among obese individuals as these abnormal findings should not attributed to obesity per se.