Longitudinal effects of obesity on pulmonary function in obese children and adolescents

The present study found that the overall BMI z-score and WC/Ht among obese children and adolescents were significantly reduced at the 1-year follow-up; however, only minor changes were observed. These minor changes in obesity indices are within normal expectations, as achieving weight control through lifestyle modifications or dietary changes—without a specific weight loss program—can be quite challenging for obese individuals. The individual differences in adherence to recommendations, as well as variations in physical activity, dietary changes, and lifestyle modifications, can also introduce confounding effects on the study results.

In terms of lung function, we observed significant increases in both the absolute and %predicted values of RMS, along with a reduction in the FEV₁/FVC and FEF_25–75/FVC at follow-up. A possible explanation for the increased RMS could be the heightened workload on respiratory muscles due to the excess weight carried by obese individuals. This heightened workload may elicit an adaptive response, prompting the respiratory muscles to strengthen as a compensatory mechanism to meet the increased oxygen demand.^18,19,20 The present study indicated a decrease in the BMI z-score but the reduction was minimal and BMI z-score remained within the obesity range. Consequently, this compensatory mechanism may continue and contribute to the increased RMS. Despite the improvements in BMI z-score and WC/Ht, the FEV₁/FVC and FEF_25–75/FVC values were found to be reduced at follow-up. This decline may be explained by ongoing dysanapsis process, which refers to the disproportionate overgrowth of lung tissue relative to airway development in obese children and adolescents²¹ due to persistent obesity.

Our study also revealed differences in the absolute value of the 6-MWT distance between baseline and follow-up. The increase in absolute value can be attributed to somatic growth, particularly an increase in height, as the 6-MWT distance is significantly correlated with height.²² However, the z-score value for the 6-MWT distance, which is adjusted for height, did not show a significant difference. This lack of change in the z-score value may be caused by the marginal changes in BMI z-score and WC/Ht at follow-up, which likely had no impact on the z-score for the 6-MWT distance.

Alterations in lung function may result in shortness of breath or exercise intolerance.^2,3 In our study, although notable changes in lung function were observed at the follow-up assessment, these changes were not considered clinically significant. This lack of clinical impacts may be attributed to the minor changes in both lung function and obesity indices, as well as the fact that lung function remained within normal ranges at the beginning of the study and at the 1-year follow-up.

The present study provides important data specific to the longitudinal effects of a change in obesity on lung function in obese children and adolescents. We found a change in WC or WC/Ht, which indicates abdominal adiposity, to be inversely correlated with a change in FEV₁/FVC, FEF_25–75% /FVC, and FEF_25–75% in children and adolescents with obesity after 1 year of follow-up. In contrast, a change in BMI did not significantly affect any spirometry parameters, RMS, or 6-MWT distance.

Our results indicate that abdominal fat deposition influences lung function change more profoundly than total adiposity, as reflected by BMI. Consistent with the findings of our study, a recent large longitudinal study in middle-aged Asian population by Park, et al.²³ reported increased WC/Ht to be significantly associated with long-term impairment of lung function. A long-term study in children and adolescents with obesity found that a change in WC had more effect on lung function than change in BMI.⁷ Many previous cross-sectional studies in children^24,25 and adults^26,27,28 also reported abdominal adiposity to be a predictor of altered lung function. The main mechanism of abdominal obesity on altered lung function is excessive fat deposition in the diaphragm and abdominal visceral organs, which exerts an adverse mechanical effect on diaphragmatic movement and lung expansion.^29,30 In our study, a change in BMI after 1 year follow-up was not significantly related to lung function change. The reason might be that BMI indicates overall fat and non-fat components of the body, but BMI does not reflect specific body fat distribution like WC or WC/Ht. Our results seem to strongly suggest WC and WC/Ht as important markers for lung function change in obesity. Accordingly, these two obesity parameters should be measured and monitored (in addition to BW and BMI) in routine clinical practice for obese individuals. Ideally, distribution of body fat and muscle mass as measured by direct methods, such as dual-energy X-ray absorptiometry or bioelectrical impedance analysis, should be used for monitoring obese individuals. However, these measurement modalities are expensive and often not available, which makes their use often impractical in routine clinical practice.

The present study included FEF_25–75% /FVC, which was rarely investigated in previous studies, and found that change in WC and WC/Ht had negative association with change in FEV₁/FVC, FEF_25–75% /FVC, and FEF_25–75%, but no association with FEV₁ or FVC.

FEV₁/FVC and FEF_25–75%/FVC are surrogate markers for dysanapsis, which is defined as disproportionate scaling of airway dimensions to lung volume, which in turn leads to low FEV₁/FVC and FEF_25–75%/FVC.^21,31 Our 1-year follow-up study supports dysanapsis growth, which can occur in obese children and adolescents.²¹ Most studies that found negative association between FEF_25–75%/FVC³² or FEV₁/FVC^4,5,25,32 and obesity indices had a cross-sectional design. The longitudinal Dutch PIAMA study⁸ and Swedish BAMSE cohort study⁹ also found persistent high BMI or obesity in children to be associated with lower FEV₁/FVC, but neither of those studies evaluated FEF_25–75%/FVC.

We also found that change in WC and WC/Ht significantly inversely affects FEF_25–75%, which reflects small airway function. This supports the findings of recent meta-analyses that found negative association between FEF_25–75% and obesity status in obese children and adolescents.^4,5 The greater effect of both WC and WC/Ht on FEF_25–75%/FVC compared to their effect on FEV₁/FVC, as indicated by a greater b-value for FEF_25–75%/FVC, may reflect that obesity has more focused adverse effect on small airways. Lung function measurement that is more sensitive than spirometry for detecting impairment of small airways, such as oscillation technique, should be used to evaluate and clarify respiratory function change in obese individuals. In contrast to these findings, a longitudinal study in obese children and adolescents by van de Griendt, et al.⁷ reported no effect of change in obesity on any spirometric parameters. The actual long-term effects of fat deposition on the respiratory system and the pathophysiology of respiratory symptoms in obesity are still being investigated and debated. Further study is, therefore, needed to improve our understanding of this issue so that we can develop and improve prevention and treatment strategies.

The present study also found sex difference in small airway growth. Boys demonstrated more negative effect on FEF_25–75% and FEF_25–75%/FVC. This result is consistent with those reported from previous studies that demonstrated airway growth of boys to be slower than that observed in girls.^21,33,34

Strengths and limitations

The strength of our study is its longitudinal design, which facilitated observation of change in the same patients over time. This is in contrast to a cross-sectional design which observes different patients at a single time point. Another strength is that we included WC and WC/Ht as obesity status variables, and FEF_25–75%/FVC, RMS, and 6-MWT as lung function variables.

This study also has some mentionable limitations. First, data specific to other variables, such as physical activity, diet, and other environmental exposures, were not collected. Second, subgroup analysis of sex and pubertal stage was not be performed because the small size of many subgroups would need yield the statistical power needed to provide reliable statistical results. Future multicenter study in a much larger patient population is needed to confirm and expand upon the results of this study.