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Obesity can significantly impact pulmonary function test (PFT) results, primarily by reducing lung volumes and increasing the risk of restrictive patterns. Excess body weight, particularly around the abdomen, can restrict the movement of the diaphragm and the expansion of the lungs during inhalation. This restriction leads to decreased total lung capacity (TLC), vital capacity (VC), and potentially inspiratory capacity (IC).

Individuals with obesity may find it challenging to take deep breaths, which can manifest as a restrictive lung pattern on spirometry. In these cases, PFT results may show reduced forced vital capacity (FVC) and a corresponding increase in the FEV1/FVC ratio because both values decrease approximately in the same proportion. This pattern is indicative of a restriction rather than an obstructive process, which would show a more disproportionate reduction in FEV1 compared to FVC.

While obesity does have various effects on respiratory mechanics, including potential complications in airflow and lung mechanics, the most significant impact in terms of PFT results typically manifests as reduced lung volumes and a higher prevalence of restrictive lung disease. Thus, reduced lung volumes and increased risk of restrictive patterns characterize the relationship between obesity and pulmonary function testing outcomes.