HbA1c and pulmonary function in type 2 diabetes mellitus: A systematic review and meta-analysis

Background The lung, although not traditionally been considered a target organ in diabetes, may undergo microvascular and connective tissue changes that are related to poor glycemic control. This systematic review and meta-analysis aimed to systematically evaluate and quantify the association between HbA1c values and pulmonary function in adults with type 2 diabetes mellitus. Methods A systematic review was conducted in accordance with the PRISMA 2020 guidelines. Studies were searched in PubMed/MEDLINE, Scopus, and Web of Science Core Collection from database inception to 25 December 2025. Eligible studies compared spirometric outcomes between adults with type 2 diabetes categorized as having good versus poor glycemic control based on HbA1c. Random-effects meta-analyses were conducted using mean differences. Results Individuals with poor glycemic control had significantly lower absolute spirometric volumes. FEV1 was lower in the poor HbA1c group, with a pooled mean difference of –0.16 L (95% CI –0.25 to –0.07; p < 0.001). Similarly, FVC was reduced, with a pooled mean difference of –0.24 L (95% CI –0.35 to –0.14; p < 0.001). In contrast, the FEV1/FVC ratio was slightly higher in the poor-control group (MD = 1.61 percentage points, 95% CI 0.50 to 2.73; p = 0.005). All pooled analyses showed no evidence of statistical heterogeneity (I 2 = 0%). Conclusion Poor glycemic control in T2DM is associated with lower absolute FEV1 and FVC. The preserved or slightly higher FEV1/FVC ratio suggests reduced spirometric volumes rather than definite obstructive impairment. These findings should be interpreted as group-level associations because the analyses were based on absolute spirometric values and observational studies.