Defining catch-up growth in lung function between adolescence and adulthood: A 25-year population-based study

Background Lung function (LF) development from adolescence to adulthood varies widely. Two related but distinct phenomena are relevant: pronounced growth (large positive change in FEV 1 z-score) and classical catch-up (crossing the lower limit of normal). We aimed to quantify both. Methods We used data from 2169 participants in the HUNT Study with spirometry measured between 13 and 42 years. Change in FEV 1 z-score between baseline and follow-up was categorized into five FEV 1 change groups: pronounced increase, moderate increase, unchanged, moderate decline and pronounced decline. A catch-up group (z≤−1.645 to >−1.645) was also identified. Results Of 82 individuals (3.8%) with pronounced LF increase from adolescence to adulthood, only 39 (48%) met the classical catch-up definition. Conversely, 48 of 87 (55%) individuals classified as catch-up did not show pronounced increase. Both groups shared a similar phenotype (female-predominant, low baseline BMI, and favorable adult metabolic profiles), but differed in adult LF: pronounced increase reached above-average FEV 1 (z=+0.65), while classical catch-up reached only partial normalization (z=−0.55). In contrast, the pronounced decline group had the highest BMI, C-reactive protein (CRP), and burden of chronic symptoms. Baseline characteristics differed between change groups, though individual distributions overlapped substantially. Conclusion Pronounced LF growth and classical catch-up identify overlapping but distinct individuals with similar phenotypes yet different degrees of adult lung-function normalization. A substantial proportion of the most favorable lung-function gains originate from apparently normal baselines and are not detected by a binary catch-up definition.