Design: systematic review and meta-analysis of longitudinal cohorts, 7,160 citations screened
Population: 50 publications, 35 cohorts, over 365,000 adults without lung disease, followed 3 to 35 years
Primary outcome: annual change in forced vital capacity with age
Effect: about 26 mL per year in women, linear; about 22 mL per year before 40 and 36 mL after in men; published estimates ranged 10-80 mL per year

Fifty publications representing 35 cohorts and more than 365,000 participants without lung disease were reviewed to establish what forced vital capacity does with age. Follow-up ran from three to 35 years, with three to twelve spirometric assessments per participant.

Across five decades of research the direction was consistent and the magnitude was not: published estimates ranged from 10 to 80 mL per year, driven by differences in design, population and analysis. The strongest single source, an individual-level pooled analysis of over 30,000 participants, found FVC peaks in the mid-20s and declines continuously with no plateau - linear in women at about 26 mL per year, biphasic in men at about 22 mL per year before 40 and about 36 mL after. The overall normative figure is roughly 20 to 30 mL annually.

The reason this matters is that attenuation of FVC decline is the primary endpoint of most interstitial lung disease trials, and until now there has been no agreed denominator. A therapy that slows annual FVC loss to 30 mL has done something quite different depending on whether normal ageing in that population is 10 mL or 40 mL. The same applies at the bedside: a 60-year-old losing 30 mL a year is ageing, and one losing 150 mL a year is not, and that distinction has been made by impression rather than by reference.

Judge an individual's FVC trajectory against roughly 20 to 30 mL per year, not against a single predicted value
Expect faster normal decline in men after 40 - about 36 mL a year - when interpreting serial spirometry
Require at least three measurements over a meaningful interval before calling a trajectory abnormal
Read trial claims of preserved lung function against this benchmark rather than against placebo alone
Use the same laboratory and technique for serial measurements; between-device variation swamps 25 mL

Why it matters

Trials of interstitial lung disease have measured slowed decline for years without an agreed figure for what unslowed normal decline is.

The statistics, in plain English

The 10 to 80 mL per year spread across the literature is the finding as much as the 20 to 30 mL headline is: most of that variation is methodological rather than biological, so any single published figure quoted without its source is unreliable. The authors give most weight to the individual-level pooled analysis of over 30,000 people, which is the right call - pooling individual data avoids the aggregation problems that produced the wider range. These are also averages in people without lung disease, so they set a reference, not a threshold for any one patient.