Design: systematic review and meta-analysis of longitudinal cohort evidence, 7,160 citations screened
Population: 50 publications from 35 cohorts, over 365,000 adults without lung disease; follow-up 3–35 years with 3–12 spirometric assessments each
Primary outcome: normal age-related rate of forced vital capacity decline
Effect: published estimates 10–80 mL/year; best individual-level evidence (>30,000 participants) gives a linear ~26 mL/year in women and biphasic ~22 mL/year before 40 and ~36 mL/year after in men, averaging ~20–30 mL/year

An older patient with a forced vital capacity lower than last year's raises a question nobody can usually answer: is that ageing or is that disease? A systematic review set out to supply the benchmark, screening 7,160 citations and including 50 publications covering 35 cohorts and more than 365,000 participants without lung disease, with follow-up from three to 35 years and between three and twelve spirometry measurements per person.

The published estimates span an unusable range — 10 to 80 mL per year — because the studies differ in design, population and analysis. The authors identify the most trustworthy source within that: an individual-level pooled analysis of more than 30,000 participants, showing that forced vital capacity peaks in the mid-twenties and declines continuously with no plateau. In women the decline is linear at about 26 mL per year. In men it is biphasic — about 22 mL per year before age 40 and about 36 mL per year after. The overall benchmark is roughly 20 to 30 mL annually.

The immediate use is interpretive. A 75-year-old whose vital capacity has fallen 30 mL over a year is doing what lungs do; one who has lost 150 mL is not, and that distinction is worth making before either reassuring or investigating. The second use is for reading trials: attenuation of forced vital capacity decline is the primary endpoint in most interstitial lung disease trials, and a treatment effect needs to be judged against the ageing trajectory rather than against zero.

Use roughly 20–30 mL per year as the expected normal decline when interpreting serial spirometry.
Expect a steeper decline in men after 40 — about 36 mL per year — and a linear one in women.
Calculate the annual rate rather than comparing two absolute values; the interval between tests matters.
Ensure serial tests use the same equipment and technique before attributing a change to disease.
When reading an interstitial lung disease trial, compare the treated decline against the ageing benchmark, not against no decline.

Why it matters

It gives a number to the question that decides whether a falling vital capacity gets investigated or reassured.

The statistics, in plain English

The 10–80 mL range across studies is not measurement noise but real methodological disagreement — different populations, different definitions, different models — which is why the authors single out one individual-level pooled analysis rather than presenting a meta-analytic average. That is the right call, and it also means the headline figures rest on 30,000 people rather than 365,000. These are population averages in people without lung disease, so an individual's normal decline can sit well away from them, and the biphasic male pattern means a single annual figure misdescribes men on either side of 40.