Objectives Restrictive spirometry pattern (RSP) suggests an impairment of lung function associated with a significantly increased risk of premature mortality. We evaluated the independent and joint associations of cardiorespiratory fitness (CRF) and body mass index with incident RSP.

Methods Data from the Aerobics Centre Longitudinal Study included 12 360 participants (18–82 years). CRF was assessed by maximal treadmill test and categorised into five groups. Body mass index was categorised into normal weight (<25.0 kg/m²), overweight (25.0–29.9 kg/m²) or obesity (≥30.0 kg/m²). RSP was defined as the simultaneous occurrence of forced expiratory volume in 1 s/force vital capacity ≥lower limit of normal and forced vital capacity <lower limit of normal.

Results There were 900 (7.3%) cases of RSP (mean follow-up: 6.9 years). Compared with category 1 (‘least fit’), HRs (95% CIs) of RSP were 0.78 (0.63 to 0.96), 0.68 (0.54 to 0.86), 0.70 (0.55 to 0.88) and 0.59 (0.45 to 0.77) in categories 2, 3, 4 and 5 (most fit), respectively, after adjusting for confounders including body mass index. Compared with normal weight, HRs (95% CIs) of RSP were 1.06 (0.91 to 1.23) and 1.30 (1.03 to 1.64) in overweight and obese, respectively. However, the association between obesity and RSP was attenuated when additionally adjusting for CRF (HR 1.08, 95% CI 0.84 to 1.39). Compared with the ‘unfit and overweight/obese’ group, HRs (95% CIs) for RSP were 1.35 (0.98 to 1.85), 0.77 (0.63 to 0.96) and 0.70 (0.56 to 0.87) in the ‘unfit and normal weight,’ ‘fit and overweight/obese’ and ‘fit and normal weight’ groups, respectively.

Conclusions Low CRF was associated with a greater incidence of RSP, irrespective of body mass index. Future studies are needed to explore potential underlying mechanisms of this association and to prospectively evaluate if improving CRF reduces the risk of developing RSP.