A Cochrane review [Abstract] 1 included 7 cross-over studies with a total of 75 subjects with mild to moderate asthma. Six studies (n=55) showed that in comparison with placebo, caffeine, even at a ‘low dose' (< 5mg/kg body weight), appears to improve lung function for up to two hours after consumption. Forced expiratory volume in one minute showed a small improvement up to two hours after caffeine ingestion (SMD 0.72, 95% CI 0.25 to 1.20), which translates into a 5% mean difference in FEV1. However in 2 studies the mean differences in FEV1 were 12% and 18% after caffeine. Mid-expiratory flow rates also showed a small improvement with caffeine and this was sustained up to four hours. One study (n=20) examined the effect of drinking coffee versus a decaffeinated variety on the exhaled nitric oxide levels in patients with asthma and concluded that there was no significant effect on this outcome. The authors state that people may need to avoid caffeine for at least four hours prior to lung function testing, as caffeine ingestion could cause misinterpretation of the results.
Comment: The quality of evidence is downgraded study quality (unclear allocation concealment) and by imprecise results (limited study size for each comparison).
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