Y-Y Han et al, 2021. Urinary caffeine and caffeine metabolites, asthma, and lung function in a nationwide study of U.S. adults, Journal of Asthma, published online.

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Coffee intake has been inversely associated with asthma in adults. We examined the relation between urinary levels of caffeine or caffeine metabolites and asthma, lung function, and fractional exhaled nitric oxide (FeNO) in adults.

Cross-sectional study of 2,832 adults aged 18-79 years in the US National Health and Nutrition Examination Survey (NHANES). Multivariable logistic or linear regression was used for the analysis of urinary levels of caffeine or each of its three major metabolites (paraxanthine, theobromine, and theophylline) and current asthma, lung function, and FeNO.

Subjects with urinary paraxanthine levels in the fourth quartile (Q4) had 53% lower odds of current asthma than those whose urinary paraxanthine levels were in the first quartile (Q1; 95% confidence =0.22 to 1.00). Among never and former smokers, subjects with urinary theophylline levels above Q1 had 41% lower odds of current asthma than those whose urinary theophylline level was in Q1 (95% CI =0.38 to 0.91). Among subjects without current asthma, each log10-unit increment in paraxanthine level was associated with a 0.83% increment in percent predicted (%pred) FEV1 and a 1.27% increment in %pred FVC, while each log10-unit in theophylline was associated with a 1.24% increment in %pred FVC. Neither urinary caffeine nor any urinary caffeine metabolite was associated with bronchodilator response or FeNO.

Our findings suggest that two caffeine metabolites (theophylline and paraxanthine) may contribute to the previously reported inverse association between coffee intake and asthma in adults.

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