Urine excretion of caffeine and select caffeine metabolites is common in the U.S. population and associated with caffeine intake

Abstract

Background: Caffeine is a widely consumed psychoactive stimulant and is of epidemiologic interest. Major sources of caffeine are challenging to standardize, and the use of biomarkers is proposed as an alternative means of assessing intake.

Objective: We described urine caffeine and caffeine metabolite concentrations (n = 2466) and excretion rates (n = 2261) in the US population ≥6 y by age, sex, race-ethnicity, and caffeine intake (from foods, beverages, and dietary supplements).

Methods: We measured caffeine and 14 of its metabolites in spot urine samples from the cross-sectional NHANES 2009-2010 by use of LC-tandem mass spectrometry.

Results: Caffeine and its metabolites were detectable in the urine of most persons, generally at concentrations ≥1 μmol/L. Median concentrations (95% CI) ranged from 0.560 (0.497, 0.620) μmol/L to 58.6 (48.6, 67.2) μmol/L; median excretion rates from 0.423 (0.385, 0.468) nmol/min to 46.0 (40.7, 50.2) nmol/min. Urine concentrations and excretion rates for 9 analytes (caffeine, theophylline, paraxanthine, 1-methylxanthine, 1-methyluric acid, 1,3-dimethyluric acid, 1,7-dimethyluric acid, 1,3,7-trimethyluric acid, and 5-acetylamino-6-amino-3-methyluracil) had moderate correlations with caffeine intake (Spearman ρ = 0.55-0.68, P < 0.0001); the remaining analytes had low correlations (ρ = 0.15-0.33, P < 0.0001). We observed larger differences in geometric mean concentrations and excretion rates between the highest vs. lowest quartiles of caffeine intake for these 9 compounds than the rest. Consistent with dietary caffeine intake, we observed that urine concentrations and excretion rates for most compounds were significantly (P < 0.05) higher in men than women, non-Hispanic whites than Hispanics and non-Hispanic blacks, and highest in persons aged 40-59 y.

Conclusion: Excretion of caffeine and its metabolites in urine is common in the US population. According to the observed associations between spot urine concentrations or excretion rates with caffeine intake, several of these compounds show promise as potential biomarkers of caffeine intake.

Keywords: LC-MS/MS; NHANES; biomarkers; caffeine; dietary intake; metabolites; phenotyping; urine.

Figure 1

Relationship of caffeine metabolism in humans to correlations observed between spot urine caffeine and caffeine metabolite concentrations and excretion rates with 24-h caffeine intake from foods and dietary supplements in U.S. persons aged ≥6 y, NHANES 2009–2010. “Group 1” consists of compounds for which Spearman correlations of 0.55 ≤ |r| ≤ 0.68 with intake were observed; “Group 2” consists of compounds for which Spearman correlations of 0.15 ≤ |r| ≤ 0.33 with intake were observed (Table 4, Supplemental Table 7). 137U, 1,3,7-trimethyluric acid; 13U, 1,3-dimethyluric acid; 17U, 1,7-dimethyluric acid; 1U, 1-methyluric acid; 1X, 1-methylxanthine; 37U, 3,7-dimethyluric acid; 3U, 3-methyluric acid; 3X, 3-methylxanthine; 7U, 7-methyluric acid; 7X, 7-methylxanthine; AAMU, 5-acetylamino-6-amino-3-methyluracil.

Figure 2

Comparison of 24-h caffeine intake from diet and supplements, spot urine 5-acetylamino-6-amino-3-methyluracil (AAMU) concentrations, and urine AAMU excretion rates s in U.S. persons aged ≥6 y, NHANES 2009–2010, stratified by demographic variables. Values are medians ± 95% CI. (A) caffeine intake, (B) AAMU concentration, and (C) AAMU excretion rate stratified by age. (D) caffeine intake, (E) AAMU concentration, and (F) AAMU excretion rate stratified by sex. (G) caffeine intake, (H) AAMU concentration, and (I) AAMU excretion rate stratified by race-ethnicity. M, male; F, female; NHW, non-Hispanic white; NHB, non-Hispanic black. Sample sizes (n) for spot urine concentration and excretion rate data appear in Supplemental Table 2. Intake data sample sizes were as follows: Age: 6–11 y, n = 358; 12–19 y, n = 381; 20–39 y, n = 575; 40–59 y, n = 589; ≥60 y, n = 505; Sex: male, n = 1178; female, n = 1230; Race-ethnicity: non-Hispanic white, n = 1028; non-Hispanic black, n = 447; all Hispanic, n = 807.