Table 1.
Intraindividual variability in the caffeine metabolic ratio reported in the literature
| Race/ethnicitya | Sex | Smoker? | Age (y) | No. of subjects | Caffeine formulation (caffeine content) | Metabolic ratio | No. of replicates within a subject | Interval of replicate measurement | CV (%) | Reference | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Mean ± SD | Description | Mean ± SD | Mean (range) | ||||||||
| Japanese | M | No | 30.4 (17‐46)b | 5 | Instant coffee (caffeine content NS) | Urinary (17U+ PX)/CAF, 4–5 h collection interval | 8.7 | 2 | 11 mo | 27.5 (0.7–66.7) | 31 |
| F | No | 31.7 (23–36)b | 3 | 2.8 | 59.5 (41.6–84.5) | ||||||
| M | Yes | 35.3 (25–50)b | 4 | 11.6 | 17.6 (6.9–37.6) | ||||||
| White | M | No | (22–61)b | 10 | Brewed coffee (~200 mg) | Urinary PX/CAF, 4–5 h collection interval | 4.5 ± 1.7 | 3 | 1 wk | 24.3 (3.0–71.5) | 28 |
| F | No | 9 | 5.5 ± 1.5 | 26.2 (4.2–60.4) | |||||||
| NS | M | No | 34.8 ± 7.9 | 10 | NoDoze (2 mg/kg) | Urinary (1U+1X+AFMU)/17U, 14–15 h collection interval | 9.4 ± 10.2 | 6 | 18.6 (4.5–45.3) | 29 | |
| F | No | 38.2 ± 9.3 | 10 | 5.7 ± 2.3 | ~2 wk | 20.7 (8.7–49.3) | |||||
| NS | M | No | Young subjects | 34 | Koffein “Dak” (200 mg) | Plasma PX/CAF at 6 h | 0.75 ± 0.22 | 2 | 18 (1–45) | 32 | |
| M | Yes | 8 | 1.6 ± 0.7 | 12–14 wk | 16 (2–46) | ||||||
| NS | NS | No | 25 ± 0.3 | 12 | Instant coffee (150 mg) | Plasma PX/CAF at 5 h | 0.45 ± 0.19 | 5 | 3 wk | 16.2 (6.9–25.7) | 33 |
| Yes | 4 | 0.76 ± 0.21 | 21.8 (18.9–26.5) | ||||||||
| No | 70 ± l .7 | 13 | 0.32 ± 0.14 | 16.4 (8.6–32.9) | |||||||
| Yes | 3 | 0.45 ± 0.07 | 15.3 (10.4–18.5) | ||||||||
AFMU, 5‐acetylamino‐6‐formylamino‐3‐methyluracil; CAF, caffeine; CV, coefficient of variation; F, female; M, male; NS, not specified; PX, paraxanthine; 1U, 1‐methyl uric acid; 17U, 1,7‐dimethyluric acid; 1X, 1‐methylxanthine. aThe studies from references 29, 32, and 33 were conducted in the United States, Denmark, and France, respectively. bMean (range) or (range).