The absolute bioavailability of caffeine in man.
Level 2 - randomized trial
Individual randomized crossover pharmacokinetic trial
PubMed 6832208 · doi:10.1007/BF00613933
What was done
Ten healthy adult male volunteers aged 18.8 to 30.0 years received a 5 mg/kg dose of caffeine as either an aqueous oral solution or an intravenous infusion in a randomized crossover design, separated by approximately one week. Plasma samples were collected over 24 hours post-dose and analyzed via high-performance liquid chromatography. Absolute bioavailability was assessed by comparing the area under the plasma concentration-time curves for intravenous and oral administration.
What was found
Oral absorption was rapid, with peak plasma concentration (Tp) reached at 29.8 +/- 8.1 min (mean +/- SEM). Maximum plasma concentration (Cmax) was 10.0 +/- 1.0 micrograms/ml. Absolute bioavailability of oral caffeine was essentially complete at F(%) = 108.3 +/- 3.6%. Plasma half-lives ranged widely among subjects, from 2.7 to 9.9 h.
Why it matters
This study demonstrates that oral caffeine in aqueous solution is completely bioavailable in humans, confirming that oral administration achieves systemic exposure equivalent to intravenous dosing.
Limits
The study was very small (n = 10) and restricted exclusively to young adult males. It evaluated only a single aqueous solution dose and did not test female participants, broader age groups, or real-world dietary forms of caffeine.
Cited by
- supports Caffeine reaches peak effectiveness about 30 minutes after consumption and impairs sleep for at least six hours.