Absorption and bioavailability of caffeine were generally similar between humans,
dogs, rabbits, rats, and mice (Walton et al. 2001). In animals and man, absorption is
characterized by rapid and complete gastrointestinal absorption (Arnaud 1976;
Pharmacokinetics and Metabolism of Natural Methylxanthines in Animal and Man 35
Arnaud and Welsch 1980a; Yesair et al. 1984). In man, 99% of the administered
dose was absorbed in 45 min (Blanchard and Sawers 1983a), mainly from the small
intestine but also 20% from the stomach (Chvasta and Cooke 1971). The absolute
bioavailability of caffeine (5 mg/kg) in healthy adult male volunteers showed a
rapid oral absorption with the time to reach peak the peak plasma concentration
(Tmax) of 29.8 8.1 min and a peak plasma concentration (Cmax) of 10.0 1.0 mg/
mL. From comparison of the caffeine area under the concentration versus time
curve (AUC) after intravenous and oral doses, a complete absolute bioavailability
of caffeine was demonstrated (Blanchard and Sawers 1983a). Caffeine pharmacokinetics
were independent of the route of administration as shown by superimposable
plasma concentration curves, suggesting that there is no important hepatic
first-pass effect. Caffeine absorption from food and beverages does not seem to be
dependent on age, gender, genetics, and disease or consumption of drugs, alcohol,
and nicotine. Caffeine absorption from cola and chocolate was delayed, with Tmax
of 1.5 2 h instead of 0.5 h for a capsule and the Cmax values were 1.57 for cola, 1.50
for chocolate, and 2.05 mg/mL for the capsule (Mumford et al. 1996). Pharmacokinetics
of caffeine (100 mg) and its dimethylxanthine metabolites studied after
inhalation in heroin users and compared with intravenous and oral administration
in healthy volunteers showed a rapid and effective absorption after inhalation with
an approximate bioavailability of inhaled caffeine of 60% in experienced smokers
(Zandvliet et al. 2005). The efficacy of percutaneous caffeine absorption has been
demonstrated in premature infants treated for neonatal apnea (Morisot et al. 1990).