1 The pharmacokinetics of caffeine (CA), paraxanthine (PX), theobromine (TB) and theophylline (TP) were studied in six healthy male volunteers after oral administration of each compound on separate occasions. 2 The total plasma clearances of CA and PX were similar in value (2.07 and 2.20 ml minkg-', respectively) as were those for TP and TB (0.93 and 1.20 ml min-' kg-', respectively).The unbound plasma clearances of CA and PX were also similar in magnitude (3.11 and 4.14 ml min-' kg-', respectively) as were those of TP and TB (1.61 and 1.39 ml min-t kg-', respectively). 3 The half-lives of TP and TB (6.2 and 7.2 h, respectively) were significantly longer than those of CA and PX (4.1 and 3.1 h, respectively).4 The volume of distribution at steady state of TP (0.441 kg-') was lower than that of the other methylxanthines (0.63-0.721 kg-1). The unbound volume of distribution of TP (0.77 1 kg-) was however the same as that of TB (0.791 kg-') whereas the unbound volume of distribution of PX (1.18 1 kg-1) was similar to that of CA (1.06 1 kg-').

The caffeine content of all tea or coffee beverages consumed by 17 healthy adults over 24 hours was measured. Plasma caffeine, theophylline, theobromine, and paraxanthine concentrations were determined over the same 24 hours. The average caffeine content per drink was 60.4 +/- 21.8 mg for instant coffee (14-fold range), 80.1 +/- 19.2 mg for brewed coffee (2.8-fold range), and 28.8 +/- 13.7 mg for tea (5.5-fold range). The number of drinks of coffee and tea consumed was a poor index of actual caffeine intake (r2 = 0.42). Caffeine intake correlated poorly with the 24-hour average caffeine concentration (r2 = 0.41), but there was a very good correlation between a single plasma caffeine concentration measured at 5 PM and the 24-hour average concentration (r2 = 0.94). The same was true for paraxanthine (r2 = 0.86). Paraxanthine accounted for 67.3% of the total dimethylxanthines in plasma, while theobromine and theophylline accounted for 24.4% and 8.3%, respectively. Mean caffeine clearance was 1.2 +/- 0.3 ml/min/kg. Plasma caffeine concentration before the first drink in the morning correlated very poorly with caffeine clearance (r2 = 0.07), even when adjusted for caffeine intake (r2 = 0.21).

Five subjects who participated in an earlier study (Lelo et al., 1986b) of the comparative pharmacokinetics of caffeine (CA) and its primary monodemethylated metabolites paraxanthine (PX), theobromine (TB) and theophylline (TP) were administered CA to steady-state. Using areas under the plasma concentration-time curves for each of the dimethylxanthines derived from CA in the steady-state study and individual plasma clearances of PX, TB and TP determined in the previous study, the fractional conversion of CA to PX, TB and TP and the individual partial clearances of CA have been defined. The mean (± s.d.) fractional conversion of CA to PX, TB and TP was 79.6 + 21.0%, 10.8 + 2.4% and 3.7 + 1.3%, respectively. When only demethylation pathways are considered PX, TB and TP accounted for 83.9 + 5.4%, 12.1 + 4.1% and 4.0 + 1.4%, respectively of the CA demethylations. The mean partial clearance of CA to PX was approximately 8-fold and 23-fold greater than those to TB and TP respectively. These data confirm earlier reports that PX is the major metabolite of CA in humans but suggest that PX formation is quantitatively more important than previously believed.

Our previous study demonstrated that curcumin, an active compound of Curcuma xanthorrhiza and C. domestica, produces a positive cholekinetic effect. A 20 mg amount of curcumin is capable of contracting the gall bladder by up to 29% within an observation time of 2 h. The aim of the current study was to define the dosage of curcumin capable of producing a 50% contraction of the gall bladder, and to determine if there is a linear relationship between doubling the curcumin dosage and the doubling of gall bladder contraction. A randomised, single-blind, three-phase, crossover-designed examination was carried out on 12 healthy volunteers. Ultrasonography was carried out serially to measure the gall bladder volume. The data obtained was analysed by analysis of variance (ANOVA). The fasting volumes of gall bladders were similar (P > 0.50), with 17.28 +/- 5.47 mL for 20 mg curcumin, 18.34 +/- 3.75 mL for 40 mg and 18.24 +/- 3.72 mL for 80 mg. The percentage decrease in gall bladder volume 2 h after administration of 20, 40 and 80 mg was 34.10 +/- 10.16, 51.15 +/- 8.08 and 72.25 +/- 8.22, respectively, which was significantly different (P < 0.01). On the basis of the present findings, it appears that the dosage of cucumin capable of producing a 50% contraction of the bladder was 40 mg. This study did not show any linear relationship between doubling curcumin dosage and the doubling of gall bladder contraction.

On the basis of the present findings, it appears that curcumin induces contraction of the human gall-bladder.