TABLE 1. Subject Characteristics by Tolerance Group

Total Population (n=97)Tolerance Groups
High (n=48)Low (n=49)
Mean±SEM shown in parentheses.
Caffeine intake reflects usual consumption from all sources based on structured interview. Screening BP is average of 3 readings over 5 minutes after 5 minutes seated. High- and low-tolerance groups were not different in numbers of males and females (χ2=1.7, P=0.2), percents with a positive parental history of hypertension (χ2=0.9, P=0.3), or in any other listed variable (t<1.85, P>0.08). Caffeine levels from saliva specimens collected at 8:00 am on each test day after 5 days on the indicated daily maintenance dose of caffeine. ANOVA on tolerance group×week for C300 and C600 weeks indicated groups did not differ in residual caffeine levels, F(1,93)=2.69, P=0.104. Morning BPs are resting values before caffeine or placebo in the laboratory. ANOVA for tolerance group×week for all 4 weeks showed no BP difference for tolerance groups, F(1,95)<1.1, P>0.31, or for tolerance groups×week, F(3,76)<2.0, P>0.13 for either BP.
Sex (M/F)49/4821/2728/21
Age, y28 (0.6)28 (0.9)29 (0.8)
Weight, kg72 (1.2)72 (1.6)72 (1.9)
Height, cm174 (1.0)172 (1.5)176 (1.4)
Body fat, %20 (0.7)20 (1.0)19 (0.9)
Body mass index, kg/m223.9 (0.3)24.4 (0.4)23.4 (0.4)
Parental hypertension, %131511
Screening blood pressure, mm Hg
    Systolic112 (1.0)111 (1.3)114 (1.4)
    Diastolic66 (0.6)65 (0.9)67 (0.9)
Caffeine intake, mg/d454 (48)400 (66)506 (68)
Morning saliva caffeine levels each week, μmol/L
    0 mg/d0.18 (0.1)0.35 (0.2)0.03 (0.03)
    0 mg/d0.36 (0.2)0.67 (0.3)0.06 (0.04)
    300 mg/d3.56 (0.5)4.21 (0.09)2.96 (0.06)
    600 mg/d12.58 (1.7)15.18 (3.0)10.14 (1.6)
Morning baseline BP levels in each study week, mm Hg
    Systolic BP
        0 mg/d113 (0.93)112 (1.13)114 (1.47)
        0 mg/d113 (0.92)112 (1.12)115 (1.46)
        300 mg/d113 (0.92)112 (1.23)114 (1.37)
        600 mg/d113 (0.92)111 (1.02)114 (1.40)
    Diastolic BP
        0 mg/d67 (0.70)66 (0.98)68 (0.99)
        0 mg/d67 (0.76)66 (1.12)68 (1.04)
        300 mg/d66 (0.70)66 (1.07)67 (0.91)
        600 mg/d67 (0.74)67 (1.09)67 (1.00)