CYP1A2 and Parkinson disease: When comparing the PD and HS groups as a whole, there were no significant differences in basal caffeine levels (1.43 ± 1.65 vs. 1.66 ± 1.80 μg/ml; U = 837, p = 0.15), caffeine absorption (0.59 ± 1.49 vs. 0.51 ± 1.95 μg/ml; U = 606, p = 0.30), or CYP1A2 activity (0.28 ± 0.26 vs. 0.28 ± 0.29; U = 700, p = 0.98).