CYP1A2 and Parkinson disease: Similarly, early PD patients and HS showed statistically similar basal caffeine levels (1.81 ± 2.10 vs. 1.11 ± 1.13 μg/ml; U = 837, p = 0.15), caffeine absorption (0.34 ± 1.47 vs. 0.94 ± 1.61 μg/ml; U = 605, p = 0.30), and CYP1A2 activity (0.29 ± 0.28 vs. 0.31 ± 0.36; U = 700, p = 0.98).