Table 1 Characteristics of the included studies

Fiszer et al.³⁴

Cross-sectional

Control (n = 12)

PD (n = 16)

Duration of PD (6.62 ± 3.79 years)

Therapy = Levodopa (715.62 ± 648.52 mg/day)

Control (F = 9; M = 3)

PD (F = 8; M = 8)

Control = 58.58 ± 8.60 years

PD = 60.5 ± 8.65 years

Unger et al.³³

Quasi-experimental

Control (n = 20)

PD (n = 19 drug-naive; n = 20 under therapy)

Duration of PD (Drug-naive = median 1.1 [95% CI: 0.6–3.1]; Under therapy = median 3.9 [95% CI: 1.9–7.3])

Therapy = levodopa monotherapy (n = 2), levodopa plus entacapone (n = 1), levodopa plus selegiline (n = 1), levodopa plus pramipexole (n = 2), levodopa plus entacapone plus pramipexole plus amantadine (n = 1), levodopa plus rotigotine (n = 2), cabergoline plus rasagiline (n = 1), pramipexole monotherapy (n = 4), pramipexole plus selegiline plus budipine (n = 1), pramipexole plus rotigotine (n = 1), ropinirol monotherapy (n = 1), and rotigotine monotherapy (n = 2)

Control (F = 8; M = 12)

Drug-naive PD (F = 7; M = 12)

Under therapy PD (F = 11; M = 9)

Control = 55.2 ± 8.8 years

Drug-naive PD = 62.4 ± 11.2 years

Under therapy PD = 63.7 ± 8.5 years

Song et al.²⁶

Quasi-experimental

Control (n = 303: 20 for postprandial)

PD (n = 291; 20 for postprandial)

Duration of PD = Not informed (PD stages 1–3)

Therapy = Not informed

Control (F = 144; M = 159)

PD (F = 131; M = 160)

Control = 66.04 ± 9.89 years

PD = 65.96 ± 9.95 years

Tarianyk et al.³⁸

Quasi-experimental

Control (n = 21)

Akinetic-rigid PD (n = 16)

Mixed PD (n = 14)

Duration of PD (Akinetic-rigid = 6.42 ± 2.13 years; Mixed = 9.13 ± 1.87 years)

Therapy = Not informed

Not informed

Control = 58.63 ± 6.16 years

Akinetic-rigid PD = 59.94 ± 7.97 years

Mixed PD = 62.13 ± 10.39 years

Pietraszko et al.³⁷

Quasi-experimental

Control (n = 30)

PD (n = 29)

Duration of PD (10.9 ± 4.7 years)

Therapy = Not informed (All medications were withheld or refused by the patient, ensuring a drug-free period of at least 12 h)

Control (F = 11; M = 19)

PD (F = 11; M = 18)

Control = 58.4 ± 11.1 years

PD = 59.4 ± 9.9 years

Hornsby et al.³⁵

Quasi-experimental

Control (n = 20)

PD (n = 20)

PD Dementia (n = 8)

Duration of PD (Not informed)

Therapy = Not informed (All medications were withheld for plasma collection)

Control (F = 9; M = 11)

PD (F = 9; M = 11)

PD Dementia (F = 1; M = 7)

Control = 74.0 ± 6.28 years

PD = 72.2 ± 5.51 years

PD Dementia = 74.75 ± 5.99 years

Siervo et al.³⁶

Quasi-experimental

Control (n = 20)

PD (n = 19)

PD-CI (n = 16)

Duration of PD (PD = 69.5 ± 69.0 months; PD-CI = 107.3 ± 59.5 months)

Therapy = Standard preparation levodopa (n = 16 PD; n = 16 PD-CI), Controlled release levodopa (n = 3 PD; n = 9 PC-CI), Dopamine agonists (n = 7 PD), MAOIB (n = 11 PD; n = 2 PD-CI), COMT inhibitors (n = 4 PD; n = 7 PD-CI), Cholinesterase inhibitors (n = 7 PD-CI), Selective anticholinergics (n = 3 PD; n = 3 PD-CI), Antidepressants (n = 7 PD-CI), Pro-appetitive medicines (n = 3 PD-CI), and Anorectic medications (n = 11 PD; n = 6 PD-CI). All medications were withheld for plasma collection.

Control (F = 9; M = 11)

PD (F = 9; M = 10)

PD-CI (F = 7; M = 9)

Control = 74.0 ± 6.2 years

PD = 72.5 ± 5.5 years

PD-CI = 74.3 ± 6.0 years

Majeed; Al-Lami; AlGawwam³⁹

Cross-sectional

Control (n = 40)

PD (n = 51)

Duration of PD (<5 years = 36 [70.6%]; 5–10 years = 12 [23.5%]; >10 years = 3 [5.9%])

Therapy = Not informed

Control (F = 25; M = 15)

PD (F = 13; M = 38)

Control = 41.25 ± 18.30

PD = 63.76 ± 12.29