Hi Mister X,
I believe the study "An open trial of high-dosage antioxidants in early Parkinson's disease" (see link below) also suggests Vit C (plus Vit D) may be beneficial. The conclusions are actually quite encouraging: "The time when levodopa became necessary was extended by 2.5 y in the group taking antioxidants" (note that tocopherol = vit D and ascorbate = vit C). It's a pity the study methodology wasn't particularly robust (the control group was from another study and the number of participants was anyway limited).
... but I've been taking Vit D and Vit C since Dx around 2 years ago and I'm not taking l-dopa yet, so I'm not going to complain...