I seem to have PD and essential tremors. Rasagiline seems to help and it delays the development of PD, and even reverts some damage. Many nutrients delay the progression of PD in animal simulations. For many of them, there is no scientific or rational basis for believing they will not have the same benefit in humans, provided it can be shown they will reach the human brain in the same concentrations as in the animals. To give a quick example of this in about 30 common nutrients, the RDA nutrients are pretty much the same across all mammals and prevent the same diseases if taken in the right dose for the species at hand. It should be assumed other nutrients will act similarly until proven otherwise, if they can reach the human brain. More complicated nutrients than the RDAs are degraded by the efficient human liver and the human blood-brain barrier blocks them more efficiently.
The number of nutrients that help prevent or reverse PD in mammals is about 100. Some of these nutrients like caffeine, nicotine, magnesium, vitamin D, vitamin A, zinc, black tea (20 mg theanine per serving), beer, uric acid, are so common in the human diet that epidemiological evidence in humans already show they prevent PD and/or slow its progression.
So if it works in animals and there is reason to believe it reaches the human brain, OR if it has been shown in epidemiological evidence, then I'm going to take it. If it works in animals and it has been shown in epidemiological evidence to help in humans, then it is obviously something to take.
Some nutrients are harmful: milk for men, iron, chocolate, and it seems DHA in fish oil.
There are 8 classes of pharmaceuticals I know of that usually help: L-DOPA-increasing (including Ritalin), MAO inhibitors, anti-depressants, anti-anxiety, an iron chelator, some statins, some NSAIDs, and tyrosine-kinase inhibitors.
So I cringe when someone says "nothing slows PD progression". With the proper knowledge of each individual's genetics and the effect of each of these compounds based on the genetics and type of PD, there is good reason to believe (in my mind) that exercise and two or three of the properly-selected safe nutrients will immediately halt over 75% of PD cases.