This lay article makes the case for eating certain vegetables (most notably bell peppers) as a safer way than smoking of obtaining nicotine for neuroprotection.

The theory is that although cigarettes contain much much more nicotine than foods, only a couple of puffs of a cigarette saturates over half the brain's nicotine receptors, so the small amount present in food may be sufficient for a clinically observable protective effect.

Furthermore, the article states that those exposed to second hand smoke are also less likely to develop PD, and second hand smoke contains very little nicotine.

The article does not go so far as to say that nicotine is the neuroprotective compound in cigarettes.

For those of us with PD, the question perhaps is: can disease progression be slowed by ingesting nicotine.

Nicotine In Nightshade Vegetables Linked To A Lower Risk Of Parkinson’s | Care2 Healthy Living

Dan