Smoking and smokeless tobacco are associated with lower Parkinson's risk, but not in those who have quit smoking. In fact, those who have quit smoking are more at risk of Parkinson's than those who have never smoked. (http://www.sciencedirect.com/science/article/pii/S1353802014004805) That suggests that 1) something in cigarettes could be accumulating in the body (I suspect cadmium), and 2) something in cigarettes partially protects against whatever (cadmium-induced?) brain insults are killing off dopaminergic neurons.
I might have come across two good candidates for the protective component of cigarettes. The first is nicotine, which in small doses--nicotine is in several commonly-eaten foods--appears to have some helpful aspects, which I'll blog about later. Unfortunately, nicotine patches appear ineffective in treating Parkinson's (https://www.ncbi.nlm.nih.gov/pubmed/14687854), so there's probably something else in cigarettes besides nicotine that is helping stave off Parkinson's. I suspect that second something is licorice. Isoliquiritigenin, a flavonoid in licorice (http://apps.who.int/medicinedocs/en/d/Js2200e/20.html), can apparently protect dopaminergic neurons from dying:
Biosci Biotechnol Biochem. 2012;76(3):536-43.
Isoliquiritigenin isolated from licorice Glycyrrhiza uralensis prevents 6-hydroxydopamine-induced apoptosis in dopaminergic neurons.
Licorice (Glycyrrhiza uralensis) is a medicinal herb containing various bioactive components implicated in antioxidative, anti-inflammatory, antiviral, and neuroprotective effects, but the effects of licorice against Parkinson's disease (PD)-related dopaminergic cell death have not been studied. In this study, we investigated the protective effects of isoliquiritigenin (ISL) isolated from Glycyrrhiza uralensis on 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in a dopaminergic cell line, SN4741. ISL (1 µM) significantly attenuated 6-OHDA (50 µM)-induced reactive oxygen species (ROS) and nitric oxide (NO) generation and apoptotic cell death. ISL pretreatment effectively suppressed 6-OHDA-mediated upregulation of Bax, p-c-Jun N-terminal kinase (JNK), p-p38 mitogen-activated protein (MAP) kinase, cytochrome c release, and caspase 3 activation. In addition, ISL significantly attenuated 6-OHDA-induced Bcl-2, brain-derived neurotrophic factor (BDNF), and mitochondrial membrane potential (MMP) reduction. Pharmacological inhibitors of the phosphatidylinositol 3-kinase (PI3K)-Akt/protein kinase B (PKB) pathway reversed ISL-mediated neuroprotection against 6-OHDA toxicity in SN4741 cells. These results provide the first evidence that ISL can protect dopaminergic cells under oxidative stress conditions by regulating the apoptotic process.
Most of the world's licorice production is used in tobacco products. (http://www.npr.org/2012/09/07/160752629/the-secrets-in-a-cigarette) Some licorice makes it way into black licorice candy and root beer, but here in the USA, licorice is a very common ingredient in cigarettes and smokeless tobacco.
As a non-smoker, I'll be doing much more research on this, for I've lived in places with bad air pollution and I prefer to avoid developing Parkinson's.