By Joseph Masters (Medical Student, Barts and the London School of Medicine and Dentistry)
It is now well established that smokers seem have a reduced risk of Parkinson’s disease and a nice summary of the evidence was published last year [1]. Unfortunately, the detrimental effects of smoking on general health almost certainly outweigh the potential benefits of reducing Parkinson’s disease risk [2].
However, some of the foods we eat – namely peppers, tomatoes and potatoes – contain tiny amounts of nicotine (an active chemical in cigarette smoke) and may suggest protection against Parkinson’s disease in a similar way to smoking, without the harmful side-effects!
The above paper [3] reported that consumption of nicotine containing vegetables (which are members of the Solanaceae family) is associated with a reduced risk of Parkinson’s disease. Some newspapers would have you believe that this is firm evidence that eating peppers can prevent you from developing Parkinson’s – if only it were that simple…
What the authors did:
This was a case-control study, meaning that a group of people recently diagnosed with Parkinson’s disease (cases) were compared with a group of people of similar age and background who did not have Parkinson’s disease (controls). Both groups were asked about their diet and cigarette smoking.
What the paper found:
The controls were found to have significantly higher levels of consumption of all Solanaceae vegetables (peppers, tomatoes and potatoes). However, there was no association between overall vegetable consumption and Parkinson’s disease, so it was just these Solanaceae vegetables which appeared to show a protective effect.
Not all Solanaceae vegetables contain the same amount of nicotine (peppers contain the most and potatoes hardly any at all) and when the authors weighted their calculations to account for the different nicotine concentrations, they found an even stronger association. This suggests that it is the nicotine content of these foods that is driving the protective effect.
Interestingly, when the researchers split the group into ‘ever’ and ‘never’ cigarette smokers, they found that the protective effect of Solanaceae consumption was much greater for non-smokers and, in fact, non-existent for current smokers. This makes sense if it is the nicotine content of these foods which is causing the protective effect, since the nicotine receptors in the brains of smokers will already be occupied by nicotine from their smoke [4], leaving the small amounts of dietary nicotine from peppers unable to have any additional effect.
Limitations of this research:
On the whole, this was good research and there were no major flaws in the study’s methods. The sample size was large enough to produce robust, statistically significant results and the authors adjusted for other known risk factors for Parkinson’s disease such as age, ethnicity, smoking and caffeine consumption. Therefore, it is fair to claim that an association between Solanaceae vegetables and Parkinson’s disease does exist.
However, correlation is not the same as causation and we most definitely cannot yet say that peppers prevent Parkinson’s disease for the following reasons:
The possibility of reverse causality: Parkinson’s disease is known to cause a wide-range of symptoms including loss of smell/taste [5] and heartburn which have been shown to affect patients’ diets [6], and these early features may even occur before Parkinson’s is diagnosed. Therefore, it may simply be the case that people with Parkinson’s (or on track to develop it) favour a plainer diet and are less likely to eat peppers and tomatoes, rather than these foods having a protective effect.
Diet was not assessed for different periods of life: It is likely that consumption of these foods has its effects over several decades but the authors just asked about current dietary patterns.
Replicability: This is the first finding that the nicotine content of food is associated with reduced risk of Parkinson’s disease. Further studies should be done to confirm that the effect really exists.
Poor understanding of the biological basis for the effect: The amounts of nicotine contained in these foods are very, very small relative to the amounts in cigarette smoke and we are uncertain about how much of it actually reaches the brain (much will be removed by the liver en route). We are not even certain whether it is actually the nicotine content of these foods which is producing the protective effect – the authors suggest that other compounds, known as capsinoids or an alkaloid called anantabine might be responsible, but this is all very speculative.
So should I eat more peppers if I want to reduce my risk of Parkinson’s disease?
The authors’ finding that there is an association between Solanaceae consumption and Parkinson’s disease is very interesting. It does not provide unequivocal proof that eating peppers actually prevents Parkinson’s disease! Nonetheless, a ‘Mediterranean style’ diet rich in fruit and vegetables has numerous beneficial health effects and eating a few more tomatoes and peppers every week most certainly cannot do any harm!
References:
[1] Noyce AJ, Bestwick JP, Silveira-Moriyama L, Hawkes CH, Giovannoni G, Lees AJ, et al. Meta-analysis of early nonmotor features and risk factors for Parkinson disease. Ann Neurol. 2012;72:893-901.
[2] The Health Consequences of Smoking: A Report of the Surgeon General. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2004.
[3] Nielsen SS, Franklin GM, Longstreth WT, Swanson PD, Checkoway H. Nicotine from edible Solanaceae and risk of Parkinson disease. Ann Neurol. 2013.
[4] Brody AL, Mandelkern MA, London ED, Olmstead RE, Farahi J, Scheibal D, et al. Cigarette smoking saturates brain alpha 4 beta 2 nicotinic acetylcholine receptors. Arch Gen Psychiatry. 2006;63:907-15.
[5] Shah M, Deeb J, Fernando M, Noyce A, Visentin E, Findley LJ, et al. Abnormality of taste and smell in Parkinson's disease. Parkinsonism Relat Disord. 2009;15:232-7.
[6] Barichella M, Cereda E, Pezzoli G. Major nutritional issues in the management of Parkinson's disease. Mov Disord. 2009;24:1881-92.
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