New research suggests that edible sources of nicotine may provide a protective effect against Parkinson’s disease. But independent experts are not convinced the effect is clinically significant.
The study published this week in Annals of Neurology indicates that eating foods that contain even a small amount of nicotine, such as peppers and tomatoes, may reduce risk of developing Parkinson’s.
US researchers analysed lifetime dietary habits of Parkinson’s disease sufferers and healthy controls, finding a slight association between diet factors and Parkinson’s disease incidence. Specifically, consumption of nicotine-containing foods seemed to give a protective effect against Parkinson’s.
Our colleagues at the UK SMC collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; smc@sciencemediacentre.co.nz).
Catherine Collins, Principal Dietitian, St George’s Hospital NHS Trust, said:
“The authors of this study add to the knowledge that a diet rich in plant antioxidants and low in inflammatory fats – a ‘Mediterranean style’ diet – is protective against Parkinson’s Disease (PD), an irreversible condition for which there is currently no cure. This study built on previous research that showed smokers to have a lower risk of PD, attributed to nicotine possibly having a protective effect on brain cell function.
“People are often surprised to know that nicotine is found in variable amounts in the ‘deadly nightshade’ family of plants (potatoes, tomatoes, peppers, chillies, and aubergine – plus cauliflower, which isn’t a member of this family group). In this study the researchers used a standard food questionnaire to examine the frequency (note – not the actual amount) of vegetable nicotine intake of PD sufferers compared to the general population.
“Despite strong suggestion that nicotine – from any source – may prove beneficial, the results of this study prove disappointing for a dietitian, with frequent pepper consumption showing a weak association with PD protection, but of insufficient robustness to promote peppers as a protective food against PD.
“Why should this be the case? Nicotine content can vary in vegetables due to growing conditions, storage, harvest and cooking method. For example, the nicotine content of a green tomato is ten times higher than a ripe tomato. Nicotine is poorly absorbed and we don’t really know enough about how other foods interact with the plant nicotine to influence its effect in the body.
“As the authors neatly explain in the discussion section, the devil is in the detail. Plants that we eat contain thousands of biologically active constituents, many of which have potential for protecting us against chronic disease. We also choose to eat plants in a variety of ways – which may also influence the effect on our health. Peppers and chillies are rarely eaten alone, and it may be the foods cooked with these vegetables that confer some protection.
“As this and other research has demonstrated over the last decade, it’s impossible to pull out one vegetable, or a group of vegetables, as independently beneficial to health. The Mediterranean diet is more than the sum of its parts. At the moment, we can’t identify exactly what it is in the diet that does us so much good, only that we know it does.”
Claire Bale, Research Communications Manager at Parkinson’s UK, said:
“The researchers studied several vegetables that contain tiny amounts of nicotine including peppers, which have the most, as well as tomatoes and potatoes. They found that people who reported eating peppers at least twice a week were around 30% less likely to develop Parkinson’s. This may sound significant, but because the risk of developing Parkinson’s in the general population is already quite low (around 1 in 500), this reduction in risk is actually smaller than it sounds.
“This was a small study which involved just over 1000 people and to produce convincing results, these types of studies really need to involve much larger numbers – tens to hundreds of thousands. It also studied a relatively young group of people, the average age was mid-60s, when Parkinson’s is a condition which often develops much later in life.
“At the moment we don’t fully understand the causes of Parkinson’s and we don’t know of anything which can prevent the condition, although there are a number of factors that may slightly lower risk including smoking cigarettes, drinking coffee and exercise.
“Although this study offers some interesting clues about how diet may affect risk of Parkinson’s developing, we need much larger studies in older populations before we know whether eating peppers can be added to this list. Currently, there’s no evidence that eating certain foods containing nicotine can slow Parkinson’s progression”.
Professor K Ray Chaudhuri, Consultant Neurologist and Professor in Neurology / Movement Disorders, King’s College Hospital, said:
“Nielsen et al report an interesting study addressing risk of Parkinson’s disease (PD) and diet. They report, in a controlled study, that eating edible vegetables from the Solanaceae family (particularly peppers), allied to dietary nicotine, reduced the risk of developing Parkinson’s. This study adds to a number of studies that have reported various factors that may reduce or increase the risk of PD or symptoms such as dyskinesias, ranging from insecticides and pesticides to coffee and beer drinking.
“Nicotine has been previously studied and has a complex relationship with PD. A neuro-protective effect has been postulated and the MJ Fox foundation has embarked on a large trial, the NIC-PD, to address whether nicotine patches may have a protective effect against PD. If supported by further studies that show the same association, this dietary study would certainly add weight to this evidence base and for the first time, provide a reasonable evidence base for dietary intervention in PD. However, as the authors state, there may be many confounders of the observed effects, most notably alternative agents contained in pepper as well as the amount of effective nicotine that may be delivered to the brain from dietary nicotine containing vegetables, given there is delayed gastric emptying and absorption issues which may be present even in the very early stages of PD. Furthermore, studies in animal models have been disappointing with respect to motor effects of nicotine and previous studies in Parkinson’s have produced contradictory results. However, the controlled design as well as inclusion of a group of well characterised patients is a strength of this study. As the authors say, we have to regard these results with interest, but have to wait till large scale studies such as NIC-PD report their data.”
Professor John Hardy, Professor of Neuroscience, UCL, said:
“The trouble is in epidemiology studies one compares many factors against many outcomes. Because so many tests are done but only the positive ones are reported it is difficult to assess their true significance. The association reported here between eating red peppers and risk of developing Parkinson’s disease is almost certainly a false positive and not worth making a fuss about.”
Before The Headlines analysis
Before The Headlines is a service provided to the UK SMC by volunteer statisticians.
Claim supported by evidence?
Eating a large amount of pepper as part of daily diet is not going to substantially reduce an individual’s risk of PD but the results support further research into the possible link between chemicals found in both peppers and cigarettes (not restricted to nicotine) and reduction in PD risk.
Summary
Results presented in the paper consistently show reductions in PD risk in those who ate more vegetables in the Solanaceae family, with stronger associations in those that ate more vegetables containing higher levels of nictotine (peppers).
It should be noted that the data were collected by retrospective self-reporting of lifetime diet, which may therefore limit the reliability of the data in the study.
In terms of absolute risk it should be noted that a 30% reduction, for example, could relate to a 0.05% reduction is absolute risk. (This is a very rough calculation based on DWP figures of incidence on PD in the UK and does not take into account other factors such as genetics and hence is probably an overestimate.)
The findings were driven by the subgroup of subjects who had never or hardly ever used tobacco. If the link between nicotine and PD risk is true then this can be explained by the fact that diet is a modest contributor to the daily nicotine dose relative to tobacco use.
This study has shown an association which is not the same as a causal effect, the authors are clear on the limitations of the study (below) and acknowledge that replication of their findings are needed.
Study Conclusions
An inverse association between PD and edible Solanaceae, especially peppers was observed but this was largely confined to subjects who have never or hardly ever smoked. The finding that peppers had the strongest association is consistent with them containing more nicotine than the other Solanaceae vegetables such as tomato, tomato juice, potato and eggplant.
Table 2 shows the relative risk of different frequencies of pepper consumption and reductions of >30% are only seen once consumption is 2-4 times a week, so it would need to be a major part of the diet to see an effect and as mentioned above this risk reduction relates to a very small absolute risk reduction.
The authors provide references to other research which support their findings.
The authors discuss that it may be other constituents other than nicotine that could be neuroprotective and which are common to both peppers and tobacco use.
Strengths
Large number of cases and controls used; matched on sex, age, race, clinic
Hypothesis tested is based on consistent findings in previous research linking reduced risks of PD and tobacco smoking. Also animal models provide further support for potential role of nicotine.
People with established cause of secondary PD such as stroke, brain tumour were excluded.
Consumption of ‘Other’ vegetables which do not contain Solanaceae were included in the analysis and no effect was found. This goes some way to adjust for an effect of ‘healthy eating’ on reducing the risk of PD and indicates that the Solanacaea vegetables are different in some way.
Limitations
Diet data collected retrospectively by self-report and were not assessed for different periods of life. As the authors acknowledge PD may affect diet and could have influenced the result, with heartburn given as a potential concern.
There is always the potential for other unknown confounders and in particular the authors acknowledge that there was a lack of environmental tobacco smoke exposure data for most participants.