Title, Date of Publication & Journal

Nicotine from Edible Solanaceae and Risk of Parkinson Disease, 9^th May 2013, American Neurological Association

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/Limitations

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.