Edible nicotine in peppers, tomatoes and potatoes may protect against Parkinson's disease, a study suggests.
The research adds to evidence associating a reduced risk of the disease with smoking.
Peppers appeared to have the biggest effect. People who ate the vegetables twice a week or more were found to be 30% less likely to develop Parkinson's. However, experts urged caution since a number of factors including the disease itself may have influenced the findings.
Nicotine, the addictive chemical in cigarettes, is found in tiny amounts in a number of edible plants related to tobacco, including peppers, tomatoes, potatoes and aubergines. Speculation that nicotine might protect against Parkinson's was fuelled by animal studies.
Experiments showed that stimulation of nicotine-sensitive receptor molecules in the brain prevents the kind of neuron damage seen in Parkinson's. Human population studies have also shown that people who smoke are less likely to develop the disease. Even passive smoking, which involves much less exposure to nicotine, seems to be protective.
For the new study 490 patients newly diagnosed with Parkinson's disease, and 644 other individuals not suffering from any neurological conditions, were questioned about their dietary habits and tobacco use.
Vegetable consumption in general was not found to affect Parkinson's risk. But the likelihood of being diagnosed with Parkinson's reduced the more people ate vegetables from the Solanaceae family, which contain nicotine. The trend was strongest for peppers, mainly in people with little or no previous exposure to tobacco.
Although the evidence pointed towards nicotine, the scientists did not rule out another chemical shared by tobacco and its cousins being responsible for the effect. One possibility was anatabine, which had anti-inflammatory properties.
Dr Searles Nielsen, from the University of Washington in Seattle, US, who led the research reported in the journal Annals of Neurology, said: "Our study is the first to investigate dietary nicotine and risk of developing Parkinson's disease. Similar to the many studies that indicate tobacco use might reduce risk of Parkinson's, our findings also suggest a protective effect from nicotine, or perhaps a similar but less toxic chemical in peppers and tobacco."
Parkinson's disease is a progressive condition in which damage to neurons in the brain affects control over motor functions, leading to tremors, rigidity and slow movement. Around 127,000 people in the UK have the disease, one in 500 of the population.