April 2, 2003 (Honolulu) -- An apple a day may not keep the doctor away, suggests a new study showing that eating a serving of fruit daily may increase the risk of Parkinson's disease.
But the researchers were quick to add that the benefits of fruit in protecting against common disorders such as heart disease outweigh any increased risk of the relatively uncommon neurological disorder marked by tremors, slow movement, and stiffness in the arms and legs.
Additionally, fruit itself might not be the culprit, says study leader Andrew Grandinetti, PhD, an epidemiologist at the Pacific Biomedical Research Center at the University of Hawaii at Manoa.
"We suspect that the increased risk may be due to food-borne toxins, pesticides, or herbicides, rather than the fruit itself," Grandinetti said at a news conference at the American Academy of Neurology 55th Annual Meeting.
About 1.2 million people in the United States and Canada suffer from Parkinson's disease.
The study is part of the ongoing Honolulu Heart Program, which enrolled 8,006 men of Japanese ancestry ages 45 to 68 residing in Oahu, Hawaii, in 1965. Over the next 34 years, 141 of the men developed Parkinson's disease.
For the current study, the researchers administered standard food questionnaires in 1990 asking the men what they ate, how much they ate, and how often they ate certain foods.
The questionnaires revealed that 1,449 of the men ate fruit or consumed fruit drinks at least once a day. Fruit drinks included canned prepared drinks such as Hawaiian Punch as well as natural fruit juices, Grandinetti says.
Their study showed that after taking into account known risk factors for Parkinson's disease such as age and caffeine intake, men who consumed fruit or drank fruit drinks at least once a day were almost twice as likely to develop Parkinson's disease as those who consumed less fruit, Grandinetti says.
Though the researchers did not analyze the types of fruit the men were eating, "there was some evidence that those with the highest intake were eating a lot of tropical fruits, such as pawpaw and custard apple, native to Hawaii," he says.
Since earlier studies had linked high intake of vitamin C to Parkinson's disease, the researchers also looked for such an association. But they failed to find one, Grandinetti said.
"We looked at vitamin C in two ways: both by estimating the amount of vitamin C in their diet and by issuing a separate questionnaire asking about supplement use," he says. "In neither case did we find any link between vitamin C and Parkinson's disease."
The researchers also found no evidence that eating vegetables increases the risk of Parkinson's disease, Grandinetti says.
"Most people eat their fruits raw, with the peels on. But especially in the 1960s, when the study began, canned or frozen vegetables were much more common than raw ones," he says.
Samuel Goldman, MD, a clinical research scientist at the Parkinson's Institute in Sunnyvale, Calif., said he believes food-borne toxins, not pesticides, are to blame.
"The study found an association between the disease and both fresh fruit and canned juices," he says. "The common denominator here is the fruit, not possible pesticide exposure."
Fruit contains isoquinolones, naturally occurring compounds that may be toxic to nerves, Goldman says.
To help resolve the issue, Grandinetti has called for further research looking into the link between food-borne toxins, pesticides, and Parkinson's disease.
In the meantime, he says, the best advice is to wash your fruit thoroughly.