A simple sniff test could predict the risk of Parkinson's disease up to 10 years before a patient is diagnosed, according to a study which found that people with a poor sense of smell were nearly five times more likely to develop the disorder.
Researchers found that older men with a poor sense of smell were more likely to develop the disease compared to women.
"One of the key differences in our study was we followed older white and black participants for an average of about 10 years, much longer than any other previous study," said Honglei Chen, from Michigan State University in the US.
"We found that there was a strong link between smell and disease risk for up to six years. After that, the link remained, but just wasn't as strong," said Chen.
The relationship between smell and Parkinson's risk in black participants also appeared not as strong as in the white participant group, he added.
"Previous studies have shown that black people are more likely to have a poor sense of smell than whites and yet may be less likely to develop Parkinson's disease," said Chen.
"We found no statistical significance for a link between poor sense of smell and Parkinson's disease in blacks, but that may have been due to the small sample size and more research is needed," he added.
The study included 1,510 white and 952 black participants with an average age of 75. The test asked people to smell 12 common odours including cinnamon, lemon, gasoline, soap and onion, and then select the correct answer from four choices.
Based on their scores, participants were divided into three groups - poor sense of smell, medium and good.
Researchers then monitored participant health through clinical visits and phone interviews for more than a decade.
Overall, 42 people developed Parkinson's during the study including 30 white people and 12 black people.
People with poor sense of smell were nearly five times more likely to develop the disease than people with a good sense of smell.
Of the 764 people with a poor sense of smell, 26 people developed the disease, compared to just seven of the 835 people whose sense of smell was good and nine of the 863 people whose sense of smell was categorised as medium.
Researchers also discovered that the results stayed the same after adjusting for other factors that could affect risk including smoking, coffee intake and history of head injury.