Limitations of the study include that people with severe Parkinson's disease were not included.
How your body metabolises coffee can help diagnose Parkinson's disease, according to a study which found that people with the neurodegenerative disorder have significantly lower levels of caffeine in their blood.
"Previous studies have shown a link between caffeine and a lower risk of developing Parkinson's disease, but we haven't known much about how caffeine metabolises within the people with the disease," said Shinji Saiki from Juntendo University in Japan.
The study involved 108 people who had Parkinson's disease for an average of about six years and 31 people of the same age who did not have the disease.
Their blood was tested for caffeine and for 11 byproducts the body makes as it metabolises caffeine. They were also tested for mutations in genes that can affect caffeine metabolism.
The two groups consumed about the same amount of caffeine, with an average equivalent to about two cups of coffee per day.
However, the people with Parkinson's disease had significantly lower blood levels of caffeine and nine of the 11 byproducts of caffeine in the blood.
In the statistical analysis, researchers found that the test could be used to reliably identify the people with Parkinson's disease, with a score of 0.98 where a score of 1 means that all cases are identified correctly.
In the genetic analysis, there were no differences in the caffeine-related genes between the two groups.
People in the study with more severe stages of the disease did not have lower levels of caffeine in the blood, suggesting that the decrease occurs from the earliest stages of the disease, said David G Munoz, of the University of Toronto in Canada.
"If these results can be confirmed, they would point to an easy test for early diagnosis of Parkinson's, possibly even before symptoms are appearing," said Munoz, who was not part of the study.
"This is important because Parkinson's disease is difficult to diagnose, especially at the early stages," he said.
Limitations of the study include that people with severe Parkinson's disease were not included, which could affect the ability to detect an association between disease severity and caffeine levels.
Munoz also noted that all of the people with Parkinson's were taking Parkinson's medication and it is possible that these drugs could affect the metabolism of caffeine.