The chemical urate, which is known to cause gout, appears to slow the progression of Parkinson's disease, US researchers have concluded.
The team found that a study confirmed their previous suspicions about urate, which occurs naturally in the blood.
Urate is a potent antioxidant and so counteracts oxygen-related cell damage thought to contribute to Parkinson's, they report in Archives of Neurology.
Trials are under way to find a safe way to raise urate levels as a therapy.
With support from the Michael J Fox Foundation, the researchers will recruit 90 recently diagnosed Parkinson's patients for treatment with a chemical which helps to produce urate - called inosine - to see if this can raise urate levels so as to slow or halt disease progression.
Diets which are rich in foods like liver, seafood and dried beans and peas, as well as alcohol, can also increase blood urate levels.
But too much urate in the blood can cause gout, a painful joint disease.
Dr Michael Schwarzschild and colleague Dr Alberto Ascherio originally made the link between urate and Parkinson's when analysing data from a previous clinical trial.
Their latest work confirms their hunch that urate is protective, they say.
They looked at samples of both blood and cerebrospinal fluid - the fluid that surrounds the brain and spinal cord - and measured urate levels.
Among the 800 Parkinson's patients in the study there was a clear trend linking higher urate levels and slower disease progression.
Dr Schwarzschild, associate professor of neurology at Massachusetts General Hospital in Boston, said: "Urate is a major antioxidant and it can protect brain cells in the lab, which makes this a compelling possibility; but we don't yet know if it's urate itself or some urate-determining factor that helps people with Parkinson's."
He said people should not take the findings to mean they should eat more urate-rich foods to guard against Parkinson's.
"Because elevated urate levels have known health risks, including gout and kidney stones, urate elevation should only be attempted in the context of a closely monitored clinical trial in which potential benefits and risks are carefully balanced," he said.
Dr Kieran Breen of the Parkinson's Disease Society said more research was needed.
"This tells us that it is possible to influence the rate of disease progression. It's a new lead to follow."
He said the findings might help find a therapy and markers to help track the disease.
He added that people treated for gout should not be concerned that the treatment would inadvertently increase their risk of Parkinson's.
"There is no evidence to suggest it would," he said.