The study, published in the Annals of Neurology, found smell impairment can precede motor symptoms in men by at least four years.

Study leader G. Webster Ross of the VA Pacific Islands Health Care System and the Pacific Health Research Institute in Honolulu said those with the lowest olfactory scores -- those with poorest odor identification -- had a five times greater risk of developing Parkinson's disease than those with the highest scores.

"One interpretation of this finding is that the relationship of olfactory deficits to higher risk of future Parkinson's disease begins to weaken beyond a threshold of approximately four years between testing and diagnosis," the study authors said in a statement.

This study and findings from earlier studies suggest that olfactory impairment begins between two and seven years prior to diagnosis and estimates from neuroimaging and pathological studies suggest that there is a period of about five to seven years between the onset of nerve loss in an area of the brain affected by Parkinson's disease and diagnosis of the disease, the researchers said.