Researchers suggest that a poor sense of smell may be an early sign of Parkinson's disease.
Researchers found that older adults who scored poorly on sniff tests were almost five times more likely to develop Parkinson's disease over 10 years of follow-up compared with individuals with a better sense of smell.
However, on looking at the results by race, the researchers found no statistically significant association between poor sense of smell and increased Parkinson's risk in black adults.
What is more, the study revealed that sense of smell appears to influence Parkinson's risk in men more than women.
Study co-author Honglei Chen, Ph.D., of the College of Human Medicine at Michigan State University in East Lansing, and colleagues recently reported their findings in the journal Neurology.
Parkinson's disease is central nervous system disorder that is characterized by problems with movement, balance, and coordination.
It is estimated that more than 10 million people across the globe are living with Parkinson's disease, and up to 1 million of these individuals live in the United States.
Previous research has suggested that a reduction in sense of smell may be an early indicator of Parkinson's, appearing years before the onset of motor symptoms. However, not all people who experience olfactory loss go on to develop the disease.
For this latest study, Dr. Chen and colleagues sought to learn more about this association - in particular, how far in advance loss of smell might predict the risk of Parkinson's disease, and whether this association varies between black and white adults.
Parkinson's risk increased almost fivefold
The study included 2,462 older adults aged 75 years, on average, who were a part of the Health, Aging, and Body Composition study. Of these adults, 1,510 were white and 952 were black.
Based on their scores, subjects were divided into three groups: a poor sense of smell, a medium sense of smell, and a good sense of smell.
The researchers followed the participants for an average of 10 years, noting any Parkinson's disease development during that time. A total of 42 subjects developed the condition during follow-up, of whom 30 were white and 12 were black.
Overall, the team found that participants who scored poorly on the smell test were almost five times more likely to develop Parkinson's disease than subjects with a good sense of smell.
The findings remained after accounting for a number of possible confounding factors, including a history of head injury, smoking, and coffee intake.
Differences by sex and race
While the link between poor sense of smell and increased Parkinson's risk persisted for the entire follow-up period, it was strongest in the first 6 years after the smell test, the researchers report.
"Earlier studies had shown prediction of Parkinson's disease about 4 to 5 years after the smell test was taken," notes Dr. Chen. "Our study shows that this test may be able to inform the risk much earlier than that."
Interestingly, the researchers found that men with a poor sense of smell were considerably more likely to develop Parkinson's disease than women with a poor sense of smell.
Additionally, the team found that the link between poor sense of smell and greater Parkinson's risk was not statistically significant for black adults.
"Reasons for this potential racial difference are unclear," say the authors. "One possibility is that, compared to white participants, the etiology of olfactory dysfunction in black participants is more diverse and complex, and that Parkinson's disease-related pathology is a relatively minor contributor."
But the team says that the findings should be interpreted with caution, and they note that further studies are required before a sniff test can be used to detect Parkinson's in clinical settings.
Still, they believe that their findings certainly pave the way for such a possibility.