People living with Parkinson’s disease may experience the world of scent very differently, according to new research suggesting the condition reduces enjoyment of pleasant smells such as lemon. Scientists say the discovery could help doctors identify Parkinson’s earlier using a simple, low-cost smell test, potentially speeding up a diagnosis that often takes years.
Loss of smell is already known to be one of the earliest symptoms of Parkinson’s disease, affecting roughly 75 to 90 percent of patients. In many cases it appears years, or even decades, before the tremors and movement problems that most people associate with the condition. However, diagnosing Parkinson’s based on smell loss alone has proven difficult because the ability to detect scents naturally declines as people age.
Researchers led by Noam Sobel at the Weizmann Institute of Science in Israel explored a different approach by examining not just whether people could smell, but how they perceived odors. Their study focused on what they describe as an “olfactory perceptual fingerprint,” which measures both how intense a scent seems and how pleasant people find it.
The research involved 94 participants, most in their late fifties to late sixties. Among them were 33 people diagnosed with Parkinson’s disease, 33 healthy individuals with no known medical conditions, and 28 people with smell dysfunction unrelated to Parkinson’s. Participants first completed standard smell detection and identification tests, followed by a specialized experiment designed to capture their perceptual response to specific odors.
In this test, participants were asked to evaluate scents coming from three jars. One jar contained a high concentration of citral, a chemical that produces a lemon-like smell. Another contained a mixture of aromatic compounds with a strong, unpleasant odor resembling fecal matter. The third jar was empty and served as a control.
The results showed that while all tests could identify when someone had reduced smell ability, only the olfactory perceptual fingerprint method could reliably distinguish between people whose smell loss was linked to Parkinson’s and those whose smell loss had other causes. The technique correctly separated the two groups with 88 percent accuracy, increasing to 94 percent when age and sex were matched among participants.
Interestingly, people with Parkinson’s detected the citrus smell as strongly as healthy participants did. However, they reported finding the pleasant scent less enjoyable. Both groups with smell impairments rated the pleasantness of the lemon scent lower than healthy individuals, but the difference in behavioral response was particularly notable among those with Parkinson’s.
When exposed to the unpleasant odor, participants with Parkinson’s also showed a different sniffing pattern. They inhaled for slightly longer when smelling the unpleasant scent than when smelling the lemon odor. In contrast, healthy participants and those with non-Parkinson’s smell disorders reduced their sniff duration significantly when encountering the unpleasant smell.
The researchers believe this pattern suggests that the nose itself may still function relatively normally in people with Parkinson’s, but the brain processes the scent signals differently. This altered processing may reduce the emotional or pleasurable response to positive smells and disrupt the usual behavioral reaction to unpleasant ones.
Scientists suspect these changes may be linked to early degeneration in parts of the brain involved in smell processing, such as the anterior olfactory nucleus. This region is thought to be one of the earliest sites affected by Parkinson’s disease and can shrink when deprived of normal smell signals.
Experts say distinguishing between normal age-related smell decline and smell changes linked specifically to Parkinson’s could be extremely valuable. Michał Pieniak from the Smell and Taste Clinic at the Dresden University of Technology notes that roughly one in ten patients who visit the clinic with unexplained smell loss eventually develops Parkinson’s disease.
Being able to identify which individuals are at higher risk would allow doctors to monitor them more closely and potentially intervene earlier. However, researchers caution that larger studies will be needed before the method can be widely used in clinical settings.
Charles Greer of Yale School of Medicine says the approach shows promise but will require testing on larger populations and long-term validation. Because smell loss can occur many years before other symptoms appear, confirming its predictive value for Parkinson’s may take time.
Despite these challenges, the findings suggest that subtle changes in how people experience everyday scents could become an important clue in identifying Parkinson’s disease at its earliest stages, potentially transforming how the condition is detected and managed.