How well you can smell could be a sign of overall brain health.

A growing body of research suggests that the two are strongly linked, though researchers are just starting to figure out how and why.

In a recent study, researchers found that a simple smell test may predict your chance of having dementia.

“Ability to smell is a window into parts of the brain related to core functions, like pleasure, emotion, and memory,” says Jayant Pinto, MD, author of the study and an otolaryngologist and head and neck surgeon at University of Chicago Medicine. The smell test, he adds, allows doctors “to see, a little earlier, a sign that problems are happening.”

In the study, researchers visited the homes of more than 2,900 adults ages 57 to 85 to test how well they could recognize five different odors: peppermint, fish, orange, rose, and leather. Five years later, researchers followed up with the older adults to find out if any of them had gotten a diagnosis of dementia since taking the smell test.

Those who couldn’t identify at least four of the five odors on the test were twice as likely as others to have dementia 5 years later.

The lower their score on the smell test, the greater their odds of having dementia. This decline in memory and thinking skills comes in several forms, including Alzheimer’s disease, Parkinson’s, and Lewy body dementia, among others. 

Pinto’s is not the only study to link a waning sense of smell -- also known as olfactory function -- with the breakdown of other parts of the brain. In fact, it happens with many conditions, known as neurodegenerative diseases, in which brain health declines over time. Obesity, which can raise you odds of having Alzheimer’s and Parkinson’s disease, might also dull the sense of smell. Studies show that exercise lowers the chance of losing this sense. But what is the connection between how well you detect and identify smells and your overall health? Doctors and scientists are trying to sniff out the answer to this crucial question.

The Nose Is a Window to the Brain

Your nose may provide a direct path for harmful substances from the environment to reach your brain. “Your olfactory nerve is sitting out there sampling air,” says Pinto. “That’s what it’s supposed to do, but it’s at risk for viruses, bacteria, whatever’s in your nose.”

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That’s especially risky since cells in the nose transmit directly to the brain. Neurons capture odors and send signals to the smell center at the base of the brain, known as the olfactory bulb. The signals then go to different areas throughout the brain, says Marie-Elyse Lafaille-Magnan, who researches the link between the sense of smell and the odds of Alzheimer’s at McGill University in Montreal.

Some of the areas the olfactory bulb transmits signals to are related to thinking and memory. 

In fact, on autopsy, researchers have found brain tangles associated with Alzheimer’s disease, known as tau, in the olfactory bulbs of people who had Alzheimer’s, Parkinson’s, and other forms of dementia linked to smell loss.

The nose’s direct line from the environment to the brain could mean that pollutants, viruses, and bacteria travel through the nasal passages to set the wheels of brain disease in motion. Some viruses, which researchers consider a possible cause of Parkinson’s disease, could reach the brain through the nose. Studies show that children and young adults who live in areas with heavy air pollution, such as Mexico City, have brain inflammation and buildup in their brains of some of the same proteins seen in older adults who have Alzheimer’s or Parkinson’s. 

These tiny particles that get into the brain through the nose may actually start the disease process, says Richard Doty, PhD, director of the Smell and Taste Center at the University of Pennsylvania Perelman School of Medicine.

It has already been shown that people who carry the APOE e4 gene mutation have a higher chance of getting Alzheimer’s. And notably, this study suggests that these people have an even higher chance of Alzheimer’s if they also live in a highly polluted area.

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