A growing body of research suggests that our olfactory function — how well we smell — is linked to our overall brain health, although researchers are just starting to figure out how and why.
Sonya Collins reports for WebMD, Oct. 11, 2017, that a recent study found that a simple smell test may predict your chance of having dementia.
Jayant Pinto, MD, author of the study and an otolaryngologist (head and neck surgeon) at University of Chicago Medicine, said: “Ability to smell is a window into parts of the brain related to core functions, like pleasure, emotion, and memory. The smell test 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: fish, leather, orange, peppermint and rose. Five years later, researchers followed up with the older adults to find out if any of them had been diagnosed with dementia since taking the smell test.
These are the findings:
- Those who couldn’t identify at least four of the five odors on the smell 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.
- The decline in memory and thinking skills comes in several forms, including Alzheimer’s disease, Parkinson’s, and Lewy body dementia, among others.
In fact, the loss of olfactory function is associated with not just dementia, but with other neurodegenerative diseases in which brain health declines over time. Obesity, in addition to raising the odds of a whole host of diseases — including Alzheimer’s, Parkinson’s, cancer and diabetes — may also dull the sense of smell. Studies show that exercise lowers the chance of losing our sense of smell.
But what is the connection between how well you detect and identify smells and your overall health?
Cells in the nose transmit signals directly to the brain — 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. Researchers have found, in autopsies, 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 your nose may provide a direct path for harmful substances, such as pollutants, viruses and bacteria to travel through the nasal passages to set the wheels of brain disease in motion, says Richard Doty, PhD, director of the Smell and Taste Center at the University of Pennsylvania Perelman School of Medicine. Dr. Pinto explains: “Your olfactory nerve is sitting out there sampling air. That’s what it’s supposed to do, but it’s at risk for viruses, bacteria, whatever’s in your nose.”
Viruses that 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.
It has already been shown that people who carry the APOE e4 gene mutation have a higher chance of getting Alzheimer’s. Dr. Pinto’s study suggests that these people have an even higher chance of Alzheimer’s if they also live in a highly polluted area.
Maybe it’s because the nose is so likely to be invaded by viruses and bacteria that olfactory cells can regenerate, even into old age, constantly repairing the sense of smell. Dr. Pinto says, “It’s possible that when that regenerative process peters out, when we get older, it’s a sign that [brain cells] can’t regenerate, and that reflects what’s going on centrally.”
That petering out of both sense of smell and thinking skills associated with dementia might result from damage to a single part of the brain called the basal nucleus of Meynert. This damage could be due to harmful substances reaching the brain through the nose, or it could be an unrelated process. Doty’s research found a strong connection between low scores on a 40-item smell test and damage to this area of the brain. His recent article in The Lancet outlines possible ways that sense of smell and overall brain health could be linked and “may be the genesis of many neurodegenerative diseases.”
In addition to dementia, a decline in the sense of smell may also weaken other systems because the nose is giving the brain less information. Pinto calls it the sensory deprivation hypothesis: As your hearing, vision and sense of smell decline, you’re getting less information. The circuits that usually bring this information in might then stop working as well, which can lead to problems elsewhere in the brain.
Weight seems to play a role, too. Obesity is known to make brain disease more likely because obese people have a weaker sense of smell than others. One possible explanation is that adipokines, chemicals given off by fat tissue, could weaken the sense of smell. The relationship could also be indirect: Weight gain can lead to diabetes and heart disease, which hurt blood flow, including to the brain, and vascular disease is a risk factor for dementia.
Research has found that regular exercise, which improves blood flow and burns fat, also helps the sense of smell in older adults.
But not everyone who loses some sense of smell goes on to develop dementia. Doty says, “As we go through life, the smell ability diminishes. Between the ages of 65 and 80, half the population has smell loss. After 85, three-quarters do.”
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