The Nose-Brain Axis: How Smell Shapes Mood and Memory

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“Smell is a potent wizard that transports you across thousands of miles and all the years you have lived.”—Helen Keller

A sense of smell is underrated. While we exalt our vision and hearing, smell seems like an optional add-on. But when we sit down to a meal, although we appreciate the sizzle of a fajita or the sight of a beautiful fruit plate, it is the heavenly smell of good food that lifts our mood. Smell brings out the hedonist in us all.

So it’s shocking that almost 20 percent of the population has a poor or non-existent sense of smell. This has an outsized impact on our mood, enjoyment of food, hygiene, social interactions, and even our sex life. Worse yet, it can affect our lifespan. A Swedish study found that olfactory dysfunction—not heart failure, stroke, liver damage, or cancer—is the best predictor of five-year mortality in older adults.

Smell is a deeply primeval sense. Even microbes, which don’t have noses, eyes, or ears, have a way to sniff their environment so they can head away from danger and toward a tasty treat.

As evolution crafted more complicated animals, the sense of smell remained central. The same basic olfactory system is remarkably similar in insects, reptiles, fish, and mammals. It takes a lot of machinery to run the system, with over 400 genes for the various scent receptors. Different odors activate unique combinations of these receptors, yielding a trillion distinct, discernible scents.

Each scent is relayed to the olfactory bulb, and after checking through our cataloged memories of smells, it determines if the scent is good or bad. It does so quickly, because getting away from a stinky situation can be critical to staying healthy and alive.

A bad smell can dial up your immune system, getting it ready to deal with potential toxins. And once you crank up the immune system, you start to stress. An overstimulated immune system can put pressure on gut microbes, potentially changing the composition of the microbiome.

Conversely, pleasant smells like flowers or food can lower inflammation and reduce stress. They can stimulate the release of endorphin and serotonin, improving mood. This may be the scientific basis of aromatherapy.

The brain connection

One of the fastest, tightest connections between any of your senses and your brain is from the nose to the olfactory bulb. But that pathway can lose function if you don’t maintain it. When it does, you lose more than smell. Your cognitive abilities also start to drop.

There is a strong connection between the loss of smell and the onset of dementia and Alzheimer’s. Olfactory tests are better predictors of impending cognitive decline than mental tests.

Scents are highly emotive and capable of evoking memories more poignantly than any other sense. Marcel Proust was able to fill an entire book with memories brought back by the smell of a madeleine cake. The loss of smell, known as anosmia, interferes with emotional control centers. People with anosmia feel cut off from the world, with deadened emotions. It can lead to loneliness and depression.

The neurons that store memories in your brain are like real estate: There’s only a limited amount available. Your brain may occasionally need to sweep out old memories to make room for new ones. A neuron that isn’t used often is a perfect target for erasure and recycling. Use it or lose it. And so, it behooves us to stimulate and rejuvenate the olfactory nerves with a little concerted sniffing. Smelling the roses can be good medical advice.

When you do this over time, your olfactory bulb increases in size and so does your hippocampus, where memories are first encoded. Surprisingly, it can even improve your verbal memory.

Some researchers keep a few vials, say of cloves, rosemary, or lemon, at their bedside for a morning snort to light up their olfactory bulb before starting the day. They say it’s like going from a grayscale world to full color. Try a little bedside potpourri to keep your sniffer in top shape.

The microbe connection

There is a unique microbiome for every part of your body that has mucus, and your nose is typically the first place you think of in this regard. Microbes love mucus. Your nose microbes are similar to the ones in your gut, and in fact millions of them make the trip from your nose to your gut every time you swallow.

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But some microbes are not welcome in the nose. Although we know that butyrate-producing bacteria like Faecalibacterium are good for your gut, they may wreak havoc on your sense of smell. That’s because butyrate, for all its benefits, stinks like rancid butter, and it may crowd out other smells.

Oral bacteria like Porphyromonas can migrate to your nose and also ruin your sense of smell. This bacterium is bad in your mouth and your nose, so better brushing may help you smell better in two ways. Bad nasal bacteria can be catastrophic, and a recent study found that they contribute to a breakdown of the blood-brain barrier, leading to major depressive disorder.

It isn’t clear why, but diet seems to affect your sense of smell as well. People with higher meat intake are more likely to suffer from dysosmia—an altered sense of smell—than vegetarians. This is yet another bonus of a Mediterranean-style diet with lots of veggies and less meat than the traditional American diet.

Although there are many connections between your nose and your gut, it is more difficult to modify your nose microbes because you can’t feed them. There are currently no good medical remedies for dysosmia. For now, your best bet is to keep smelling the roses.