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Nose-brain connection: The surprising link between allergies and mental health revealed

allergy & immunology May 30, 2023
Rear view of a person in a beige hoodie standing amidst lush greenery, with hands gently cradling their head, against a backdrop of vibrant yellow blossoms on a sunny day.

Artificially separating the body from the mind has been one of the biggest missteps of modern medicine. Over the last few years, we have increasingly read studies confirming the mind-body connection, the gut-brain axis, and now we are learning more about the nose-brain connection.

Given the direct proximity between our nose and our brain, this connection shouldn’t be much of a surprise. The olfactory nerve, cranial nerve 1, wires directly to the amygdala, hippocampus, and orbitofrontal cortex – our emotional centers. Smells, our most ancient sense phylogenetically, have been a critical part of our ability to sense threats, perceive rewards, and maintain homeostasis.

Over the years, within the field of allergy/immunology, we have anecdotally noticed the toll that living with chronic congestion, nasal obstruction, loss of smell and taste, and headaches takes on our patients. Missed work, foggy thinking, and interrupted sleep all take their toll. With allergies affecting between 10 to 30 percent of U.S. adults and costing upwards of $3 to 4 billion per year, allergic rhinoconjunctivitis has a profound impact on society. Studies have long confirmed our suspicions that anxiety and depression occur at higher rates among allergic patients, and we see a similar phenomenon in autoimmune conditions like lupus, rheumatoid arthritis, and Sjogren’s disease.

For years, allergists have pondered the question: which comes first? Does the chronic symptom burden cause mental health symptoms? Is it an alteration in sleep quality? Is it something driven by another pathophysiologic mechanism? Do these patients share genetic quirks that predispose them to both?

As with most things in medicine, it isn’t straightforward but recently published studies have demonstrated that high levels of inflammatory mediators such as IL-1beta, IL-6, and TNF alpha can penetrate the blood-brain barrier and activate major brain regions regulating emotions, including those in the limbic system.

We also know that the hypothalamic pituitary adrenal (HPA) axis plays a significant role in the connection between our brain, hormones, and inflammation responses. The HPA axis plays a role in the generation of anxiety and depression in patients with chronic inflammation, including allergies. Allergic inflammation results in the production of excess glucocorticoids, which play a role in mood regulation in the hippocampus and limbic systems. Over time, this can result in hormone resistance and further dysregulation.

Not only can we see effects on our emotions, but we can also see changes in thinking speed, memory, and learning related to allergies as well. Additionally, some of the medications commonly used to treat allergies can compound this effect, such as 1st generation antihistamines, also known as “Benadryl brain”! This is why allergists are increasingly vocal about encouraging the use of second-generation antihistamines that have much lower anticholinergic side effects and do not cross the blood-brain barrier as readily.

I am frequently asked how one can “boost” or “balance” their immune system, and this begs the question: What can we do?

1. Severity of allergies correlates directly with increased rates of anxiety and depression. It is reasonable to postulate that treating symptoms before they get out of hand may lessen this risk. Allergen avoidance, intranasal corticosteroids, long-acting second-generation antihistamines, and allergen immunotherapy are the mainstays of allergy treatment.

2. Optimize other factors that increase the risk of allergies, anxiety, and depression, including sleep routines, nutrition, and exercise! Aiming for 7 to 9 hours of quality sleep, eating a diverse plant-forward diet, and getting 150 minutes of moderate activity weekly are all beneficial for our immune system health.

3. Increasingly, data supports the avoidance of substances that impair our epithelial barriers to prevent the development of allergic and autoimmune conditions, including harsh soaps and surfactants, indoor and outdoor air pollution, dishwasher rinse aids, and certain food additives.

With longer growing seasons, we are seeing record-high pollen counts again this year. Safe and effective treatment strategies are available that can significantly improve our quality of life in ways that we may not have even realized before.

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