If you’re human and you’ve been on earth the past two years, you’ve likely had periods of feeling down. You're not alone. A study out of Brown University, found that rates of depression tripled during the first year of COVID.
The study found that 27.8% of adults experienced depressive symptoms in early pandemic months, compared to 32.3 in 2021 and 8.3% before the pandemic. The depression disproportionately impacted lower income households and single mothers. The rates were related to the number of stressors experienced.
And now with recent events in the Ukraine, life feels tenuous.
While we can't control the giant outside stressors, we can take steps to mitigate the impact of stress on our bodies and minds.
For some time I’ve been fascinated with the connection between food and mood. The brain influences our perceptions and responses to stress. The stress response impacts on our mood and how we digest our food and the environment in our gastrointestinal (GI) tract. The GI tract microbiota is a delicate balance of beneficial bacteria that keep out harmful bacteria. Problems with gut bacteria can lead to leaky gut problems.
It’s phenomenal to see evidence showing the relationship is bi-directional, meaning your brain influences your gut and your gut influences your brain.
This relationship is powerful. By influencing your response to stress you can impact your gut health. And in turn, a healthy gut encourages a healthy mind.
In this post, I'll be your guide as we navigate a safari into the hypothalamic-gut-brain axis. We’ll learn how it impacts stress, our mood and our gut health. I’ll touch on the foods and habits you can put into place now.
I promise to keep things simple and as clear.
The Master Regulator: Hypothalamic Pituitary Adrenal Axis
Our central nervous system controls how we perceive the world and then how we react to external stimuli. The frontal cortex of the brain perceives stress and sends signals to the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS).
The hypothalamus is a master regulator, receiving environmental inputs from sources like photo neural cells in the eyes for circadian rhythms, stress signals, sexual signals and more. We are often unaware of background sensory mechanisms as they search for danger. When our limbic system perceives danger, it sends stress signals to the anterior pituitary gland, which then sends signals to the adrenals to make cortisol. At the same time, the sympathetic nervous system sends signals to the adrenal medulla, stimulating the release of epinephrine and norepinephrine (adrenaline).
Stress And The Gut
The HPA and the sympathetic nervous system impact gut function by increasing inflammation in the colon, decreasing mucus and surface cell integrity, and altering the gut microbiota in favor of unhelpful bacteria. Together, this increases output to a branch of a pathway which increases neuro-inflammation inducing depressive and anxiety-like behaviors (reference here).
Stress-induced activation of the SNS also induces changes in gut bacteria. In vitro studies show a log-fold increase in growth of gram-negative bacteria when exposed to noradrenaline. Gram negative bacteria are the “bad” gut bacteria, opposed to favorable gut bacteria like lactobacillus. In addition, the SNS impacts the enteric nervous system, the nervous system of the gut, causing decreases in motility and increases in inflammation.
This sounds scary. But we have a very big defense mechanism against unchecked inflammation: the vagus nerve.
The Vagus Nerve
The Vagus nerve is classified as the tenth cranial nerve, yet it wanders down to innervate a diverse array of organs, including our throat (uvula), small amounts of taste, smooth muscle in the trachea and bronchi, heart, stomach and intestines. Its name means “wanderer” like a vagabond.
In contrast to the HPA axis, the vagus nerve is the primary actor for the parasympathetic nervous system. It has many functions: slowing down the heart rate, increasing gut motility, secretions and decreasing inflammation. In the gut the vagus strengthens intestinal barrier function.
Stress stops the vagus nerve. This allows the sympathetic nervous system to inflame the gut.
Compromised intestinal integrity brought about by stress has a detrimental impact on the gut flora. This can allow bacteria to enter circulation, leading to systemic inflammation with further negative consequences for brain health. Afferent fibers of the vagus nerve send signals to the brain, relaying the status of the intestinal environment and regulating the HPA axis.
Most of the enteric nervous system input comes from the vagus nerve. Chronic stress inhibits the anti-inflammatory potential of the vagus nerve – increasing pro-inflammatory molecules (cytokines) promoting neuro-inflammation (reference here).
Serotonin and Your Gut
One of the major pathways of depression involves the neurotransmitter serotonin. You’ve undoubtedly heard of prozac. Antidepressants work by increasing the amount of serotonin available to the brain. Inflammation and disruption of the serotonin pathway in the gut is a major cause of depression. The gut has a major effect on the amount of serotonin synthesized by specific enzymes that convert the precursor tryptophan to serotonin. Chronic stress and inflammation dysregulates the production of serotonin in the gut.
Impact of Gut on the Brain: Intestinal Barrier
Several studies have shown inflammation in the gut leads to “leaky gut” allowing bacteria to enter the body. Our immune system recognizes the invader and sets off inflammation signals. Stress and the gut microbiota can regulate the permeability of the BBB, these inflammatory cytokines may migrate to the brain and activate immune cells of the brain. This also leads to neuro inflammation and may contribute to depression.
The pathway is bidirectional: stress signals from our brain influence our gut and our gut influences our brain. We can improve the health of both our brain and our gut by optimizing our diet and putting the vagus nerve in charge.
Diets For Gut Health
Diets high in fiber and nutrients and low in refined carbohydrates and saturated fats have positive effects on mood and may prevent the onset of depression. To improve the health of your microbiome, include fiber from fruits and vegetables and whole grains. Anti-inflammatory fats from omega-3 fatty acids also improve gut health. Choose walnuts, flax and oily fish like salmon or sardines.
Diets high in these gut friendly foods improve stress, anxiety and depression. One study found that
“Habitual diets rich in dietary fiber and omega-3-polyunsaturated fatty acids may be linked to reduced risk of developing symptoms of depression, anxiety, and stress;” Reference here.
In addition to diet, specific exercises make your vagus nerve more dominant than your SNS.
How to Put the Vagus In Charge
The vagus nerve can calm our mind and our gut. Promoting dominance of the vagus nerve decreases inflammation in our gut and calms our minds. Calming our minds will decrease the signals of stress from the hypothalamus to the pituitary and the adrenals.
This is the tip of the iceberg. In future posts we’ll explore ways to empower the vagus nerve.
Let's Flip the Script on Stress
Though the story of how your brain and gut conspire to cause inflammation can seem scary, we can flip the script. The cool thing about this connection is that we have so many tools to calm our mind and put the vagus in charge. You don’t have to try a crazy diet. Instead you can start with a couple minutes of deep breathing each day. When you notice your stress rising, realize that YOU HAVE A CHOICE in HOW YOU REACT. You can’t control your environmental inputs, but you can control your reaction.
If you’d like a method to boost your health check out my 4-Module Course.