The Gut-Brain Connection and the Link to Depression

The gut is increasingly being referred to as our “second brain”. There are over 500 million neurons in our intestines, which connect to the brain via the vagus nerve.  If you have ever felt butterflies in your stomach or experienced nausea or other digestive issues when anxious, you likely have some idea of how thoughts and emotions can impact the gut. Simply thinking about eating triggers the release of digestive enzymes in our gastrointestinal system. However, the vagus nerve is a 2-way street. In fact, research shows that approximately 80% of the neurons in the vagus nerve are afferent, meaning that they travel from the gut to the brain. 

It makes sense then, that just as thoughts and emotions in our brain impact our gut, our gut also impacts our brain. Research in this area has found a direct link between dysbiosis of the microbes in the gut and depression, along with a long list of other conditions that impact the brain. Unfortunately, because this is a relatively new and emerging area of study, it is not something traditionally addressed by most physicians.  

The Gut Microbiome

There are over 35 trillion micro-organisms (mostly bacteria) living in our gastrointestinal tract, and they make up around 5 pounds of our body weight!  The type and diversity of these microbes is based upon a variety of factors, including our diet, medications, genes, sleep, stress, exercise and toxins, and they can be beneficial or detrimental to overall health. A healthy and diverse gut microbiome produces and responds to over 30 neurotransmitters, including serotonin, dopamine, GABA and norepinephrine. In fact, more than 90% of serotonin, the happiness hormone most antidepressants directly work to increase, is metabolized in the gut. Thus, ensuring a healthy gut microbiome is extremely important, particularly for individuals with depression. 

Good Gut Bacteria

While bacteria are most commonly thought of as bad, most of the bacteria in our body are actually beneficial to our health. However, our digestive system, particularly our large intestine, has both beneficial and harmful bacteria. Ideally, we want a wide diversity of beneficial bacteria in our gut for optimal health. The primary determinant of the type and diversity of bacteria in our gut is the foods we consume. The beneficial gut bacteria thrive off of unprocessed, fibre-rich plants. Fibre rich plants contain prebiotics, which serve as fuel for the healthy gut bacteria, causing them to thrive. Furthermore, different types of bacteria prefer different types of plants, making the diversity of plants consumed important as well. When good bacteria flourish, they crowd out the bad bacteria. Other factors that have been found to increase healthy gut bacteria include fermented foods (such as sauerkraut and tempeh), regular exercise, adequate sleep and managed stress.

When the healthy gut bacteria ferment prebiotic foods they produce postbiotics, which is a category of health promoting compounds such as short chain fatty acids. Short chain fatty acids help to reduce inflammation by maintaining tight junctions in the intestinal wall so pathogens, toxins, and undigested food cannot enter the bloodstream. They are also an energy source for the cells in the intestinal wall, increase absorption of some minerals, and inhibit the growth of bad bacteria. A healthy gut microbiome is associated with a strong immune system, improved mood, and better digestion. 

Gut Dysbiosis

An altered gut microbiome (often referred to as gut dysbiosis), is one that is populated by predominantly unhealthy bacteria, and has a low number and diversity of beneficial bacteria. Foods that promote the growth of unhealthy gut bacteria include processed foods, refined sugar and grains, industrial seed oils (such as sunflower and canola oil), meat (especially meats that are processed), dairy, MSG, and alcohol. This is not to say you must avoid all of these, however, they should not make up a bulk of your diet. Frequent antibiotic use, stress, lack of sleep, smoking and a sedentary lifestyle also contribute to dysbiosis of the gut. 

Gut dysbiosis leads to increased intestinal permeability where the junction between cell walls of the colon permit the passage of endotoxins (produced by harmful bacteria), food particles and bacteria, which enter the bloodstream and trigger systemic inflammation. It also impacts the production of serotonin and other neurotransmitters which contributes to digestives issues and chronic health problems such as depression.  

Optimizing Gut Health

The best way to promote a healthy gut microbiome and optimize the gut-brain connection is to consume a plant predominant diet focusing on fruit (particularly berries), fermented foods, greens, whole grains, omega-3 rich seeds, aromatics (onions and garlic), legumes, and sulforaphane (found in cruciferous vegetables, particularly broccoli sprouts).  As well, it is important to ensure adequate hydration, sleep, time in nature, socialization and exercise, and to learn to manage stress and avoid inflammatory foods. 

Although the exact mechanisms of how the gut impacts depression are not all fully understood, the link is well established and research has found that improving gut health through diet and lifestyle modifications can result in statically significant improvements in symptoms. 

For more information on gut health, the book Fibre Fueled by Dr. Will Bulsiewicz is a great reference.  If you are looking for guidance to manage your depression and/or anxiety through evidence-based diet and lifestyle modifications, book a free consultation with Christy De Jaegher to learn more.