Most people are aware of the fact that their mental state can have a real and profound impact on their gut and its daily functioning. This connection tends to be most noticeable during times of stress and is manifested in symptoms like nausea or cramping. Less appreciated is the fact that this relationship works in the reverse direction as well. That is to say, the general health and functioning of your gut can have a direct impact on your mood and well-being. While there is still much unknown about this complex relationship, some of what is known will be discussed below.
It is first important to understand the anatomical basis by which the gut and brain communicate and thereby influence one another. A fundamental way by which the gut "talks" to the brain is through its own complex nervous system, called the enteric nervous system (ENS). The ENS is constantly active and amazingly, most of its functioning is completely independent of input from the central nervous system, involving coordination of the day-to-day digestive functions necessary for survival. Besides managing digestion, the ENS constantly sends information to the brain via the vagus nerve, which provides a direct line of communication between the two. What sort of information does the ENS share? It turns out your gut has a vast network of receptors to sense everything from the type of food you last ate, to the types of bacteria living in your gut and if there are foreign invaders present. The ENS also relays information about the gut reactions you may have to daily situations, essentially acting as a mirror for your emotional state. In this way your brain is constantly monitoring the activity of your gut and thus learning about your health and mood, even though you are most often unaware of it.
Many of those suffering from depression and anxiety have heard of serotonin in the context of medications that alter its impact in the brain. However, most people are not aware that the majority of the body's serotonin (over 90%) is actually located in the gut. In fact, specialized cells in the gut, called enterochromaffin cells, are constantly releasing serotonin to coordinate the activities of your GI tract. However, it is now believed that some of this serotonin is also feeding back on the brain via the vagus nerve and thereby affecting your general mood. Research has shown that eating foods which promote serotonin production, specifically complex carbohydrates, can improve mood. This may explain why some women crave carbohydrates during the mood fluctuations experienced before their menses and why research has shown that dieters can experience a worsened mood when limiting their carbohydrate intake.
Your gut can affect your mood in more indirect ways as well. For example, an intestinal wall compromised by a pro-inflammatory diet, stress, alcohol, or certain medications (including NSAIDs and antibiotics) becomes inappropriately permeable. When your intestinal lining is too permeable ("leaky"), it allows access of inflammation-causing food and bacterial components deeper into the gut wall where they stimulate the immune system. This causes the release of inflammatory mediators called cytokines, which enter the general bloodstream eventually making their way to your brain. Consider if one is consistently assaulting their gut lining with one or more of the above listed factors; one's brain would be nearly constantly bathed in these inflammatory mediators. It is now believed that this chronic, low-grade brain inflammation plays a significant role in depression. In fact, many studies have shown that decreasing pro-inflammatory foods such as meat and increasing anti-inflammatory foods such as leafy greens in one's diet can relieve symptoms of depression.
A discussion about the gut-brain axis is not complete without considering the role of the microbiome. Consisting of the trillions of bacteria and other microbes that live (mostly) in our large intestine, the microbiome plays a large role in our mental and overall well-being. These organisms regularly produce metabolites including hormones and neurotransmitters. For example, there are bacteria known to produce and regulate levels of the neurotransmitters serotonin, dopamine, and GABA, all of which affect mood. When exposed to fiber inside the colon, some of these bacteria produce something called butyrate, a neuroactive short-chain fatty acid which has been linked to decreased anxiety and depression. Furthermore, some bacteria directly stimulate the vagus nerve, thus having a direct line to your central nervous system.
Just as certain types of bacteria can have a positive effect on your mood, the wrong types of bacteria can cause problems. An example of this is gram-negative bacteria, which are present in animal foods and more numerous in the microbiomes of people who habitually consume foods high in animal fat. These organisms contain something called endotoxins, or more specifically lipopolysaccharides, as part of their cellular wall, which stimulate the immune system located in the intestinal wall. The resulting immune response causes increased intestinal permeability and, as these inflammatory mediators make their way into systemic circulation, generalized inflammation called endotoxemia. Unfortunately, if you are regularly eating foods that naturally contain or promote the growth of these bacteria, you are regularly provoking your immune system leading to a chronic, low-grade inflammation which, as discussed above, promotes depression.
The good news is that you have control over many of the factors discussed above and therefore can have a meaningful impact on your mood and well-being. Perhaps most important is the type of foods your GI tract is interfacing with on a daily basis. Eating foods high in complex carbohydrates (whole plant foods like sweet potatoes, beans, and oatmeal) supports the beneficial and mental health-promoting bacteria that require fiber for survival. Eating foods that are anti-inflammatory, particularly fruits and vegetables, can quench the inflammation produced in our bodies on a day-to-day basis. In addition, limiting animal products will minimize the gut-harming effects induced by their saturated fat and bacterial toxins. (See "Everyday Habits That May Be Damaging Your Gut" to learn more about protecting this very important health mediator.)
It is also important to manage stress. Excess stress sets up a vicious cycle in which your mental state negatively affects your gut, which in turn negatively affects your mood. Two ways stress can negatively affect your gut include by decreasing the amounts of beneficial bacteria in your large intestine and by increasing the permeability of your intestinal lining. Healthy stress management techniques can break this cycle and may include regular exercise, getting adequate sleep, and mindfulness practices like meditation.
Rather than being shrouded in mystery, as mental illness is often portrayed to be, it is time to recognize that our mental health, or lack thereof, is often an extension of our physical health and thus many of the actionable steps we can take to improve our physical health will likewise promote our mental health. While suffering with a mental health issue such as anxiety or depression can feel hopeless, the above information demonstrates that one can have a significant impact on their mental well-being by making simple and healthy choices on a day-to-day basis.