The Link Between Gut Health and Mental Health

Laura Lewis, MS

Winter holidays are often a time of indulgence. For some of us it's the spread of treats at the company holiday party. For some of us it’s a comfort food binge when holiday overwhelm strikes or the season change triggers our low mood. Either way, our regular rules over healthy eating often get relaxed at the end of the year. Unfortunately, eating outside of our normal diet can wreak havoc on our gut health, and our gut health and mental health are tightly linked.  

Read on for a reminder about why moderating champagne, latkes, and Christmas cookies is so important to keeping your mood balanced this winter. 

Gut-Brain Axis 

The relationship between our gut bacteria, gut health, and mental health is often referred to as the gut-brain connection or gut-brain axis.  

One point of the axis is the enteric nervous system (ENS); a highly complex system of nerves found throughout the lining of the gut. The ENS generally communicates with the brain through the vagus nerve. 

An obvious example of this communication is triggered by the sight and/or smell of a meal. The parasympathetic nervous system (part of the central nervous system) sends signals to the ENS. These signals cause a release of digestive secretions in both the mouth and stomach. These signals also prepare muscles of the abdomen for the peristalsis that moves food through the digestive tract. 

In the other direction, when you are full from a meal, the gut sends signals to the brain to slow your body for rest, so that energy can be directed towards digestion. Unfortunately, sometimes this signaling system is thrown out of whack. This can cause uncomfortable symptoms of indigestion, missed opportunities for nutrient absorption, and the loss of important microbes whose secretions contribute to mood balance. 

Scientists are delving deeper into the relationship between the gut, gut bacteria, and mental health. It has become clear that the diversity and abundance of certain microbial populations living within the gastrointestinal (GI) tract have a significant influence on the brain. Because of this, a more accurate title for the phenomenon of gut-brain communication is the “microbiome-gut-brain axis”.  

The Influence of Gut Bacteria on Mental Health 

An imbalanced gut microbiome (a condition also known as “dysbiosis”) can cause physical and chemical changes in the brain. These changes affect behavior, mood, and cognition. 

Compelling evidence for such an interaction was published almost 30 years ago. It was observed that a course of oral antibiotics resulted in dramatic mental improvement in patients attributed to a decrease of toxins from the gut entering the circulation [1]. When antibiotics ended dysbiosis of the gut, a rapid change in neural effects soon followed. It demonstrated an important relationship between changing microbial populations of the GI and brain function. 

Since then, data surrounding gut bacteria and mental health have been collected showing a potential role of microbiota in influencing mental health [2]. Normal brain development is partially dependent on microbiotic presence in the gut [3]. Animal studies suggest that social development also requires a certain level of health and balance in the GI microbiome [4,5]. 

We still need more research to elucidate the mechanisms creating a relationship between gut bacteria and mental health. We do know that the gut produces about 90% of the body’s serotonin. Serotonin is a neurotransmitter that promotes a sense of well-being. Bacteria living in the GI can induce enteric secretion of serotonin, along with other signaling molecules (such as dopamine, glutamine, and GABA) and activation of their receptors [6]. 

The brains of research animals with altered or absent gut microbiota present various molecular differences including brain-region-specific changes in levels of brain-derived neurotrophic factor (BDNF), which is known to be modulated in certain mental health issues [7]. Due to this evidence, healing dysbiosis and caring for your microbiome through diet and the use of probiotic and prebiotic fiber supplements are likely effective measures to support your mental health in combination with your digestion.  

The Brain Influences Microbiota 

Conversely, psychological stress such as worry, fear, frustration, grief, and sadness can cause physiological changes that, in turn, alter the bacterial populations in our gut. Some gut problems, like stress-induced diarrhea, can flush out beneficial bacterial species too.  

As diet has the greatest effect on microbiotic health in the gut [8], eating more unhealthy comfort foods during times of stress can exacerbate dysbiotic conditions, worsening disrupted motility, reducing nutrient absorption, and even negatively affecting immune response. Over time these changes can lead to neurotransmitter imbalances and increased strain on our natural stress response. 

When we experience stress, the sympathetic nervous system responds by engaging the body’s “fight or flight” mode. This deactivates signals for digestion, rest, or relaxation, as these physiological functions are not relevant to self-defense or fleeing from an emergency. 

When stress becomes chronic, the digestive system becomes confused, and symptoms such as gas, bloating, nausea, reflux, or motility issues crop up. The cause of these symptoms, at least in part, are changes to the microbiome responding to stress [8]. Even events such as a social defeat or a reduction in social diversity have been documented to precede a change to GI microbial species known to affect neurotransmitter synthesis [8].  

Healing and Maintaining a Healthy Microbiome 

Knowing that the microorganisms living inside your gut can hurt or hinder your mental state along with your physical health is important. Caring for gut bacteria is caring for mental health. 

But it can be difficult to determine whether your unhealthy gut is the root of a low mood or occasional anxiety, or if your stress level is hurting the health of your gut. Either way, changing your diet and lifestyle to support healthy digestion will benefit your well being.  

Probiotics 

Animal studies involving microbiome manipulations have not only confirmed hypotheses about connections between gut populations and behavior, they also show supplementing with probiotics can correct some of these effects [7].  

Taking a daily probiotic supplement is an efficient way to start improving the diversity and abundance of beneficial microbial species in the GI. While many fermented foods contain probiotic species, it is difficult for those probiotics to reach and populate the gut due to enzymes and stomach acids. A high-quality capsulated supplement, by design, withstands early stages of digestion. Thus, it can deliver maximum colony forming units (CFUs) of specifically selected beneficial bacteria and yeast strains directly to the intestine. 

As there are only so many adherence sites for bacteria in the walls of the gut, it may take some time for the beneficial strains to take root and begin to multiply. As they do they compete with less beneficial species, which are slowly sloughed through natural bowel movement. This is why it is important to make a probiotic supplement part of your daily health regimen, rather than expecting one capsule to make a noticeable difference. 

You may start to notice a positive difference in areas like digestive comfort within as little as two days, but secondary benefits like mood may take a couple of months or more to notice a significant change [9,10]. Don’t give up! But do make sure you’re taking the right strains for your needs, a dose that is appropriate for your body condition, that your probiotic is from a trustworthy source, has not surpassed its expiration date, and that you are following that particular product's instructions along with advice from your healthcare practitioner.  

Prebiotics 

Another important piece to the probiotic puzzle is the amount of prebiotic fiber in your diet. As living organisms, probiotics need to eat. In order to survive and multiply, the microbial species that benefit the gut require oligosaccharides and fibers considered indigestible by the human gut. Too much sugar can drive the microbiome towards less beneficial species. Therefore, avoiding a high-sugar diet while taking a prebiotic supplement is an effective way to nurture the growth and maintenance of desired bacterial species aiding in their dominance over less helpful species.   

Food Intolerances and Sensitivities 

If adding probiotics and prebiotics to your daily health regimen does not change GI discomfort over time, you may benefit from a deeper analysis of your diet. While an allergic reaction to a food is likely to cause a histamine response such as a rash or sudden inflammation, a food sensitivity or intolerance is more likely to present as a GI issue (bloating, gas, diarrhea, etc.) and may be a bit more difficult to pinpoint. In some cases, the body isn’t producing enough natural enzymes for the abundance of certain food proteins one consumes. 

These proteins bypass digestion and ferment in the gut causing uncomfortable or inconvenient symptoms. Common intolerances include casein, lactose, and gluten. You can take a natural digestive aid containing enzymes that help break down these proteins (along with fats and other components). This may increase absorption of their nutritional value and avoid their fermentation (as well as its side effects). If the sensitivity is more serious, an elimination diet can help to determine the culprit so you know which foods to skip completely in the future. 

Leaky Gut 

Leaky gut is a different story. In order for absorption to occur, the intestinal wall acts as a sieve, allowing water and nutrients to move through the wall into the bloodstream. Toxins and undigested materials are too big to fit through, so they stay in the intestine, destined for excretion from the body. Sometimes, the holes in the intestinal wall (called “tight junctions”) loosen, allowing toxins into the bloodstream along with nutrients and water. This phenomenon is “leaky gut” or “intestinal hyperpermeability”. 

Maintaining mucosal integrity in the gut is important to overall health and maintaining normal inflammatory balance. Leaky gut may affect skin, immune, metabolic and brain health [11-14]. This can occur naturally, as some people's genetics predispose them to make more of the chemical zonulin, which loosens tight junctions. Gluten, however, can stimulate zonulin overproduction in some people who have an otherwise healthy gut, causing leaky gut syndrome [15].  

Gluten is not the only potential cause of leaky gut. There are certain bacteria that also stimulate the overproduction of zonulin, and alcohol and refined sugar have also been linked with the permeability of tight junctions [15-17]. If you think you may be suffering from a leaky gut, your doctor will likely recommend some combination of a low FODMAP diet, pre- and probiotics, and self-care behaviors to destress your system to heal your gut. 

Disclaimer:
The information provided is for educational purposes only. Consult your physician or healthcare practitioner if you have specific questions before instituting any changes in your daily lifestyle including changes in diet, exercise, and supplement use.

Laura Lewis is a clinical researcher and science communicator with a Master of Science degree and a background in nutrition science and integrative health. She brings a decade of experience translating complex clinical data into accessible insights for healthcare practitioners and patients alike.

Laura has contributed to educational programming and strategic content development across the integrative and functional medicine space. Her strengths lie in evidence synthesis, practitioner engagement, and crafting communications that bridge the gap between scientific rigor and real-world application. At ARG, Laura supports clinical content development and educational initiatives that elevate practitioner confidence and product credibility.