Exploring the Gut-Mind Connection: A Healthy Gut Microbiome, Anxiety and Depression

According to the WHO 2022 statistics, mental health disorders are among the top ten leading causes of disease burden worldwide with one in every eight people suffering from them. Emerging research suggests that the gut microbiome could play a role in mental health. Researchers have discovered links between imbalances in the gut microbiome and conditions such as depression, anxiety, and schizophrenia. In this article, we dive deep into this fascinating connection and explore how a healthy diet can improve mental health.
Frederika Malichová

Frederika Malichová

Dementia Researcher at the University Of Cambridge.

An image showing a link between the gut and the brain.

What Is The Gut Microbiome?

The gut microbiome is a collection of microorganisms consisting of trillions of bacteria, fungi and other organisms residing in our digestive system. It is formed shortly after birth and of the many organisms, bacteria are the most abundant and diverse group within the gut microbiome [1].

This collection of organisms helps in the digestion and absorption of nutrients from our food, ensuring that we receive the necessary energy and building blocks for our body. It also produces essential vitamins and metabolizes dietary compounds that our body cannot process on its own [2].

Interestingly, the microorganisms in the gut microbiome don't only interact with each other, but they also interact with the host in various ways [1,3,4]. For example, the gut microbiome interacts with our immune system, helping to regulate and train it, and plays a role in maintaining a healthy gut barrier function [2,5]. It is a complex and diverse ecosystem that plays a crucial role in maintaining overall health [1,4].

Researchers are still only discovering the profound impact that the gut microbiome has on various processes in the body. In fact, this focus is part of a wider trend in medical research. We are slowly learning about surprising connections between seemingly unrelated parts of the human body. For example, we recently wrote a blog post about how poor dental hygiene may cause Alzheimer’s Disease. In this article, we will explore the fascinating connection between gut health and mental health, and analyze how you can improve your mental well-being by having a good diet.

Mental Health Disorders

Mental health disorders, also known as mental illnesses, are conditions that affect a person's thinking, feeling, mood, response to stress and overall behaviour. They can be occasional or long-lasting (chronic) and can significantly impact a person's ability to relate to others and function in their daily life.

There are many different types of mental disorders, including anxiety disorders, depression, bipolar disorder, eating disorders, personality disorders, post-traumatic stress disorder (PTSD), and psychotic disorders such as schizophrenia [6,7]. In this article, we will only look more closely at anxiety and depression.

Anxiety

Anxiety is a feeling of fear, dread, and uneasiness. It can be a normal reaction to stress, but for people with anxiety disorders, the fear is not temporary and can be overwhelming [8, 9]. Symptoms of anxiety can vary from person to person, but common symptoms include:

  1. Excessive worry or fear: Feeling constantly worried or fearful about everyday situations or events.
  2. Restlessness or feeling on edge
  3. Fatigue: Feeling tired or lacking energy, even without physical exertion.
  4. Difficulty concentrating: Having trouble focusing or experiencing mind going blank.
  5. Irritability
  6. Muscle tension: Having tense or tight muscles, often in the neck, shoulders, or jaw.
  7. Sleep disturbances
  8. Physical symptoms: Experiencing physical symptoms such as rapid heartbeat, sweating, trembling, shortness of breath, chest pain, dizziness, or gastrointestinal issues.

Depression

Depression is a serious medical illness that goes beyond feeling sad or "blue" for a few days. It is characterized by persistent feelings of sadness, loss of interest in activities, changes in appetite and sleep patterns, fatigue, feelings of hopelessness or guilt, and thoughts of death or suicide [10,11,12]. Symptoms of depression include:

  1. Persistent sadness or low mood
  2. Loss of interest or pleasure in activities once enjoyed
  3. Changes in appetite or weight (either increased or decreased)
  4. Sleep disturbances (insomnia or excessive sleeping)
  5. Fatigue or loss of energy: Feeling tired or lacking energy, even without physical exertion.
  6. Feelings of worthlessness or excessive guilt
  7. Difficulty concentrating or making decisions
  8. Restlessness or slowed movements
  9. Recurrent thoughts of death or suicide

To answer this question, we will look into the science. We will first consider studies conducted on animals, where researchers were able to establish a very strong link between the gut microbiome and mental health.

Animal studies

One way to examine the influence of the gut microbiome on mental health in animals is to use so-called germ-free models. These are animals with gut microbiomes completely free of microorganisms, making the gut microbiome practically non-existent. Researchers study the behaviour of these models and compare it to that of animals with a healthy microbiome.

A natural question to ask is how does one measure the mental health of animals? Since animals are not able to fill out well-being questionnaires, in practice, researchers look at the so-called "stress response" of the investigated organism. The reasoning behind that is that an impaired reaction to stress is a key symptom of mental illnesses, and this stress response is something that can be quantified.

For example, in 2014, a study conducted by Crumeyrolle-Arias et al. examined anxiety-like behaviour in rats. They confirmed that germ-free rats had an exacerbated response to stress (hypothalamic-pituitary-adrenal axis) [13], and discovered that the germ-free rats displayed a greater anxiety-like behaviour. This confirmed results from another study on germ-free mice [14].

Importantly, when the researchers improved the gut composition of the mice by inserting a Bifidobacterium species into the gut, the stress response of the mouse was restored to normal.

Another study looked at how the disruption of the gut microbiota affects mice. The researchers discovered that when they disturbed the gut microbiome of the mice, they observed depressive-like behaviour in the mice. When they subsequently treated the mice with a probiotic called Lactobacillus casei, the behaviour was reversed back to normal [15].

These studies show that the impaired response to stress, anxiety and depressive-like behaviour is heavily linked to the well-being of the gut microbiome. However, we must keep in mind, that the studies are only considering animal models, and the results do not directly extend to humans. Nevertheless, they validate the idea that the gut microbiome may affect seemingly unrelated processes in the body, even mental health.

Human studies

In human studies, researchers usually compare the gut microbiome of individuals suffering from mental health disorders to that of healthy individuals, or study the contribution of the gut microbiome diversity to the overall mood, happiness and well-being.

It turns out that the gut microbiome of patients with depression, anxiety, and other mood disorders is significantly different compared to healthy controls [16, 17].

For example, a study analysing stool samples from female patients with major depressive disorders detected specific differences in the composition of the gut microbiota. In particular, they detected that phylum Bacteroidetes, proteobacteria and Fusobacteria were increased in patients suffering from major depressive disorder. On the contrary, the phylum Firmicutes and Actinobacteria were decreased compared to healthy controls [17].

In another experiment, researchers placed three healthy adults into an isolated life support system for 105 days. Throughout the experiment, they altered the gut microbiome of the subjects and analyzed their mood. They discovered that the microbiome composition was strongly correlated with mood states. For example, bacteria such as Roseburia, Phascolarctobacterium, Lachnospira were correlated with positive mood, while Faecalibacterium, Bifidobacterium, Bacteroides, Parabacteroides, and Anaerostipes were correlated with negative mood [19]. Another correlation study of the gut microbiome and personality found that high neuroticism, a personality trait characterized by emotional instability, anxiety and self-doubt, was correlated with a high abundance of Gammaproteobacteria[20].

These studies, and many others, suggest that the gut-brain connection is indeed not a myth, and offers a fascinating angle for understanding mental health disorders. Even though more research is needed to develop treatments for mental illnesses using the gut microbiome, there is more and more evidence suggesting that having a healthy gut microbiome is absolutely crucial to leading a healthy and fulfilling life.

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How Can We Keep Our Gut Microbiome Healthy?

Many nutrition specialists recommend consuming probiotics for a healthy gut. These are live microorganisms such as bacteria or yeasts that are similar to the beneficial microorganisms naturally found in the human gut. Probiotics have various health benefits when consumed in adequate amounts, and they are believed to improve gut health.

These claims are backed by science. For example, a 2015 study examined whether regular intake of multispecies probiotics can affect our mood. 20 healthy participants received probiotic supplements over a 4-week period, after which they completed cognitive assessments to determine their mood. The study found that the participants receiving probiotics had significantly better results than the 20 patients receiving placebo. This suggests that probiotics supplementation can influence cognitive mechanisms involved in mood disorders [21].

Other studies found that supplementation with Lactobacillus rhamnosus may reduce anxiety and depressive symptoms [22]. In addition, supplementation with Bifidobacterium longum has also been associated with improvements in mood and a reduction in depressive symptoms [22, 23].

When examining the effects of probiotics supplementation in older adults, it has been determined that probiotics have a positive influence on their cognitive abilities as well as mental health [24]. However, a meta-analysis analysing clinical trials using probiotics in depression and anxiety proposed that the effects of the probiotic treatment are modest [25].

Where can we get probiotics in our normal diet? We can ask MediSearch for a quick overview:


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Conclusion

Nowadays, as the connection between healthy gut microbiome and mental health strengthens, strategies to improve the gut microbiome, such as dietary changes, probiotics, and prebiotics, are recommended. They have shown potential in improving mental health outcomes. Understanding the role of the gut microbiome may provide new approaches for preventing, treating, or reducing symptoms of mental illness. However, further research is needed to fully understand the complex relationship between the gut microbiome and mental health.

Disclaimer

This article does not offer health advice. Always consult a medical professional regarding your condition.

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Frederika Malichová

Frederika Malichová

Frederika is a postgraduate researcher at the University of Cambridge, where she investigates new biomarkers for Frontotemporal Dementia and other tauopathies. Her research has been published at prestigious conferences such as the Alzheimer’s Association International Conference 2023. She obtained her BSc in Biomedical Sciences from UCL, where she worked closely with the UK Dementia Research Institute.