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The Gut-Brain Axis: Connection Between Our Gut and Our Mental Health

Our guide to the mind-gut connection aims to help people understand the complex relationship between our gut and our brain.

Woman tasting a smoothie

The gut does more than just digest food, it also plays an important role in the gut-brain axis which research is showing has an impact on our mental health.

The gut-brain axis is a complex system that describes how the gastrointestinal system and brain are connected both physically and biochemically.

Mental health issues such as depression and anxiety are common in the UK. It is reported by Mind that 1 in 4 people, or 25%, will experience a mental health problem each year.

We created this guide to help you understand the connection between the gut and brain; and why eating a healthy, balanced diet is just as important for mental health as it is for physical health.

How Are the Gut and the Brain Connected?

The gut-brain axis is made up of the nervous system, endocrine system, immune system as well as neurotransmitters, and gut microbes.  We take a look at each of these to understand how the gut interacts with the brain and how this can impact our emotions.

Gut-Brain Connection

The gut and the brain are connected by the central nervous system, localised enteric nervous system which is the nervous system of the gut - often referred to as the ‘second brain’ - and the autonomic nervous system.

The autonomic nervous system is divided into the sympathetic nervous system best known for creating our ‘fight or flight’ response and the parasympathetic nervous system responsible for relaxing the body.

The vagus nerve also makes up the gut-brain axis as it runs from the brain to the stomach and represents the main component of the parasympathetic nervous system.

The vagus nerve is the longest and most widely distributed nerve within the body with nerve branches throughout all the main organs in the body including the stomach.  The vagus nerve oversees a vast array of crucial bodily functions, including control of mood, immune response, digestion, and heart rate.

Some research suggests stress can inhibit the vagus nerve from functioning optimally. Other research has focused on vagus nerve stimulation to support mental health conditions.[1]

Hormones

Another key element of the gut-brain connection is the endocrine system which is made up of the thyroid gland, adrenal gland, and pituitary gland. It is responsible for making hormones that control mood, growth, development, metabolism, and reproduction. It also controls the pattern in which hormones are released.

The interplay between the hypothalamus (small region at the base of the brain) pituitary gland and adrenal gland - known as the HPA axis - coordinates the body’s response to stress including the emotional centres in the brain's limbic system.

The gut-brain axis involves complex two-way communication between the HPA axis, the immune system, and the autonomic nervous system.

Gut Microbes & Neurotransmitters

There are over 100 trillion microbes in the gut, which support the body in a vast range of functions, including the production of neurotransmitters and short-chain fatty acids which affect brain function in various ways such as mood and reducing appetite.

There is increasing evidence[2] that gut microbes can have an impact on the brain and therefore mental health issues such as anxiety and depression.

Neurotransmitters are produced by the brain and the rest of the nervous system, together with the gut, and are chemical messengers that transmit messages between nerve cells or from nerve cells to muscles.

There are an estimated 100 billion neurons in the brain and 500 million neurons in the gut.

Neurotransmitters help control feelings and emotions.  The main neurotransmitters that affect our mood are:

Serotonin

Serotonin is associated with improved mood and feeling happy, helping to relieve depression and anxiety. It is the precursor to melatonin, which helps support circadian rhythm, which regulates wake and sleep cycles. The gut produces 95% of the serotonin in the body.

GABA

GABA (gamma-aminobutyric acid) which is another neurotransmitter produced by gut microbes and also produced in the brain. GABA contributes to reducing anxiety, stress, and fear.

Dopamine

Dopamine is known as the ‘feel-good’ neurotransmitter, associated with reward and pleasure. It also helps to regulate the production of insulin in the pancreas. 

Epinephrine

Epinephrine is best known as adrenaline. It prepares your body for ‘fight-or-flight’ situations by elevating heart rate and stimulating glucose production. 

Immune System

Gut microbes also play an important role in our immune system and inflammation within the body. Infection and your body’s immune response to infection can lead to inflammation.

It is now understood that the central nervous system can show signs of inflammation in response to injury, infection, or disease. There is now increasing interest[3] in the role that inflammation in the central nervous system plays in some psychiatric disorders such as depression, anxiety, and schizophrenia.

Why Is the Gut-Brain Axis Important in Mental Health?

The gut and the brain are closely interlinked, and the gut-brain axis helps maintain a stable internal environment to support normal bodily functions.  If the axis is knocked off balance it can cause physical as well as mental health problems.

The gut-brain axis links the areas of the brain relating to emotional and cognitive functions to the gastrointestinal system. To support a healthy, balanced gut ecosystem (eubiosis) the digestive system has to maintain a stable, self-regulating environment.

When the gut microbiome ecosystem is not in a healthy balance (dysbiosis), bacteria can release inflammatory toxins such as lipopolysaccharide. High levels of this toxin are linked to depression.[4]

In addition, short-chain fatty acids such as butyric acid, propionic acid, and acetic acid are bacterial metabolites that stimulate the sympathetic nervous system and influence serotonin release. If serotonin levels are low it can affect the production of melatonin and as a result, impact sleep.  Low serotonin levels are also associated with depression.[4] 

Stress

The pituitary gland and adrenal gland play key roles in how the body responds to stress. These glands interact through the HPA axis, which coordinates the body’s response to stress. 

To respond to stress, the hypothalamus or the ‘command centre’ in the brain, stimulates the sympathetic nervous system branch of the autonomic nervous system. The autonomic nervous system carries this stress signal to the adrenal glands, which release adrenaline.

The HPA axis helps the body sustain a stress response. To do this, the pituitary gland releases an adrenocorticotropic hormone, which then prompts the adrenal glands to release cortisol.

Cortisol along with adrenaline prepares the body for the ‘fight or flight’ response by triggering rapid heart rate and breathing, and also affects mood.

Some studies have shown that people who release high amounts of cortisol in response to stress - high reactors - have a greater snack intake in relation to stress than those who are low reactors. Therefore, stress and cortisol may play a role in weight gain and obesity.[5]

In addition, sustained elevated cortisol levels can cause the immune system to resist the hormone’s effects, and increase the production of cells that promote inflammation. Because cortisol is stimulated by the sympathetic nervous system, neurotransmitters released in the autonomic nervous system can bind to immune cells, which affect immune response. 

Having consistently high cortisol levels can also interfere with sleep. Under a normal sleep pattern cortisol release peaks when we wake up in the morning and reduces throughout the day.  Elevated cortisol released throughout the day can lead to light sleep or sleep disturbances and poor sleep quality is linked to an increase in anxiety and depression.

When we are stressed or anxious, we can often lose our appetite or feel sick and not want to eat or perhaps eat foods that are high in fat or sugar. Stress also interferes with our gut causing digestion to slow down or speed up, which results in bloating, loss of appetite, constipation, or diarrhoea. While research is still emerging, stress is thought to exacerbate symptoms of IBS and IBD.

Some healthcare professionals support patients with gastrointestinal disorders with stress reduction techniques to manage symptoms. This can include using psychological therapies alone, or alongside other treatments.

Examples of psychological therapies used for gastrointestinal disorders are:

  • Cognitive behavioural therapy (CBT)
  • Relaxation therapy
  • Medical hypnotherapy specific to the gastrointestinal system

Nutrition and Mental Health

The gut-brain axis highlights the importance of good nutrition when it comes to mental health.  Having a healthy diet will help support a healthy, balanced gut ecosystem and in turn, support the gut-brain axis and mental health.

Certain food groups help improve our mental health including omega-3, foods that are high in fibre, foods rich in polyphenols such as olive oil, green tea, and cocoa, and foods rich in the amino acid tryptophan which is converted into serotonin.  Eggs, cheese, and turkey all contain tryptophan.

Other nutrients that are proving key to good mental health include iron, vitamin D, B vitamins particularly B12 and B9 (folate), zinc, and magnesium.

Conclusion

The gut-brain axis intricately connects digestive and mental health. It is a relatively new field of science and one that researchers are still understanding. This has implications for the role of nutrition, psychological therapy, and stress reduction techniques to support mental health and gastrointestinal disorders.

References

  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4176918/ 
  2. https://www.nature.com/articles/nrn3346
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1760754/
  4. https://www.healthline.com/nutrition/gut-brain-connection
  5. Newman, E., O’Connor, D, B and Conner, M. (2007). Daily Hassles and Eating Behaviour: The Role of Cortisol Reactivity Status. Psychoendocrinology: 32, pp 125-132.
Medically Reviewed
Dr Nicky Keay
Chief Medical Officer, BA, MA (Cantab), MB, BChir, MRCP.​
This article has been medically reviewed by Forth's Chief Medical Officer, Dr Nicky Keay.
Nicky has extensive clinical and research experience in the fields of endocrinology and sport and exercise medicine. Nicky is a member of the Royal College of Physicians, Honorary Fellow in the Department of Sport and Exercise Sciences at Durham University and former Research Fellow at St. Thomas’ Hospital.

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