BY BENJAMIN MAKEHAM December 14, 2021
The digestive tract produces serotonin not for the brain to use, but to act locally and promote regular bowel movements. It’s now thought that an imbalance in this hormone may contribute to irritable bowel syndrome (IBS). In this blog article, we discuss the important role of serotonin for gut health, and how the gut microbiome may influence serotonin balance.
Serotonin is a neurotransmitter and a hormone that is well known for its ability to make people feel happy. In fact, many people have heard that over 90% of the body’s serotonin is produced in the gut and that’s why gut health is so important for mood and mental health. However, the serotonin produced in the gut is actually used by the gut (not the brain) and assists with the function of the digestive system.
While it is true that the serotonin released by nerve cells in the brain acts as a neurotransmitter, helping to regulate our mood, the serotonin produced in the gut (gut-derived serotonin) acts as a hormone and plays an important role in a wide variety of mechanical and sensory processes - in the gut (1).
Specialised cells in the lining of your digestive tract, called enterochromaffin (EC) cells, produce and release serotonin. When serotonin binds to receptors on nerve cells in the digestive tract, this helps to stimulate muscle fibres which regulate bowel movements and also transmit sensory signals that allow us to feel sensations such as pain (2).
Normal serotonin signalling in the gut prevents functional disorders like irritable bowel syndrome by ensuring that the smooth muscles contract as they should and create regular bowel movements as a result. It also ensures that pain is felt at an appropriate level.
Studies have found that people with IBS have disturbances in the way gut-derived serotonin is released and how long its effects last, which is thought to contribute to irregular and alternating bowel habits and a sensitive digestive tract that more easily feels pain (3).
One of the leading hypotheses that is attempting to explain why this happens suggests that abnormalities in the gut microbiome may be responsible (3).
The gut microbiome, and the metabolites that it produces, are heavily involved in the regulation of hormones and nerve cells in the gut and have been found to impact gut-derived serotonin. Short chain fatty acids, which are released by gut microbes, influence the rate of serotonin synthesis and release by EC cells and the gut microbiome has been shown to impact the clearance of serotonin from the nerve cells that it is acting on (3, 4).
When the gut microbiome is out of balance, which is often observed in patients with IBS, the balance of serotonin and its effects on gut function can be significantly altered and give rise to uncomfortable symptoms, such as diarrhoea or abdominal pain (3).
The effects of the gut microbiome in IBS may help to explain why some probiotic strains, which impact the gut microbiome, can offer symptom relief. For example, Lactobacillus rhamnosus GG is a specific probiotic strain that has demonstrated an ability to relieve the symptoms of IBS in human clinical trials, and early-phase in-vitro studies have shown that these benefits may be, at least partly, due to its effects on gut-derived serotonin signalling (3, 6).
The gut is called the body’s second brain, not because of its ability to produce serotonin for the brain to use, but because it also has its own complex nervous system that is needed to keep it functioning well (called the enteric nervous system). The serotonin produced by the gut largely stays in the gut and may play a bigger role in IBS than mental health. Although gut-derived serotonin may have less of an impact on our mood than people think, the gut is really important for our mental health in lots of other ways and you can read more about that here.
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