Serotonin: The Happy Hormone Produced In Our Gut
Our nervous system is the ultimate human supercomputer; it can evaluate an enormous amount of information coming from the various reaches of our body while sending out specific, targeted instructions that direct the body’s functions and responses to its environment. Your nervous system knows that you’ve burned your finger before you are fully able to comprehend what’s occurred. Similarly, while you may not be actively thinking about what’s going on in your digestive tract as you eat your dinner, your nervous system is coordinating an intricate process that is critical to your overall health and wellbeing.
Your brain and gut are in constant communication, connected by an information highway known as the vagus nerve. Given that your entire body relies on your brain for its basic operating instructions, you might be shocked to learn that far more communication reaches the brain from the gut than the other way around! If you’ve ever heard the gut called ‘the second brain,’ there’s a good reason: the number of nerves in your gut outnumbers your spinal cord.
In fact, our guts can act semi-independently of the brain through a variety of reflex circuits that govern its movement, its secretions and the vessel dilation required for absorption of nutrients into the blood stream1. You can think of your nerve cells – neurons – as the hardware and the neurotransmitters as the software of this supercomputer. Different neurotransmitters have different effects on downstream neurons; the most well-known of all neurotransmitters, serotonin, is thought to govern messages of wellbeing, contentment, satiety, anxiety, and fear throughout the nervous system.
The Feel Good Hormone
Often called the ‘feel good hormone,’ serotonin acts as both a hormone and a neurotransmitter in our body. Low mood, depression, anxiety, and even autism are associated with altered serotonin levels. For all of its importance to mental wellbeing, you might expect that the brain is where we find most of the body’s serotonin, but it’s not. In fact, the gut contains the vast majority of the serotonin in our body.1,2. Serotonin, or 5-hydroxytryptamine, is produced by the enterochromaffin (EC) cells lining the digestive tract1,2.
Given that our gut is the primary site of serotonin production, the ability of gut-derived serotonin to impact mental wellbeing is a hot topic; so is the role that gut bacteria play in regulating said production. At this current time, it is believed that our gut bacteria can both, directly and indirectly, influence serotonin levels. In the lab, certain gut bacteria have been shown to produce their own serotonin5; however, they can also interact with the nervous system to alter its production of neurotransmitters2. Remember the enterochromaffin cells we just mentioned? They can respond to chemical or mechanical stimuli in the gut – which is where your gut bacteria come in. Using an extreme example, if you were to eat food contaminated with the harmful bacteria E.coli, the EC cells would release a flood of serotonin causing you to expel the contents of your gut to help rid the bacteria.
On the other side of the spectrum, beneficial microbes produce short-chain fatty acids like butyrate that can influence the production of serotonin in the enterochromaffin cells4. Microbes can also alter the availability of tryptophan, which is the amino acid building block required for serotonin production4.
Not too much, not too little
As with many things in the body, there appears to be an optimal range of serotonin production for good health. The same transporter – serotonin selective reuptake transporter, or SERT, - that ushers serotonin into the brain tissues is also found in the gut cells1.
Serotonin released within the gut has many effects locally, including regulating peristalsis, which is the normal rhythmic movement of the gut muscle that helps move contents along the way1,3. Serotonin also regulates digestive secretions and the perception of pain or nausea. Excessive serotonin production, as in the food poisoning example, can cause significant gastrointestinal distress.
With irritable bowel syndrome, the increased sensitivity to pain and the altered movement of the gut is evidence that serotonin is involved. With IBS, changes to both the level of circulating serotonin and the levels of SERT have been noted, but there are still many questions left to answer1,3. Other research has shown that the pain and hypersensitivity of IBS seem to transfer to other animals via fecal transfer, pointing to a microbial link5.
Depression is linked to lower than normal serotonin activity; a common medication class for depression known as selective serotonin reuptake inhibitors, or SSRIs, works by increasing the serotonin activity between neurons2 – but they are known to have gastrointestinal side effects. Due to this, studies have looked at using these medications for IBS, however only mixed results have been found.3,4.
Probiotics and Serotonin
Research has established associations between gut microbes, digestive function, and mental wellbeing6,7. In irritable bowel syndrome, we tend to see lower levels of Lactobacillus strains versus healthy people3 whereas one study found that giving Lactobacillus acidophilus improved hypersensitivity5. Using a probiotic with Lactobacillus strains, such as Bio-K+, may help to regulate and restore a healthy community of Lactobacilli in the gut and improve digestive wellbeing.
Fermentation end-products of Lactobacillus have also been found to increase the expression of SERT, which is important for the regulation of serotonin levels in the gut3. Many of the markers of depression are also known to be modifiable by gut microbes4. Probiotics have been shown effective in supporting a healthy mood, with one trial even demonstrating that probiotics were more effective than a common anti-depressant medication, although we are still in the early stages of research7. For this reason, a clinical strength probiotic such as Bio-K+ is one of my top strategies, along with a healthy, plant-rich diet, for nourishing a healthy gut-brain connection.
The Gut-Brain Axis
The health of our brain and our digestive tract are intertwined; what goes on in one greatly affects the function of the other. Our thoughts can have a significant impact on our digestive function: a case of nerves can lead to butterflies in the stomach, and significant daily stress can trigger flares of IBS5, Crohn’s disease, and ulcerative colitis. And yet, our mental wellbeing is similarly dictated by our digestive wellbeing. Those with irritable bowel syndrome tend to have far higher rates of mental illness4. For example, the connection between depression, anxiety, and irritable bowel syndrome was examined in one trial with fascinating results6. Subjects were one of either two groups: diagnosed first with IBS or diagnosed with anxiety or depression. The subjects were then followed for 12 months to see what happened. In the next year, the likelihood that someone with IBS would be diagnosed with depression or anxiety was significantly higher than someone without IBS, and it was also significantly more likely that someone with depression or anxiety would be diagnosed with IBS6. For roughly 2/3 of the participants in this study, gut-level issues came first.
As you can see, nourishing a strong gut-brain connection means taking a holistic approach to wellbeing, one that does not forget that bacteria have a role to play in regulating digestive and nervous system function. Taking steps to manage stress, exercise, eat a healthy, plant-filled diet alongside a daily probiotic will help you build a healthier, more resilient mind and body.
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