Introduction
Have you ever felt a "flutter" in your stomach when you are nervous, or perhaps noticed that your digestion seems to grind to a halt when you are under significant stress? Most of us have experienced these sensations, but we often dismiss them as mere quirks of the body. However, modern science is increasingly suggesting that these experiences are evidence of a sophisticated, two-way communication system known as the gut-brain axis. Central to this conversation is serotonin, a chemical messenger often dubbed the "happy hormone" because of its profound influence on our mood and emotional well-being.
What might surprise you is that the vast majority of your body's serotonin—roughly 90% to 95%—is not found in your brain at all. Instead, it resides in your gastrointestinal tract. This has led researchers to ask a pivotal question: how much of this "brain chemical" is actually managed by the trillions of microscopic inhabitants living in our intestines? Specifically, what gut bacteria produces serotonin, and how does this affect how we feel every day?
At Blue Horizon, we believe that understanding your health requires looking at the bigger picture. Whether you are struggling with "mystery symptoms" like persistent low mood, brain fog, or sluggish digestion, the answer rarely lies in a single isolated factor. Instead, it is often found in the interplay between your biology, your lifestyle, and your gut environment. For a broader look at thyroid markers and test choices, our guide to which blood test is for thyroid function is a useful companion read.
In this article, we will explore the specific strains of bacteria that have been identified as serotonin producers, the mechanisms they use to influence your nervous system, and how you can take a structured, clinical approach to managing your health. We advocate for a phased journey: first, consulting your GP to rule out underlying medical conditions; second, using self-tracking to understand your unique patterns; and third, considering professional blood testing to gain a clear snapshot of your internal health.
Safety Note: While exploring the gut-brain connection is valuable, sudden or severe symptoms—such as the swelling of the lips, face, or throat, difficulty breathing, or a sudden collapse—warrant urgent medical attention. If you or someone else experiences these, please call 999 or attend your nearest A&E immediately.
The Second Brain: Why Serotonin Starts in the Gut
To understand which bacteria produce serotonin, we first need to understand why the gut is so central to our neurochemistry. The gut is lined with an extensive network of neurons called the enteric nervous system (ENS). Often called the "second brain," the ENS contains more neurons than the spinal cord. It operates independently of the brain in many ways, managing the complex mechanics of digestion, but it is also in constant contact with the central nervous system via the vagus nerve.
Serotonin (scientifically known as 5-hydroxytryptamine or 5-HT) acts as a vital bridge in this communication. In the gut, it serves several essential roles:
- Peristalsis: It helps trigger the muscle contractions that move food through the digestive tract.
- Sensory Signalling: It transmits signals of pain, nausea, or fullness to the brain.
- Immune Response: It plays a role in how the gut reacts to potential threats or inflammation.
While the brain produces its own serotonin to regulate mood, sleep, and appetite, the serotonin produced in the gut cannot cross the "blood-brain barrier." This means that the serotonin in your intestines doesn't directly enter your brain. However, the gut-produced serotonin influences the signals sent to the brain through the vagus nerve and affects the availability of the raw materials (like the amino acid tryptophan) that the brain needs to make its own serotonin. If you want a plain-English overview of thyroid markers and test choices, our guide to what blood tests for thyroid function are best is a helpful companion read.
This is where your gut bacteria come in. They are not just passive residents; they are active chemical factories that can either produce serotonin themselves or signal your own gut cells to do it for them.
Identifying the Bacteria: What Gut Bacteria Produces Serotonin?
Recent breakthroughs in microbiology have identified several key players in the microbial world that are directly or indirectly involved in serotonin production. Research is ongoing, but several specific genera and species have emerged as significant.
1. Limosilactobacillus mucosae and Ligilactobacillus ruminis
Perhaps the most direct evidence of bacterial serotonin production comes from studies involving these two specific strains. Researchers have discovered that when these two bacteria work together, they can produce bioactive serotonin.
Interestingly, neither bacterium seems to produce much serotonin on its own in a laboratory setting, but when they are co-isolated (grown together), they become highly effective. They use an enzyme to convert a precursor called 5-HTP (5-hydroxytryptophan) into serotonin. In clinical studies, the presence of these bacteria has been linked to improved intestinal transit times and healthier nerve connectivity in the colon.
2. Turicibacter sanguinis
The genus Turicibacter has gained attention for its unique relationship with serotonin. Researchers at UCLA found that Turicibacter sanguinis not only helps signal the gut to produce more serotonin but also possesses "machinery" to detect and transport serotonin into its own cells. If you want a closer look at the wider testing context behind symptoms like these, our guide to what is included in a thyroid function test is a helpful place to start.
This suggests a "symbiotic" feedback loop: the bacteria encourage the host (us) to make more serotonin, and the bacteria then use that serotonin to thrive and grow. This relationship is so strong that when certain antidepressants (SSRIs) are introduced, which block serotonin transport, the growth of Turicibacter can be significantly inhibited.
3. Spore-Forming Bacteria (Clostridia)
A significant portion of gut serotonin is produced by specialised cells in the gut lining called enterochromaffin (EC) cells. While these are human cells, they are heavily influenced by the bacteria around them.
Research has shown that a specific group of about 20 species of spore-forming bacteria, largely from the Clostridia class, produce metabolites (byproducts of their metabolism) that "instruct" the EC cells to ramp up serotonin production. In studies where these bacteria were absent, serotonin levels in the gut dropped by over 50%. When the bacteria were reintroduced, serotonin levels returned to normal.
4. Coprococcus and Dialister
In the context of mental health, the presence or absence of certain bacteria can be just as telling as direct production. Large-scale human studies, such as the Flemish Gut Flora Project, have identified that two genera of bacteria—Coprococcus and Dialister—are consistently depleted in individuals suffering from depression. For a simple overview of home sample options, our finger-prick blood test kits explain how collection can work in practice.
While researchers are still determining if these bacteria produce serotonin directly, they are known to have "neuroactive potential," meaning they produce substances that interact with the nervous system to influence mood and quality of life.
How the Process Works: From Tryptophan to Happiness
The production of serotonin is a multi-step chemical journey. It starts with an essential amino acid called tryptophan, which we must obtain from our diet (found in foods like turkey, eggs, cheese, and oats).
The journey usually follows this path:
- Tryptophan Intake: You eat protein-rich foods containing tryptophan.
- The Crossroads: Tryptophan in the gut can be used in different ways. It can be turned into serotonin, or it can be diverted down a different path (the kynurenine pathway) which is often associated with inflammation and stress.
- Bacterial Influence: This is the crucial moment. Certain "good" bacteria encourage the conversion of tryptophan into 5-HTP and then into serotonin. Other bacteria might produce enzymes that "steal" the tryptophan for other purposes, potentially leaving less available for serotonin production.
- Signalling the Brain: The resulting serotonin in the gut acts on the vagus nerve, sending messages to the brain that can influence mood, stress levels, and cognitive function.
This explains why some people feel "hangry" or low when their diet is poor or their gut health is compromised. If your microbial "factory" isn't functioning correctly, the supply chain for your neurotransmitters can be disrupted.
The Symptom Picture: When the Balance is Off
When the relationship between your gut bacteria and serotonin is disrupted, it rarely results in just one symptom. Instead, it often presents as a cluster of "mystery symptoms" that can feel frustratingly vague. You might experience:
- Digestive Changes: Because serotonin controls movement in the gut, a lack of it can lead to constipation, while an excess can lead to diarrhoea or urgency. This is a common factor in Irritable Bowel Syndrome (IBS).
- Low Mood and Anxiety: While gut serotonin doesn't enter the brain, the "gut-brain" signals it generates are a major component of our emotional baseline.
- Sleep Disturbances: Serotonin is a precursor to melatonin, the hormone that regulates sleep. A disruption in the gut can sometimes echo into your sleep patterns.
- Food Cravings: Your bacteria may influence your cravings for carbohydrates or sugars as they seek out the raw materials needed for their own survival and chemical production.
At Blue Horizon, we see many patients who come to us with these exact symptoms. Often, they have been told by their GP that their standard tests are "normal," yet they know something isn't right. This is where a more structured approach can provide clarity.
The Blue Horizon Method: A Clinical Path to Understanding
If you suspect your gut-brain axis is out of balance, it can be tempting to reach for the latest supplement or a restrictive diet. However, we advocate for the Blue Horizon Method—a phased, clinically responsible journey that ensures you aren't chasing ghosts or ignoring a more serious underlying issue.
Step 1: Consult Your GP
Your first port of call should always be your GP. Symptoms like fatigue, low mood, or digestive changes can be caused by many things—anaemia, thyroid dysfunction, or clinical depression, for example. It is essential to rule these out first. Your GP can perform standard NHS tests to ensure there isn't a clear medical diagnosis that needs immediate attention.
Step 2: Structured Self-Checking
Before jumping into advanced testing, start a diary. For two to four weeks, track the following:
- Symptom Timing: When do you feel most tired or low? Is it after certain meals?
- Digestive Patterns: Note the frequency and consistency of your bowel movements.
- Lifestyle Factors: How much are you sleeping? What are your stress levels like?
- Dietary Habits: Are you eating enough fibre to feed those serotonin-producing bacteria?
This data is incredibly valuable. It helps you see patterns that might not be obvious day-to-day and provides a much more productive "starting point" for a conversation with a healthcare professional.
Step 3: Targeted Blood Testing
If you have consulted your GP and tracked your lifestyle but still feel "stuck," this is where a Blue Horizon blood test can provide a structured snapshot. If you are unsure where to begin, our article on what blood test is used to check thyroid explains the options in more detail.
While we do not offer "gut bacteria" tests (as the science on stool testing for specific bacteria is still emerging and often lacks clear clinical "normal" ranges), we do offer comprehensive panels that check the effects of your internal health environment. Many symptoms of gut-brain disruption overlap with other hormonal and nutritional issues.
For example, if you are experiencing the low mood and fatigue associated with serotonin issues, it is often wise to check your thyroid function and key vitamin levels.
Choosing the Right Snapshot: Our Thyroid and Health Tiers
When people feel "off," they often find that their thyroid—the master regulator of metabolism—is involved. Since thyroid hormones and gut function are closely linked, checking these markers is a common step for our clients. We offer a tiered approach so you can choose the level of detail that fits your situation.
Bronze Thyroid Check
This is a focused starting point. It includes the base thyroid markers: TSH (Thyroid Stimulating Hormone), Free T4, and Free T3.
- TSH is the signal from your brain telling the thyroid to work.
- Free T4 and Free T3 are the actual hormones produced. At Blue Horizon, we also include our "Extras": Magnesium and Cortisol. These are cofactors that influence how you feel and how your thyroid functions—markers that are rarely included in standard tests but are vital for a "bigger picture" view. You can see the full details on our Thyroid Premium Bronze test page.
Silver Thyroid Check
This tier includes everything in the Bronze test but adds Thyroid Peroxidase Antibodies (TPOAb) and Thyroglobulin Antibodies (TgAb). These help identify if an autoimmune process is affecting your health, which can often be linked to gut permeability and inflammation.
Gold Thyroid Check
This is one of our most popular choices for those with "mystery" fatigue or mood issues. It builds on the Silver tier by adding a broader health snapshot: Ferritin (iron stores), Folate, Active Vitamin B12, C-Reactive Protein (CRP) for inflammation, and Vitamin D. If your gut bacteria are struggling, your absorption of these key nutrients might also be affected. You can compare the full marker list on our Thyroid Premium Gold profile.
Platinum Thyroid Check
Our most comprehensive profile. It includes everything in Gold, plus Reverse T3, HbA1c (a 3-month average of blood sugar), and a full Iron Panel. This gives the most detailed metabolic and hormonal map available and requires a professional blood draw (venous sample) due to the complexity of the markers. If you need the most detailed option, the Thyroid Premium Platinum profile is the best place to start.
Note on Testing: We recommend a 9am sample for these tests to ensure consistency and to align with your body's natural hormone fluctuations. You can view current pricing on our thyroid blood tests collection.
Supporting Your Serotonin-Producing Bacteria
Once you have ruled out major clinical issues with your GP and used testing to check your nutrient levels, you can focus on "feeding" the bacteria that support your serotonin production.
The goal isn't necessarily to "add" more bacteria (via probiotics), but to create an environment where the right ones can thrive. Here is how you can practically support your gut-brain axis:
1. Focus on Prebiotic Fibres
Bacteria like Clostridia and Lactobacillus thrive on complex carbohydrates that your body cannot digest on its own. These are called prebiotics. Focus on:
- Inulin-rich foods: Onions, garlic, leeks, and asparagus.
- Resistant starches: Cooked and cooled potatoes or rice.
- Diverse plant life: Aim for 30 different types of plants a week (including nuts, seeds, herbs, and spices).
2. Prioritise Tryptophan
Ensure your "factory" has the raw materials it needs. Include moderate amounts of tryptophan-rich proteins in your diet. Interestingly, consuming these alongside some healthy carbohydrates can help the tryptophan cross the blood-brain barrier more effectively, supporting brain-level serotonin as well.
3. Manage Stress
The gut-brain axis is a two-way street. High levels of cortisol (the stress hormone) can change the environment of the gut, making it less hospitable for "friendly" bacteria. Simple daily practices like walking in nature, breathwork, or consistent sleep schedules can physically change the "signals" your gut receives. For more practical context on thyroid-related timing and diet, our guide to what foods interfere with thyroid medication absorption is a useful companion read.
4. Be Cautious with Medication
If you are taking medication for your mood, such as an SSRI, or for your thyroid, never adjust your dosage based on private test results or gut health theories. Always work closely with your GP or specialist. As we saw with the Turicibacter study, medications can significantly impact your gut microbes, and any changes should be managed medically.
A Note on Persistence
Health is rarely a "quick fix," especially when it involves the complex world of the microbiome. If you have made lifestyle changes and your symptoms persist, it is important not to ignore them. Persistent bloating, changes in bowel habits, or a mood that doesn't lift despite your best efforts should always be re-evaluated by a medical professional.
Your blood test results from Blue Horizon are designed to be a tool for a better conversation. When you take your results to your GP, you aren't just saying "I feel tired"; you are saying "I feel tired, and here is a snapshot of my T3, B12, and Cortisol levels to help us figure out why."
Conclusion
The discovery that specific gut bacteria—like Limosilactobacillus mucosae, Turicibacter sanguinis, and certain Clostridia—are responsible for producing or stimulating serotonin has transformed our understanding of mental and digestive health. We are no longer just looking at the brain for answers to mood, nor just the stomach for answers to digestion. We are looking at the delicate, chemical conversation happening between them.
If you are feeling out of balance, remember the phased approach:
- See your GP to rule out clinical conditions.
- Track your symptoms and lifestyle to find your own patterns.
- Consider a structured blood test if you need a clearer internal snapshot to guide your next steps.
By understanding the "bigger picture"—from the bacteria in your gut to the hormones in your blood—you can move away from chasing symptoms and toward a more informed, proactive way of managing your well-being.
FAQ
Can I test my gut for serotonin-producing bacteria?
While there are private stool tests that can identify various strains of bacteria, the clinical application of these results is still in its infancy. There is currently no "gold standard" for what a perfect microbiome looks like, and levels can fluctuate daily. At Blue Horizon, we focus on blood markers (like vitamins, minerals, and hormones) that have established clinical ranges, as these provide a more reliable basis for a conversation with your GP about your symptoms.
Does taking probiotics increase my serotonin?
Some specific probiotics, often called "psychobiotics," have shown promise in research for supporting mood. However, the effect is often temporary and depends on your existing gut environment. It is usually more effective to "feed" the beneficial bacteria you already have by eating a wide variety of fibre-rich plant foods, which provides the fuel they need to produce serotonin naturally.
Why does my GP only test TSH if the gut-brain axis is so complex?
The NHS typically uses TSH as a first-line screening tool because it is an efficient way to spot major thyroid dysfunction. However, as we have discussed, symptoms like fatigue and low mood are often more nuanced. This is why we offer broader panels—such as our Gold and Platinum tiers—which include Free T3, Free T4, and vitamins like B12 and D, to provide the "bigger picture" that a standard screening might miss.
Can diet alone fix a serotonin deficiency in the gut?
Diet is a powerful tool for supporting the gut-brain axis, but it is not a cure-all. Factors like chronic stress, genetics, and underlying medical conditions also play a significant role. If you suspect your serotonin levels are low, use diet as a foundation, but ensure you are also consulting with a healthcare professional to address all aspects of your health.
