The Gut-Brain Connection and Mood: What the Evidence Shows

The Gut and Brain Are Always Talking

Most people think of the gut as a digestive organ and the brain as a cognitive one — two systems that operate independently. Research suggests they are more connected than that. The gut and brain communicate in both directions through what researchers call the gut-brain axis, and this communication is an active area of research into mood, cognition and stress.

The gut-brain axis is a real set of nerve, hormone and immune pathways. However, many of the proposed links to mood have been studied mainly in laboratory and animal experiments or in observational human studies, which cannot show cause and effect⁹. This article sets out what is known, what is still uncertain, and what human trials of probiotics and other supplements have found. It is general information, not a substitute for care: depression and anxiety need assessment and treatment from a doctor or mental health professional, and supplements are not a treatment for them.

What Is the Gut-Brain Axis?

The gut-brain axis is a bidirectional communication system connecting the central nervous system (CNS) — the brain and spinal cord — with the enteric nervous system (ENS), a complex network of an estimated 168 million neurons¹⁰ embedded in the walls of the gastrointestinal tract. The ENS is sophisticated enough to function independently of the brain and is sometimes called the "second brain" — it can coordinate digestion, sense chemical changes in the gut, and regulate gut function without direct input from the central nervous system.

Communication between the gut and brain travels through several pathways simultaneously:

• The vagus nerve — a direct neural pathway that carries signals in both directions between the brainstem and the gut. About 80% of its fibers are afferent, meaning they carry information toward the brain (from the gut and other organs) rather than away from it¹¹, so the gut is continuously informing the brain about its state.
• Neurotransmitter signaling — gut cells and some gut microbes produce or influence signaling molecules such as serotonin and GABA; whether this affects brain function or mood in people is still being studied
• Immune signaling — the gut contains a large amount of immune tissue, and inflammatory signals from gut immune activity may reach the brain through the bloodstream and neural pathways; how much this matters for mood in people is not established
• The HPA axis — stress activates the hypothalamic-pituitary-adrenal axis, which releases cortisol. Stress is also linked to changes in gut function and the gut microbiota¹¹, so stress and gut symptoms can influence each other

Researchers are studying whether changes in these pathways contribute to mood and anxiety disorders. Observational studies have found that the gut microbiome of adults with major depression differs on average from that of people without depression, but the findings are inconsistent between studies¹², and these studies cannot show whether microbiome changes cause depression, result from it, or reflect other factors such as diet or medicines.

Three Proposed Mechanisms: How Gut Health Might Relate to Mood

1. Neurotransmitter Production in the Gut

Neurotransmitters are the chemical messengers that regulate mood, cognition, anxiety, and emotional processing. What many people do not realize is that most of the body's serotonin is found outside the brain.

Most of the body's serotonin is found outside the central nervous system¹, and almost all of that peripheral serotonin is made by specialized enterochromaffin cells in the gut lining². In mice, spore-forming bacteria from the gut microbiota promoted serotonin production by these cells, with effects on gut motility and platelet function¹³. Serotonin cannot readily cross the blood-brain barrier, so the gut and brain serotonin pools are anatomically separate². A 2025 review proposes that gut-derived serotonin may signal to the brain by activating vagus nerve fibers¹⁴, but no human trial showing that changing gut serotonin or the gut microbiome improves mood was found. Popular claims that most serotonin is made in the gut should not be read as meaning that the gut controls brain serotonin or mood.

GABA (gamma-aminobutyric acid) is the brain's main inhibitory neurotransmitter. In laboratory tests of 91 bacterial strains from the human intestine, five (one Lactobacillus strain and four Bifidobacterium strains) produced GABA¹⁵, but it is not known whether GABA made by gut bacteria reaches the brain or affects mood or anxiety in people. A 2021 mini-review describes the microbiota-gut-brain axis as communication through the vagus nerve, the immune system, microbial metabolites such as short-chain fatty acids, the enteric nervous system and hormones¹⁶; it does not show that gut microbes change mood in people.

What this means in practice is still unclear. These findings show that gut cells and microbes can produce signaling molecules, but not that changing the microbiome changes brain chemistry or mood in people.

2. Inflammation and Brain Function

Inflammation is one of the factors being studied in depression. A meta-analysis found that about 27% of people with depression had low-grade inflammation (C-reactive protein above 3 mg/L), and that low-grade inflammation was more common in people with depression than in matched healthy controls (odds ratio 1.46)¹⁷. This does not show where the inflammation comes from, and whether the gut contributes is not established. A 2013 review discussed the idea that gut microbes and gut permeability might influence mood through inflammatory pathways, and called for controlled clinical studies to test it⁹.

One proposed mechanism is that inflammatory signals from the gut, such as the cytokines IL-6, TNF-α and IL-1β, could reach the brain and affect mood. This is a hypothesis, and no human trial showing that reducing gut inflammation improves depression was found.

For this reason, diet, probiotics or supplements should not be seen as a way to treat depression by reducing gut inflammation. Anyone with symptoms of depression should be assessed by a clinician, who can discuss treatments that have established evidence.

3. Intestinal Permeability — The "Leaky Gut" Mechanism

The intestinal lining serves as a selective barrier — allowing nutrients to pass into the bloodstream while keeping bacteria, toxins, and undigested particles out. The barrier can become more permeable than it should be in some circumstances, such as illness, heavy alcohol use or some medicines; the roles of diet, stress and the gut microbiome in people are still being studied.

A 2018 secondary analysis of a study of 43 healthy married couples (86 people) found that more hostile behavior during a discussion of a marital disagreement was associated with higher blood levels of LPS-binding protein (LBP), and that hostility combined with a history of mood disorder was associated with a higher LBP to sCD14 ratio (both are markers of exposure to bacterial endotoxin). Higher LBP and LBP/sCD14 were associated with higher C-reactive protein, and higher sCD14 with higher interleukin 6³. The study shows associations only. It did not measure the brain or neuroinflammation, and it cannot show that gut permeability causes low mood or depression. Whether endotoxin from the gut contributes to low mood in people remains a hypothesis.

Supplements and Foods Studied for the Gut-Brain Axis

Probiotics — The Most Studied Option, With Mixed Results

Probiotics are live microorganisms. Their effects depend on the specific strain and the condition studied, and results for one strain should not be assumed for another. Probiotics have more human mood data than the other supplements discussed here, but the results are mixed. A 2017 systematic review of 10 human studies found that the majority reported improvements in depressive symptoms, but the strains, doses and durations varied widely, and the authors called for more double-blind randomized trials in clinical populations¹⁸. A 2021 meta-analysis of seven randomized trials in 404 people with depression found a benefit when probiotics were added to antidepressant medicine (standardized mean difference 0.83) but not when they were used alone (-0.02). Its authors disclosed research support from a probiotics company and described the evidence for mechanisms such as changes in BDNF and C-reactive protein as limited⁴. A 2023 pilot trial of 50 adults with major depression who were already taking antidepressants and had an incomplete response found greater improvement over 8 weeks with a multistrain probiotic than with placebo, with no serious adverse reactions; the authors described the results as promising and called for a definitive trial¹⁹.

Later meta-analyses are mixed. A 2026 meta-analysis of 27 randomized trials found no significant effect of probiotics on depression in short (under 6 weeks) or medium (6 to 12 weeks) trials, and a possible benefit in only two longer trials, a finding based on limited evidence⁵. A 2025 meta-analysis of 72 trials reported larger average reductions in depression (standardized mean difference -0.53) but cautioned about high heterogeneity and limited methodological quality⁶. How probiotics might act on mood is still being studied.

The 2017 review noted that strains varied widely between studies. We did not find evidence that identifies a best strain for mood, or that shows multi-strain products work better than single-strain products. As discussed in our article on probiotic labels (CFU versus survivability), effects are strain-specific, and a higher CFU count does not by itself mean a better result.

Probiotics are generally well tolerated by healthy people, but safety outcomes are inconsistently reported in trials²⁰, and rare serious infections have been described in people with weakened immune systems or serious illness, including a case of sepsis after high-dose probiotic use in a burn patient^20,21. Talk to a doctor before taking a probiotic if you have a weakened immune system, a central venous catheter, a serious illness or a serious bowel condition, or if you are pregnant or breastfeeding or are choosing a product for a child.

Prebiotics — Feeding the Right Bacteria

Prebiotics are defined as substrates that are selectively used by gut microbes to confer a health benefit; most are non-digestible dietary fibers²². Unlike probiotics, which introduce live bacteria, prebiotics are intended to feed bacteria that are already present in the gut.

A 2022 review describes how gut microbes break down prebiotic fibers into short-chain fatty acids (SCFAs), which it presents as key molecules in communication between the microbiota and the host, and notes that people respond to prebiotics in personalized ways; the human studies it reviews concern metabolic disease, not mood²². For depression, a 2026 meta-analysis of randomized trials found no significant effect of prebiotics⁵, while a 2025 meta-analysis reported benefits for prebiotics in subgroup analyses, with high heterogeneity and limited study quality overall⁶. Prebiotic supplements such as inulin and fructooligosaccharides (FOS) can also contribute to gas, bloating and other symptoms in susceptible people, and the same substances are restricted as FODMAPs in irritable bowel syndrome²³. If you have IBS or are prone to bloating, ask a clinician or dietitian whether a prebiotic supplement is suitable.

Products that combine probiotics and prebiotics are called synbiotics. We did not find evidence that combining them works better than either alone for mood.

Digestive Enzymes — Where the Evidence Is and Is Not Established

Digestive enzymes (amylase, protease, lipase, and others) catalyze the breakdown of food into absorbable nutrients. When the pancreas does not make enough of them (pancreatic insufficiency), food is not fully digested and nutrients may be poorly absorbed; this is a medical condition that needs to be diagnosed and treated by a clinician.

A 2016 non-systematic review describes animal-derived (pancreatic) enzymes as an established standard of care in some digestive disorders, such as cystic fibrosis, and notes that enzyme supplementation in diseases not related to enzyme deficiency is still being evaluated²⁴. We did not find trials showing that digestive enzyme supplements improve mood, or that they increase the absorption of amino acids or B vitamins needed for neurotransmitter production in people with normal digestion. Persistent digestive symptoms need a medical evaluation rather than self-treatment.

Omega-3 Fatty Acids — Mixed Evidence for Mood

Omega-3 fatty acids (EPA and DHA) are found in fatty fish and fish oil. DHA is a structural component of neuronal cell membranes, and EPA and DHA are being studied for effects on inflammation. These are mechanisms, not proof that omega-3 supplements improve mood or gut-brain signaling in people.

Trials of omega-3 for depression have mixed results. A Cochrane-based review of 20 trials in adults with major depressive disorder found a small-to-modest average benefit over placebo (standardized mean difference -0.32), but described it as unlikely to be clinically meaningful, imprecise and probably biased, and concluded that there was not enough evidence to determine the effects of omega-3 as a treatment⁷. A 2025 meta-analysis of 36 trials found a small average benefit (standardized mean difference -0.26) but significant heterogeneity and publication bias, and no difference from placebo in response or remission rates⁸. Separately, a 2018 systematic review of omega-3 supplementation in people with Alzheimer's disease found that most placebo-controlled trials did not show a significant benefit²⁵.

In the 2025 meta-analysis, adverse events were not more frequent than with placebo⁸. However, a meta-analysis of seven large cardiovascular trials (81,210 patients) linked marine omega-3 supplements to a higher risk of atrial fibrillation (hazard ratio 1.25), with a larger increase in trials testing more than 1 g/day (hazard ratio 1.49)²⁶. A meta-analysis of 11 randomized trials found no overall increase in bleeding, but high-dose purified EPA was associated with a 50% relative increase in bleeding (about 0.6 percentage points in absolute terms)²⁷. Ask a doctor or pharmacist before taking omega-3 supplements if you take blood thinners or other prescribed medicines, or have a heart rhythm problem.

Practical Steps

Evidence for gut-targeted strategies is limited. General healthy-eating steps are low-risk, and in a 2019 meta-analysis of 16 randomized trials (mostly in people without a clinical diagnosis of depression), dietary improvement was associated with a small reduction in depressive symptoms but not in anxiety²⁸. Those trials tested overall dietary patterns, not supplements or gut-specific effects. Practical options include:

1. Diversify dietary fiber — different gut bacteria feed on different fiber types, and a variety of vegetables, legumes and whole grains is a sensible part of a healthy diet. Whether greater microbiome diversity improves mental health has not been shown. If extra fiber causes gas or bloating, increase it gradually
2. Include fermented foods — yogurt, kefir, kimchi, sauerkraut and miso contain live microbes. In a 17-week randomized trial in healthy adults (18 per diet group), a diet high in fermented foods increased microbiome diversity and decreased inflammatory markers²⁹; the outcomes reported were microbiome and immune measures, so this does not show an effect on mood
3. Manage psychological stress — stress and gut symptoms are linked in both directions¹¹, and stress management is worthwhile for mood in its own right. If stress, low mood or anxiety is persistent, talk to a doctor or mental health professional
4. Limit ultra-processed foods and excess alcohol — a 2025 review reports that ultra-processed foods are associated with lower microbial diversity and with changes linked to gut dysbiosis and increased gut permeability, and calls for more studies to clarify the mechanisms³⁰. Keep alcohol within national guidelines
5. Consider whether a probiotic is right for you — the evidence for mood is mixed (see above). A study in mice and human volunteers found that probiotics delayed the recovery of the gut microbiome after antibiotics compared with spontaneous recovery³¹, although the clinical significance is unclear. Talk to a doctor or pharmacist first, especially if you are unwell, are pregnant or breastfeeding, have a weakened immune system, or take antidepressants or other prescribed medicines
6. Include fatty fish — salmon, mackerel and sardines can be part of a varied diet. Omega-3 supplements are optional and have safety considerations (see above); ask a doctor or pharmacist before taking them if you take blood thinners or have a heart rhythm problem

When to Talk to a Clinician

Depression and anxiety are medical conditions that need assessment and treatment from a doctor or mental health professional. Probiotics, prebiotics, digestive enzymes, omega-3 supplements and dietary changes are not a treatment for depression or anxiety and should not be used in place of professional care. If low mood, worry or loss of interest is persistent or affects daily life, talk to a doctor or another qualified clinician.

Do not stop, reduce or change antidepressants or any other prescribed medicine without advice from the prescriber. If you take prescribed medicines, are pregnant or breastfeeding, have a weakened immune system, a heart rhythm problem or a bleeding disorder, or are choosing supplements for a child, ask a doctor or pharmacist before starting any supplement. Persistent digestive symptoms also need medical evaluation rather than self-treatment.