IBS symptoms: what's causing your gut issues and which tests can help

Gut microbiome dysbiosis

Research consistently shows that people with IBS have measurably different gut microbiome compositions compared to healthy controls, characterised by reduced bacterial diversity and shifts in the relative abundance of key species. A 2024 cross-cohort metagenomic study covering more than 9,000 samples identified a reproducible IBS-associated microbial signature for the first time, suggesting that dysbiosis is not merely a correlate of IBS but a likely mechanistic driver. Lower levels of butyrate-producing bacteria (particularly Faecalibacterium prausnitzii and Roseburia) are consistently found in IBS, reducing the short-chain fatty acid production that maintains gut barrier integrity, modulates immune responses, and regulates gut motility.

Altered gut-brain signalling

IBS is classified by gastroenterologists as a disorder of gut-brain interaction (DGBI). The enteric nervous system in the gut contains more neurons than the spinal cord, and its signalling is intimately linked to the central nervous system through the vagus nerve, serotonin pathways, and immune system intermediaries. People with IBS show increased visceral hypersensitivity, meaning the gut's pain threshold is lowered even when the gut lining itself shows no structural abnormality. Serotonin, 90 per cent of which is produced in the gut and directly influenced by gut bacteria, plays a central role in regulating gut motility, and disruptions in serotonin signalling explain many of the motility differences between IBS subtypes (IBS-D, predominantly diarrhoea; IBS-C, predominantly constipation; IBS-M, mixed).

Post-infectious IBS

A significant proportion of people develop IBS following a gastrointestinal infection such as gastroenteritis or food poisoning. Post-infectious IBS (PI-IBS) occurs when the acute inflammatory response of an infection disrupts the gut microbiome in a way that persists beyond recovery. Studies suggest that between 10 and 30 per cent of people who experience a gut infection go on to develop PI-IBS, with risk factors including the severity of the original infection, female sex, and the presence of anxiety or depression at the time of infection.

Food sensitivities and fermentable carbohydrates

IBS symptoms are strongly influenced by diet, particularly the intake of fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs). These carbohydrates are incompletely absorbed in the small intestine and rapidly fermented by gut bacteria in the large bowel, producing gas and drawing water into the colon. The degree to which FODMAPs trigger symptoms depends heavily on individual microbiome composition: different bacteria ferment different FODMAP components at different rates, which is why the same dietary trigger produces severe symptoms in one person and none in another with identical food intake.

Stress, anxiety, and psychological factors

IBS and anxiety are bidirectionally linked through the gut-brain axis. People with IBS have elevated rates of anxiety and depression, and people with anxiety have elevated rates of IBS. Stress directly worsens IBS symptoms by altering gut motility, increasing gut permeability, and shifting the microbiome toward dysbiotic compositions. This is not a suggestion that IBS is "all in the head": the psychological and biological components are mechanistically interconnected, and addressing both simultaneously produces better outcomes than treating either in isolation.

Small intestinal bacterial overgrowth (SIBO)

SIBO occurs when bacteria colonise the small intestine in excessive numbers, a region that normally has comparatively few bacteria. An estimated 10 to 15 per cent of the general population may have SIBO, and prevalence among IBS patients may be as high as 80 per cent in some studies, though estimates vary considerably across research methods. SIBO produces IBS-like symptoms including bloating, gas, abdominal pain, and irregular bowel movements through bacterial fermentation of carbohydrates in the small intestine before they can be absorbed. A hydrogen and methane breath test is the standard assessment for SIBO, and the condition is distinct from large bowel dysbiosis measured by standard microbiome testing.

Undiagnosed conditions that cause IBS-like symptoms

Several conditions produce symptoms identical to IBS but require different management: coeliac disease (immune-mediated damage to the small intestinal lining triggered by gluten), inflammatory bowel disease (Crohn's disease and ulcerative colitis), microscopic colitis, bile acid malabsorption, hypothyroidism (which slows gut motility and causes constipation), and iron deficiency anaemia (which causes fatigue and altered gut function). Ruling these out before accepting an IBS diagnosis ensures that a treatable condition is not missed and that dietary interventions are not applied to a condition that requires medical treatment.

There is currently no definitive biomarker test for IBS itself. IBS is diagnosed clinically using the Rome IV criteria, which require recurrent abdominal pain at least once per week for the previous three months, associated with two or more of: pain related to defecation, change in stool frequency, or change in stool form. Blood and stool tests are used to exclude other diagnoses rather than to confirm IBS.

The most useful testing approach for someone with IBS symptoms covers two complementary areas: blood markers that exclude other diagnoses and provide metabolic context, and gut microbiome testing that assesses the specific bacterial patterns associated with the symptom profile.

Full blood count identifies anaemia (iron deficiency, B12 deficiency, folate deficiency) which can cause fatigue that accompanies or worsens gut symptoms, and checks for raised white blood cell counts that suggest infection or inflammatory disease.

Coeliac antibodies (tTGA IgA) screen for coeliac disease, which affects approximately 1 per cent of the UK population and produces symptoms indistinguishable from IBS. Coeliac testing must be done while gluten is being consumed: removing gluten before testing renders the antibody test unreliable.

Thyroid function (TSH, Free T4, Free T3) identifies hypothyroidism, which slows gut motility and can cause or worsen constipation-predominant symptoms in ways that dietary intervention alone will not resolve.

CRP (C-reactive protein) and calprotectin distinguish functional gut disorders from inflammatory ones. Elevated CRP or calprotectin suggests gut wall inflammation that warrants further investigation for IBD or other structural conditions.

Ferritin, B12, and vitamin D assess nutritional status, which is commonly compromised in people with long-standing gut symptoms due to impaired absorption, altered dietary intake, and microbiome-related nutrient metabolism changes.

Gut microbiome analysis using DNA sequencing of a stool sample is the only investigation that directly assesses the bacterial composition driving functional symptoms. In people with confirmed IBS, microbiome testing can identify specific dysbiotic patterns, guide targeted probiotic and dietary interventions, and provide a baseline for tracking whether changes are producing measurable improvement in microbiome health over time.

Low-FODMAP dietary approach (short-term)

The low-FODMAP diet involves temporary elimination of highly fermentable carbohydrates, followed by systematic reintroduction to identify individual triggers. It is supported by strong clinical trial evidence, with approximately 70 per cent of IBS patients reporting significant symptom improvement during the elimination phase. It is designed as a short-term diagnostic tool (two to six weeks of elimination, then structured reintroduction), not a permanent dietary pattern: long-term restriction of FODMAPs reduces bacterial diversity and is not recommended indefinitely. Working with a dietitian experienced in the low-FODMAP approach maximises outcomes and prevents unnecessary long-term restriction.

Gut microbiome-targeted dietary changes

Building dietary diversity (30 or more plant foods per week), increasing soluble fibre (oats, psyllium husk, legumes), and adding fermented foods supports the microbiome shifts most beneficial for IBS. Unlike the low-FODMAP approach, which is symptom-focused, microbiome-targeted nutrition is mechanism-focused: it directly addresses the dysbiosis patterns that clinical research identifies as drivers of IBS-associated symptoms. Knowing your specific microbiome composition allows these general principles to be personalised to your bacterial profile.

Targeted probiotic supplementation

Specific probiotic strains have clinical evidence for IBS symptom reduction, including Lactobacillus rhamnosus GG, Bifidobacterium infantis 35624, and multi-strain formulations containing Lactobacillus and Bifidobacterium species. A 2024 meta-analysis of 20 randomised controlled trials involving more than 3,000 patients confirmed that probiotics outperformed placebo for overall IBS symptom improvement, abdominal pain reduction, and quality of life. The evidence is stronger for shorter treatment durations (four to eight weeks) and multi-strain formulas, and varies significantly by IBS subtype, which is why knowing your microbiome profile can inform which specific strains to prioritise.

Stress management and gut-brain axis interventions

Interventions with specific evidence for IBS management through the gut-brain axis include cognitive behavioural therapy (CBT), gut-directed hypnotherapy (which has NICE approval for IBS), mindfulness-based stress reduction, and regular aerobic exercise. These are not peripheral adjuncts to IBS management but mechanistically direct interventions that reduce visceral hypersensitivity, lower cortisol, improve vagal tone, and shift the microbiome toward less inflammatory compositions.