IBS vs IBD vs SIBO: What's the Difference?

IBS, IBD, and SIBO are three different gut conditions that share overlapping symptoms — bloating, abdominal pain, unpredictable bowel habits — which means they’re frequently confused, misdiagnosed, and mistreated.

Understanding the distinction isn’t just academic. Getting the right diagnosis determines whether your treatment involves dietary changes, antibiotics, immunosuppressants, or something else entirely.

IBS: Irritable Bowel Syndrome

IBS is a functional gut disorder — meaning it’s characterised by symptoms without any structural or inflammatory changes visible in the gut. There are no abnormalities on colonoscopy, endoscopy, or blood tests. This doesn’t mean it’s “all in your head” — the mechanisms are real and documented.

What’s actually happening in IBS:

• Visceral hypersensitivity: The gut’s pain sensing is amplified. Gas volumes that wouldn’t be noticed by most people cause significant discomfort in someone with IBS.
• Altered gut motility: The intestines contract too fast (IBS-D, diarrhoea-predominant), too slow (IBS-C, constipation-predominant), or unpredictably (IBS-M, mixed).
• Gut-brain axis dysregulation: Stress, anxiety, and the central nervous system directly alter gut function through the bidirectional gut-brain connection.

Common symptoms:

• Abdominal cramping, especially in the lower abdomen
• Bloating and gas
• Alternating diarrhoea and constipation, or predominance of one
• Urgency
• Incomplete evacuation
• Symptoms often triggered or worsened by stress, hormonal changes, and specific foods

Diagnosis: Based on symptom criteria (Rome IV: recurrent abdominal pain at least one day per week for three months, associated with defecation or change in stool frequency/form), after ruling out structural conditions.

Treatment: Dietary modification (low-FODMAP diet is first-line), stress management, gut-directed hypnotherapy, and targeted medications (antispasmodics, laxatives, loperamide depending on subtype). No cure, but symptoms are very manageable for most people.

IBD: Inflammatory Bowel Disease

IBD is a group of autoimmune and inflammatory conditions that cause actual structural damage to the digestive tract. The two main types are Crohn’s disease and ulcerative colitis.

Crohn’s disease

Can affect any part of the digestive tract from mouth to anus, though it most commonly affects the terminal ileum (end of the small intestine). Inflammation is transmural (affects all layers of the gut wall) and can be patchy. Associated with fistulas, abscesses, and strictures.

Ulcerative colitis

Affects only the colon and rectum. Inflammation is continuous (no skip lesions) and limited to the innermost layer (mucosa). Rectal bleeding is a hallmark symptom.

Common IBD symptoms:

• Rectal bleeding (especially ulcerative colitis)
• Persistent diarrhoea (often with blood or mucus)
• Significant unintentional weight loss
• Fatigue
• Fever during flares
• Abdominal pain that doesn’t clearly relate to meals
• Extraintestinal symptoms: joint pain, skin rashes (erythema nodosum), eye inflammation (uveitis)

Key difference from IBS: IBD causes measurable inflammation. Blood tests show elevated CRP, ESR, and faecal calprotectin. Endoscopy shows visible mucosal damage, ulceration, or bleeding. These markers distinguish IBD from IBS where tests are normal.

Diagnosis: Colonoscopy with biopsy, cross-sectional imaging, and blood/stool biomarkers.

Treatment: Immunosuppressants, corticosteroids, biologics (anti-TNF agents, integrin inhibitors, IL-12/23 inhibitors), and sometimes surgery. Ongoing management by a gastroenterologist is essential.

SIBO: Small Intestinal Bacterial Overgrowth

SIBO occurs when excessive bacteria colonise the small intestine — a region that should normally have relatively few bacteria compared to the colon. These bacteria ferment carbohydrates before they’re properly absorbed, producing hydrogen and/or methane gas.

What’s actually happening in SIBO: The small intestine relies on several protective mechanisms to stay relatively bacteria-free: acid secretion, bile, immunoglobulins, and the migrating motor complex (MMC — a pattern of contractions that sweeps the gut clean between meals). When any of these fail — from low stomach acid, motility disorders, anatomical abnormalities, or prior surgery — bacterial counts rise.

Common SIBO symptoms:

• Bloating that starts quickly after eating (within 30–90 minutes, rather than the 4–6 hours of colonic fermentation)
• Distension that worsens throughout the day
• Excessive gas, belching, or flatulence
• Diarrhoea (hydrogen-dominant SIBO) or constipation (methane-dominant)
• Brain fog, fatigue, and nutrient deficiency (particularly B12, fat-soluble vitamins) in chronic cases

Key difference from IBS: SIBO has a structural cause (bacterial overgrowth in the wrong location), produces early-onset bloating (rather than 4–6 hours post-meal), and responds to antibiotics — not just dietary changes. Many people diagnosed with IBS who don’t respond to standard treatment may have undiagnosed SIBO.

Diagnosis: Hydrogen/methane breath testing. Some practitioners also use small intestinal aspirate and culture (more definitive but invasive).

Treatment: Antibiotics (rifaximin, neomycin depending on gas type). Dietary support (low-FODMAP, low-fermentation diets during treatment). Treating underlying predisposing factors is critical to prevent relapse.

How they overlap — and why it matters

These conditions frequently coexist. People with IBD have significantly higher rates of SIBO. IBS and SIBO overlap substantially — some researchers estimate that 30–80% of IBS patients may have SIBO, though diagnostic criteria and testing methods vary widely.

The practical implication: if you’re following a low-FODMAP diet and managing stress but still experiencing significant symptoms — particularly early-onset bloating and gas — SIBO testing is worth discussing with your GP or gastroenterologist.

Getting the right diagnosis is the fastest path to the right treatment.