IBS Is Not One Condition

Irritable Bowel Syndrome affects 10-15% of the global population, making it one of the most common diagnoses in gastroenterology. But IBS is not a single, uniform condition. The Rome IV criteria — the international diagnostic standard — classify IBS into subtypes based on the predominant stool pattern, because each subtype has different underlying mechanisms and responds to different treatments.

Identifying your subtype is the first step toward effective management.

IBS-D: Diarrhoea-Predominant

In IBS-D, more than 25% of bowel movements are loose or watery (Bristol Stool Types 6-7), and fewer than 25% are hard or lumpy.

Typical Presentation

  • Urgent need to have a bowel movement, often immediately after eating (gastrocolic reflex hyperactivity)
  • Multiple loose stools per day, particularly in the morning
  • Abdominal cramping that is temporarily relieved by bowel movements
  • Anxiety about access to bathrooms, which can significantly restrict daily activities
  • Mucus in stool (without blood)

Common Underlying Mechanisms

IBS-D is often associated with:

  • Post-infectious IBS — approximately 10-15% of people who experience food poisoning develop IBS-D due to anti-vinculin antibodies that impair the migrating motor complex
  • Bile acid malabsorption — affects up to 30% of IBS-D patients; excess bile acids in the colon draw water into the lumen and accelerate transit
  • Hydrogen-dominant SIBO — bacterial overgrowth that accelerates small intestinal transit
  • Visceral hypersensitivity — lowered pain threshold in the gut, causing normal stimuli to be perceived as painful
If you developed IBS-D symptoms within 6 months of a bout of food poisoning or gastroenteritis, mention this to your doctor — post-infectious IBS has specific diagnostic markers (anti-CdtB and anti-vinculin antibodies) and treatment implications.

IBS-C: Constipation-Predominant

In IBS-C, more than 25% of bowel movements are hard or lumpy (Bristol Stool Types 1-2), and fewer than 25% are loose.

Typical Presentation

  • Infrequent bowel movements (often fewer than 3 per week)
  • Straining, incomplete evacuation, and sensation of rectal blockage
  • Bloating that is often constant and does not fully resolve after bowel movements
  • Abdominal distension that worsens throughout the day
  • Nausea and reduced appetite

Common Underlying Mechanisms

  • Methane-dominant overgrowth (IMO) — methane directly slows colonic transit by up to 59%. This is a treatable cause of IBS-C
  • Pelvic floor dysfunction — dyssynergic defaecation where the pelvic floor muscles contract rather than relax during attempted bowel movements
  • Slow transit constipation — reduced colonic motility often related to impaired serotonin signalling in the gut
  • Inadequate fibre or fluid intake — though this is often overemphasised; many IBS-C patients worsen with insoluble fibre supplementation

IBS-M: Mixed Type

In IBS-M (also called IBS-A for "alternating"), more than 25% of bowel movements are both hard/lumpy AND loose/watery. Patients alternate between constipation and diarrhoea, sometimes within the same day.

Typical Presentation

  • Unpredictable bowel patterns that cycle between constipation and diarrhoea
  • Symptoms that are hardest to manage because they shift
  • Often the most distressing subtype due to the unpredictability
  • May represent overlap between hydrogen and methane gas types

Why Subtyping Matters for Treatment

Treatment approaches differ significantly between subtypes:

IBS-D Management

  • Low-FODMAP diet (particularly reducing fructans and excess fructose)
  • Soluble fibre (psyllium husk) to add bulk to loose stools
  • Bile acid sequestrants (cholestyramine) if bile acid malabsorption is confirmed
  • Rifaximin for post-infectious IBS and hydrogen SIBO
  • Gut-directed hypnotherapy for visceral hypersensitivity

IBS-C Management

  • Methane-targeted antimicrobials if IMO is confirmed on breath testing
  • Osmotic laxatives (polyethylene glycol) for stool softening
  • Prokinetic agents to improve colonic transit
  • Pelvic floor physiotherapy if dyssynergic defaecation is diagnosed
  • Soluble fibre (not insoluble, which can worsen bloating)

IBS-M Management

  • Comprehensive breath testing to identify both hydrogen and methane components
  • Dietary management that avoids extremes of both restriction and fibre loading
  • Gut-directed psychological therapies, which have the broadest evidence across all subtypes
  • Antispasmodics for pain management

GutIQ's assessment evaluates bowel pattern, symptom timing, and associated factors to help identify your likely IBS subtype and the mechanisms that may be driving it. This targeted approach is far more effective than generic IBS advice.