IBS and IBD

IBS-D

Also called: IBS with diarrhoea, diarrhoea-predominant IBS

IBS-D is the diarrhoea-predominant subtype of irritable bowel syndrome. Loose or watery stools more than 25 percent of the time, hard stools less than 25 percent, with abdominal pain. About a third of IBS sits in this subtype. A meaningful portion of IBS-D is actually bile acid diarrhoea, which is treatable with bile acid binders. Worth ruling out before assuming it's IBS.

How IBS-D is diagnosed

Rome IV criteria apply: recurrent abdominal pain at least 1 day a week for 3 months, related to defecation, with mostly loose stools (Bristol types 6 to 7) and rarely hard. Coeliac, inflammatory bowel disease, microscopic colitis, and bile acid diarrhoea need to be ruled out with blood tests, faecal calprotectin, and a SeHCAT scan or colonoscopy if red flags are present.

What it feels like

  • Urgency, often within 30 minutes of eating.
  • Loose, watery or explosive stools, sometimes with mucus.
  • Abdominal pain that improves after going.
  • Bloating, often more in the lower abdomen.
  • More than 3 bowel movements a day on bad days.
  • Symptoms can be triggered by specific foods, stress, or illness.

Bile acid diarrhoea, the most missed cause

Up to a third of people diagnosed with IBS-D actually have bile acid diarrhoea (BAD). The liver makes bile acids, releases them into the small intestine to digest fats, and the small intestine reabsorbs about 95 percent. In BAD that reabsorption is broken, and excess bile acids reach the colon and pull in water. The classic clue is morning urgency and loose stools that ease through the day, often worse after fatty meals. The test is a SeHCAT scan. The treatment is a bile acid binder (colestyramine, colesevelam) which often works dramatically.

What helps if it's true IBS-D

  1. Try a 4-week low-FODMAP elimination under a dietitian. Around 70 percent of IBS patients improve overall, with the strongest signal in the diarrhoea-predominant subtype.
  2. Eat smaller, more frequent meals to reduce gastrocolic reflex.
  3. Limit caffeine, alcohol and fatty meals during flares.
  4. Soluble fibre (oats, psyllium) helps form stools. Insoluble fibre (wheat bran) often worsens.
  5. Loperamide before activities that need predictability. Use sparingly.
  6. Antispasmodics (mebeverine, peppermint oil) for the pain.
  7. If basics fail: consider rifaximin under specialist care for SIBO overlap, or low-dose tricyclics for gut-brain calming.

Red flags worth a GP visit

  • Blood in stool or stools that look black and tarry.
  • Unintentional weight loss.
  • Symptoms that wake you at night.
  • New IBS-D after age 50.
  • Family history of bowel cancer or inflammatory bowel disease.
  • Iron-deficiency anaemia on a blood test.

Common questions

Is IBS-D the same as IBD?
No. IBS-D is a functional disorder; the gut looks normal under a camera. IBD (Crohn's or ulcerative colitis) is inflammation that shows on biopsy and faecal calprotectin. They share symptoms but are very different conditions.
Can stress cause IBS-D?
Stress amplifies IBS-D, but there's almost always an underlying gut sensitivity. The vagus nerve and HPA axis directly affect motility and pain sensitivity. Cognitive behavioural therapy, gut-directed hypnotherapy, and stress reduction all have evidence for IBS-D.
Why does my IBS-D get worse after coffee?
Coffee speeds up colonic motility, the gastrocolic reflex effect. For people with already-sensitive guts, this triggers urgency. Decaffeinated coffee also stimulates motility (research shows both regular and decaf trigger gut motility within minutes; hot water doesn't), so caffeine isn't the only factor; switching to decaf may not eliminate the urgency.
Should I get tested for SIBO?
Worth considering if FODMAP elimination doesn't help and stools are very loose with strong post-meal bloating. The hydrogen breath test is the standard. Treatment is rifaximin (sometimes plus diet), accessed through gastroenterology.

Sources