Bile acid diarrhoea

Overview

Bile acid diarrhoea (BAD), also called bile acid malabsorption, is a condition in which excess bile acids reach the large bowel, causing chronic watery diarrhoea, urgency and sometimes incontinence. Bile acids are made in the liver to help digest fats and are normally reabsorbed in the last part of the small intestine.

BAD is surprisingly common and often misdiagnosed as irritable bowel syndrome with diarrhoea. It can occur on its own or after bowel surgery, gallbladder removal, Crohn's disease, radiotherapy or other conditions. Once recognised, it usually responds well to medicines called bile acid sequestrants, which bind bile acids in the gut.

Many people with bile acid diarrhoea live with symptoms for years, sometimes avoiding social events or long journeys because of urgency. Getting the right diagnosis can make a big difference, as treatment often works within days.

Causes and risk factors

• Primary (idiopathic) BAD: no obvious cause; thought to be due to overproduction of bile acids
• Secondary BAD:
• Crohn's disease affecting the terminal ileum
• Surgery removing the terminal ileum
• Gallbladder removal (cholecystectomy)
• Pelvic radiotherapy
• Coeliac disease, microscopic colitis and small intestinal bacterial overgrowth
• Chronic pancreatitis
• Diabetes medicines such as metformin

How it develops (pathophysiology)

Normal bile acid circulation

Bile acids are made from cholesterol in the liver, stored in the gallbladder and released into the small intestine after meals, where they help digest and absorb fats. About 95 percent are reabsorbed in the terminal ileum (the last part of the small intestine) by a specialised transporter (ASBT) and returned to the liver through the portal vein. This cycle, the enterohepatic circulation, happens several times a day.

When bile acids are reabsorbed in the ileum, they stimulate ileal cells to release a hormone called fibroblast growth factor 19 (FGF19), which travels to the liver and tells it to make fewer bile acids. This keeps production balanced.

What goes wrong

1. Excess bile acids reach the colon, through either: - Reduced reabsorption: disease or removal of the terminal ileum prevents bile acids from being reabsorbed. - Overproduction: in primary BAD, low FGF19 signalling means the liver is not told to slow down, so it makes too many bile acids, overwhelming reabsorption.
2. Effects on the colon: in the colon, bile acids, particularly chenodeoxycholic acid and deoxycholic acid: - Stimulate the colon lining to secrete water and salts - Increase colonic movement (motility) - Increase mucosal permeability
3. Diarrhoea: the result is watery stools, urgency and frequent bowel movements.
4. Fat malabsorption: if bile acid loss is large, there may not be enough bile acids to absorb fats, causing fatty stools (steatorrhoea).

Why the symptoms happen

• Watery diarrhoea and urgency result from water secretion and increased colonic movement.
• Bloating and abdominal pain reflect gut irritation.
• Faecal incontinence occurs with urgency.
• Pale, greasy stools may occur with fat malabsorption.
• Symptoms worse after fatty meals, which trigger bile release.

How treatment acts on the mechanism

Bile acid sequestrants (colestyramine, colesevelam, colestipol) bind bile acids in the gut, preventing them from irritating the colon, so they pass out in stools. Treating the underlying cause, such as Crohn's disease, reduces bile acid loss. A lower-fat diet reduces bile release.

Types

• Type 1: secondary to ileal disease or resection
• Type 2: primary (idiopathic)
• Type 3: secondary to other conditions, such as cholecystectomy or coeliac disease

Signs and symptoms

• Chronic watery diarrhoea
• Urgency to open the bowels
• Frequent bowel movements, sometimes at night
• Bloating and wind
• Abdominal cramps
• Faecal incontinence
• Pale, greasy or foul-smelling stools in some cases

Diarrhoea is often worse in the morning and after meals, especially fatty ones. Some people have urgent diarrhoea several times a day, while others have milder, intermittent symptoms.

How it is diagnosed

• SeHCAT scan: a capsule containing a radioactive bile acid is swallowed, and retention is measured after seven days; low retention indicates BAD
• Serum C4 (7-alpha-hydroxy-4-cholesten-3-one) blood test, where available
• Faecal bile acid measurement in some centres
• Trial of a bile acid sequestrant in places without access to tests
• Tests to exclude other causes: coeliac disease, inflammatory bowel disease, infections, thyroid disease

Goals of treatment

Treatment aims to control diarrhoea, improve quality of life and treat underlying causes.

Medicines

• Colestyramine (powder mixed with water)
• Colesevelam (tablets), often better tolerated
• Colestipol
• Loperamide to reduce bowel movements
• Treatment of underlying causes, such as Crohn's disease
• Fat-soluble vitamin supplements if deficient

Bile acid sequestrants can cause constipation, bloating and an unpleasant taste or texture. Starting with a low dose and adjusting it helps. They should be taken at least one hour before or four hours after other medicines.

Other treatments

• Lower-fat diet, eating smaller meals
• Dietitian support

Living with it and prevention

• Take bile acid sequestrants separately from other medicines, as they can reduce their absorption
• Follow a lower-fat diet if it helps
• Report new or worsening symptoms

Keeping a food and symptom diary can help identify foods that make diarrhoea worse.

Monitoring and follow-up

Response to treatment is reviewed. Long-term use of sequestrants may need monitoring of vitamin levels.

Outlook

Most people with BAD improve significantly with treatment, though long-term medicine is often needed.

Many people find their symptoms are well controlled once the cause is recognised, and quality of life often improves a great deal. Keeping a food and symptom diary, reviewing fat intake with a dietitian and having regular check-ups help keep the condition under control.

Special considerations

• People with IBS-type diarrhoea: up to a third may have BAD
• After gallbladder removal: diarrhoea may be caused by BAD
• After pelvic radiotherapy: BAD is a common cause of diarrhoea