Malabsorption: Causes, Investigation and Management

Introduction

Malabsorption describes the impaired absorption of one or more nutrients across the small intestinal mucosa. It is helpful to distinguish it from maldigestion, the failure to break food down into absorbable components, although the two frequently coexist and produce a similar clinical picture, so they are usually considered together.¹

Normal absorption requires three things to work: adequate luminal digestion (pancreatic enzymes and bile salts), an intact mucosal absorptive surface, and functioning transport away from the gut via lymphatics and portal blood. Malabsorption results from failure at any of these three stages, and this framework is the most useful way to structure both the differential diagnosis and the investigation.

Causes

In UK practice, coeliac disease, chronic pancreatitis, Crohn's disease, small intestinal bacterial overgrowth and bile acid malabsorption account for the majority of cases presenting to gastroenterology.

Clinical features

Gastrointestinal features

The classic presentation is steatorrhoea: pale, bulky, greasy, offensive-smelling stools that float and are difficult to flush, reflecting unabsorbed fat. Other features include chronic diarrhoea, abdominal bloating and distension, excessive flatus, and abdominal discomfort.

Weight loss despite an adequate or even increased appetite and food intake is characteristic and an important discriminator from simple reduced intake.

Features of specific deficiencies

Much of the clinical picture comes from the resulting deficiency states, and the pattern helps localise the lesion:

Additional clues in the history include a family history of coeliac disease, a history of alcohol excess or recurrent pancreatitis, previous bowel or bariatric surgery, foreign travel (tropical sprue, giardiasis), and coexisting autoimmune disease.

Investigations

Establishing that malabsorption is present

• Full blood count and blood film: anaemia, which may be microcytic, macrocytic or dimorphic
• Haematinics: ferritin, vitamin B12 and folate
• Urea and electrolytes, calcium, magnesium, phosphate and vitamin D
• Liver function tests and albumin: hypoalbuminaemia reflects protein loss or malnutrition
• Coagulation screen: a prolonged prothrombin time reflects vitamin K deficiency
• Faecal fat estimation is rarely performed now, having been superseded by more practical tests

Identifying the cause

• Coeliac serology: IgA tissue transglutaminase antibodies with total IgA, in every patient - and remember the patient must be eating gluten for the test to be valid
• Faecal elastase: the practical test for pancreatic exocrine insufficiency; a low level indicates reduced enzyme output
• Faecal calprotectin: raised in inflammatory bowel disease, normal in coeliac disease and functional disorders
• Stool microscopy and culture, including for Giardia, particularly with relevant travel history
• Hydrogen breath testing: for small intestinal bacterial overgrowth (glucose or lactulose breath test) and for lactose intolerance
• SeHCAT scan: for bile acid malabsorption, particularly with watery diarrhoea after cholecystectomy or ileal resection
• Thyroid function, HIV testing and immunoglobulins where clinically indicated

Endoscopy and imaging

• Upper GI endoscopy with duodenal biopsies: for coeliac disease and other causes of villous atrophy, and to sample duodenal aspirate for Giardia
• Colonoscopy with ileal intubation and biopsies: for Crohn's disease and microscopic colitis
• MR enterography: for small bowel Crohn's disease, strictures and fistulae
• CT abdomen: for pancreatic disease, including calcification of chronic pancreatitis and pancreatic malignancy
• Capsule endoscopy: for suspected small bowel pathology not identified by other means

Management

Treat the underlying cause

Definitive management is directed at the specific diagnosis, and effective treatment of the cause usually reverses the malabsorption:

• Coeliac disease: lifelong strict gluten-free diet
• Chronic pancreatitis or pancreatic insufficiency: pancreatic enzyme replacement therapy (e.g. Creon) taken with meals and snacks, together with a proton pump inhibitor to reduce acid degradation of the enzymes²
• Crohn's disease: induction and maintenance of remission with steroids, immunomodulators and biologics
• Small intestinal bacterial overgrowth: a course of antibiotics such as rifaximin, and treatment of any underlying motility disorder or blind loop
• Bile acid malabsorption: a bile acid sequestrant such as colestyramine or colesevelam
• Giardiasis: metronidazole or tinidazole
• Lactose intolerance: lactose restriction, or lactase supplementation

Nutritional support and replacement

Replace identified deficiencies: iron, vitamin B12 (parenterally if ileal absorption is impaired), folate, calcium and vitamin D, and the other fat-soluble vitamins A, E and K. Magnesium and zinc are replaced where deficient. Bone density should be assessed in patients with prolonged fat malabsorption, given the risk of osteomalacia and osteoporosis.

Dietitian involvement is essential. Depending on the cause, this may involve a gluten-free diet, a low-fat diet with medium-chain triglyceride supplementation (which is absorbed directly into the portal circulation without requiring bile salts or lymphatics), oral nutritional supplements, or in severe cases enteral or parenteral nutrition. Patients with short bowel syndrome require specialist intestinal failure input and may need long-term parenteral nutrition.

Complications

• Malnutrition, cachexia and sarcopenia, with impaired wound healing and immunity
• Anaemia, which may be microcytic, macrocytic or mixed
• Metabolic bone disease: osteomalacia and osteoporosis with fragility fractures
• Neurological sequelae: peripheral neuropathy and subacute combined degeneration from B12 deficiency, ataxia from vitamin E deficiency
• Coagulopathy from vitamin K deficiency
• Oedema and ascites from hypoalbuminaemia
• Growth failure and delayed puberty in children
• Renal oxalate stones in fat malabsorption, where unabsorbed fatty acids bind calcium, leaving oxalate free to be absorbed and excreted
• Gallstones, particularly with terminal ileal disease and bile salt loss
• Refeeding syndrome if nutrition is reintroduced too rapidly in a severely malnourished patient - monitor and replace phosphate, potassium and magnesium

Red flags

Prognosis

Prognosis depends almost entirely on the underlying cause. Where the cause is treatable - coeliac disease, giardiasis, bacterial overgrowth, bile acid malabsorption, or pancreatic insufficiency managed with enzyme replacement - symptoms and nutritional status usually improve substantially, deficiencies correct with replacement, and the long-term outlook is good.¹

Outcomes are less favourable where malabsorption results from irreversible structural loss, such as extensive short bowel syndrome or advanced chronic pancreatitis, in which case management becomes long-term nutritional support rather than cure. Malabsorption secondary to malignancy carries the prognosis of the underlying tumour. Across all causes, early recognition matters, because prolonged deficiency produces complications - particularly metabolic bone disease and neurological damage from B12 deficiency - that may not fully reverse even after the underlying cause is treated.