Malabsorption

Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal (GI) tract.

Impairment can be of single or multiple nutrients depending on the abnormality. This may lead to malnutrition and a variety of anaemias.

Classification
Some prefer to classify malabsorption clinically into three basic categories:
 * (1) selective, as seen in lactose malabsorption.
 * (2) partial, as observed in a-Beta-lipoproteinaemia.
 * (3) total as in coeliac disease.

Pathophysiology
The main purpose of the gastrointestinal tract is to digest and absorb nutrients (fat, carbohydrate, protein, and fiber), micronutrients (vitamins and trace minerals), water, and electrolytes. Digestion involves both mechanical and enzymatic breakdown of food. Mechanical processes include chewing, gastric churning, and the to-and-fro mixing in the small intestine. Enzymatic hydrolysis is initiated by intraluminal processes requiring gastric, pancreatic, and biliary secretions. The final products of digestion are absorbed through the intestinal epithelial cells.

Malabsorption constitutes the pathological interference with the normal physiological sequence of digestion (intraluminal process), absorption (mucosal process) and transport (postmucosal events) of nutrients.

Intestinal malabsorption can be due to:
 * Mucosal damage (enteropathy)
 * Congenital or acquired reduction in absorptive surface
 * Defects of specific hydrolysis
 * Defects of ion transport
 * Pancreatic insufficiency
 * Impaired enterohepatic circulation

Clinical features
They can occur in a variety of ways and features might give a clue to the underlying condition. Symptoms can be intestinal or extra-intestinal - the former predominates in severe malabsorption.


 * Diarrhoea, often steatorrhoea is the most common feature. Watery, diurnal and nocturnal, bulky, frequent stools are the clinical hallmark of overt malabsorption.  It is due to impaired water, carbohydrate and electrolyte absorption or irritation from unabsorbed fatty acid. Latter also results in bloating, flatulence and abdominal discomfort.  Cramping pain usually suggests obstructive intestinal segment e.g. in Crohn's disease, especially if it persists after defecation.
 * Weight loss can be significant despite increased oral intake of nutrients.
 * Growth retardation, failure to thrive, delayed puberty in children
 * Swelling or oedema from loss of protein
 * Anaemias, commonly from vitamin B12, folic acid and iron deficiency presenting as fatigue and weakness.
 * Muscle cramp from decreased vitamin D, calcium absorption. Also lead to osteomalacia and osteoporosis
 * Bleeding tendencies from vitamin K and other coagulation factor deficiencies.

Diagnosis
There is no specific test for malabsorption. As for most medical conditions, investigation is guided by symptoms and signs. A range of different conditions can produce malabsorption and it is necessary to look for each of these specifically. Many tests have been advocated, and some, such as tests for pancreatic function are complex, vary between centers and have not been widely adopted. However, better tests have become available with greater ease of use, better sensitivity and specificity for the causative conditions. Test are also needed to detect the systemic effects of deficiency of the malabsorbed nutrients (such as anaemia with vitamin B12 malabsorption).

Blood tests

 * Routine blood tests may reveal anaemia, high CRP or low albumin; which shows a high correlation for the presence of an organic disease. In this setting, microcytic anaemia usually implies iron deficiency and macrocytosis can be caused by impaired folic acid or B12 absorption or both. Low cholesterol or triglyceride may give a clue toward fat malabsorption. Low calcium and phosphate may give a clue toward osteomalacia from low vitamin D.
 * Specific vitamins like vitamin D or micro nutrient like zinc levels can be checked. Fat soluble vitamins (A, D, E & K) are affected in fat malabsorption. Prolonged prothrombin time can be caused by vitamin K deficiency.
 * Serological studies
 * Specific tests are carried out to determine the underlying cause.
 * IgA Anti-transglutaminase antibodies or IgA Anti-endomysial antibodies for Coeliac disease(gluten sensitive enteropathy).

Stool studies

 * Microscopy is particularly useful in diarrhoea, may show protozoa like Giardia, ova, cyst and other infective agents.
 * Fecal fat study to diagnose steatorrhoea is rarely performed nowadays.
 * Low fecal pancreatic elastase is indicative of pancreatic insufficiency. Chymotrypsin and pancreolauryl can be assessed as well

Radiological studies

 * Barium follow through is useful in delineating small intestinal anatomy. Barium enema may be undertaken to see colonic or ileal lesions.
 * CT abdomen is useful in ruling out structural abnormality, done in pancreatic protocol when visualising pancreas.
 * Magnetic resonance cholangiopancreatography (MRCP) to complement or as an alternative to ERCP

Interventional studies

 * OGD to detect duodenal pathology and obtain D2 biopsy (for coeliac disease, tropical sprue, Whipple's disease, abetalipoproteinaemia etc.)
 * Enteroscopy for enteropathy and jejunal aspirate and culture for bacterial overgrowth
 * Capsule Endoscopy is able to visualise the whole small intestine and is occasionally useful.
 * Colonoscopy is necessary in colonic and ileal disease.
 * ERCP will show pancreatic and biliary structural abnormalities.

Other investigations

 * 75SeHCAT test to diagnose bile acid malabsorption in ileal disease or primary bile acid diarrhea.
 * Glucose hydrogen breath test for bacterial overgrowth
 * Lactose hydrogen breath test for lactose intolerance
 * Sugar probes or 51Cr-EDTA to determine intestinal permeability.

Obsolete tests no longer used clinically

 * D-xylose absorption test for mucosal disease or bacterial overgrowth. Normal in pancreatic insufficiency.
 * Bile salt breath test (14C-glycocholate) to determine bile salt malabsorption.
 * Schilling test to establish cause of B12 deficiency.

Management
Treatment is directed largely towards management of underlying cause:
 * Replacement of nutrients, electrolytes and fluid may be necessary. In severe deficiency, hospital admission may be required for parenteral administration, often advice from dietitian is sought. People whose absorptive surface are severely limited from disease or surgery may need long term total parenteral nutrition.
 * Pancreatic enzymes are supplemented orally in insufficiencies.
 * Dietary modification is important in some conditions:
 * Gluten-free diet in coeliac disease.
 * Lactose avoidance in lactose intolerance.
 * Antibiotic therapy will treat Small Bowel Bacterial overgrowth.
 * Cholestyramine or other bile acid sequestrants will help reducing diarrhoea in bile acid malabsorption.

Jejunostomy is a surgical procedure done to allow placement of feeding tube as one treatment option for a variety of causes of malabsorption.