Malabsorption

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Malabsorption is manifest as weight loss, abdominal distension, and loose, bulky stools.
If fat is not being absorbed, stools are pale, foul-smelling and characteristically float in water.
Anaemia is common.

The small intestine provides an environment for absorption of nutrients from food.
There are four main elements to absorption:
The pancreas secretes digestive enzymes into the gut lumen, which are necessary for breakdown of
macromolecules.
The liver secretes bile acids needed for solubilization and absorption of fats.
The mucosa is specialized for absorption: transverse
mucosal folds and finger-like villi provide a vast surface
area.
The mucosa is the site of a set of enzymes, located on the brush border, which hydrolyse large molecules
for absorption, especially complex sugars (e.g. sucrase and lactase).

Absence of any of these elements leads to impaired digestion of food and malabsorption.
This manifests as weight loss, abdominal distension, and loose, bulky stools.
If fat is not being absorbed, stools are pale, foul-smelling and characteristically float in water.
Anaemia is common.
If the pancreas fails to produce or secrete the hydrolytic enzymes responsible for food
breakdown, nutrients are not absorbed.
The most common causes are cystic fibrosis, chronic pancreatitis, carcinoma of the pancreas,
and pancreatic surgery.

The most important small intestinal cause of malabsorption in the Western World is
coeliac disease; in the Third World it is probably severe parasitic and worm infestation.
Loss of absorptive surface of the bowel is common and is due to a number of diseases.
Extensive resection of the small bowel (e.g. for infarction or Crohn's disease) reduces
the absorptive area markedly. Crohn's disease, if very severe, leads to extensive submucosal
oedema and mucosal flattening, which reduces the surface area for absorption. Disease of the
terminal ileum prevents absorption of vitamin B12.
If secretion of bile into the gut is impaired, solubilization of fats cannot take place, resulting
in impaired fat absorption. This is also manifest in malabsorption of fat-soluble vitamins A, D, E, and K.
Damage to enterocytes in the bowel leads to replacement by cells that lack differentiation
and fail to express brush-border enzymes such as lactase or sucrase (disaccharidase deficiency).
Disaccharides in the diet cannot be broken down, and their presence in the gut causes diarrhoea,
as well as bacterial overgrowth. This may become manifest after an episode of infective diarrhoea.
Congenital lack of disaccharidases is also seen in infants, in the absence of mucosal
damage, and is a cause of failure to thrive.

Whipple's disease is a rare but important cause of malabsorption, in which the mucosa
is infiltrated by macrophages full of infective actinomycete organisms.
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