View source for 7. Specific malabsorption syndromes (level or substrate of disorder)

After the food has been digested into small particles by luminal digestion (pancreatic) and surface digestion (brush border enzymes) can the particles be absorbed. Particles get into the luminal surface of the villi through the brush border by diffusion, facilitated diffusion, active transport or endocytosis.

Absorption mainly takes place at the tip of the villi while secretion occurs at their base. Recall from earlier that the countercurrent circulation of the villi means that the tips of the villi are the most sensitive to problems with the circulation. This means that in any case of circulatory deficiency, like hypoxia, hypoperfusion or shock, will always affect absorption before it affects secretion.

Glucose and galactose are absorbed by active transport while fructose is absorbed by diffusion. Oligosaccharidases like lactase, maltase and sucrase are needed to break down and absorb these oligosaccharides.

L-amino acids and oligopeptides are absorbed by active transport. Enteropeptidase is a brush-border enzyme that actives trypsinogen into trypsin.

Fatty acids are absorbed by active transport while glycerol is absorbed by diffusion.

== Malabsorption ==
Malabsorption may occur secondary to maldigestion as improperly digested food can’t be absorbed properly.

A malabsorption ''syndrome'' is a condition where the malabsorption is the leading or only symptom.

Malabsorption can be categorized according to the cause:

* Gastric
* Pancreatic
* Hepatobiliary
* Intestinal

It can also be categorized depending on which nutrient is affected:

* Substrate-specific malabsorption syndrome – a single substrate or nutrient group is affected
* Complex malabsorption syndrome – all nutrient groups are affected

According to these classifications is lactose intolerance an intestinal substrate-specific malabsorption syndrome while enteritis is an intestinal complex malabsorption syndrome.

The causes of malabsorption are:

* Maldigestion
** Gastrectomy
** Enzyme deficiencies
* Decreased bile acid concentration
** Liver disease
** Antibiotics
** Disorder of the enterohepatic circulation
* Decrease of absorptive surface
** Gut resection
** Bypass surgery
* Lymphatic obstruction
* Cardiovascular disorders
** Mesenteric congestion
** Ischaemia
* Primary mucosal damage
** Inflammation
** Coeliac disease
** Salmonella
* Endocrine/metabolic disorders
** Diabetes
** Hypothyroidism
** Addison’s disease

Let’s look at some substrate-specific malabsorptions.

=== Substrate-specific malabsorptions ===
Carbohydrate malabsorption: This occurs in cases where the enzymes needed for the absorption of carbohydrates are deficient. Oligosaccharidases may be deficient not only due to genetics (primary) but also due to environmental factors (secondary) like gastroenteritis, inflammatory bowel disease and other diseases that affect the brush border. When carbohydrates go undigested through the <abbr>GI</abbr> tract will bacteria digest them, producing hydrogen gas and osmotic diarrhoea.

Protein malabsorption: Protein-specific malabsorption is rare. Enteropeptidase deficiency exists. In chronic pancreatitis is there maldigestion of proteins, however that’s not a protein-''specific'' malabsorption. Bacteria feed on unabsorbed protein to produce a putrid and not-so-watery diarrhoea. Hypoproteinaemia may occur. Methane gas is produced.

Fat malabsorption: Pancreatic lipase deficiency leads to impaired absorption of long-chain fatty acids and steatorrhea but normal absorption of cholesterol, bile acids and fat-soluble vitamins.

Bile deficiency decreases not only fat absorption but the absorption of all fat-soluble substances. Fat remaining in the gut will form soap with Ca2+, which increases the absorption of oxalate.

95% of bile salts are usually reabsorbed as part of the enterohepatic circulation. If the bile acid absorption is deficient will a lot of bile acids reach the colon which stimulates colonic secretions, causing diarrhoea.

=== Consequences ===
The consequences occur either because of the decreased absorption of the substrate in question or because of the presence of the unabsorbed substrate in the large intestine.

Bacteria react differently to different kinds of substrates; a common feature is that osmotically active particles are produced, which retain water and cause osmotic diarrhoea. Gases are also produced, which may cause meteorism, where the abdomen is distended due to the gas content of the bowels. Other consequences include:

* Protein deficiency
* Weight loss
* Hypovitaminoses
* Anemia and bleeding tendencies
* Water loss, salt loss, hypokalemia and hypocalcemia

[[Category:Pathophysiology 2]]