5. Specific malabsorption syndromes (level or substrate of disorder)

Intestinal absorption?

Food has been digested into small particles by:

- Luminal digestion (pancreatic)

- Surface digestion (brush border enzymes)

It can now be absorbed:

- Particles get into the luminal surface of the villi through brush border by diffusion. facilitated diffusion, active transport or endocytosis

Where does absorption occur?

Mainly takes place at the tip of the villi

- The countercurrent circulation of the villi means that the tips are the most sensitive to circulatory problems.

= In any case of circulatory deficiency, like hypoxia, hypoperfusion or shock, the absorption will always be affected before the secretion.

What is absorbed by active transport?

- Glucose and galactose

- L-amino acids

- Oligopeptides

What is absorbed by diffusion?

Fructose

What enzyme activates trypsinogen?

Enteropeptidase (a brush-border enzyme)

How is fat absorbed?

Fatty acids are absorbed by active tranpsport

Glycerol is absorbed by diffusion

How can malabsorption occur?

Can occur due to several factors including mucosal damage, enzyme deficiencies, and altered gut motility.

Conditions such as celiac disease, pancreatic insufficiency, or infections can impair nutrient absorption in the intestines.

How do we categorize malabsorption according to where it occurs?

- Gastric

- Pancreatic

- Hepatobiliary

- Intestinal

How can malabsorption be categorized depending on which nutrient is affected?

1. Substrate-specific malabsorption syndrome - a single substrate or nutrient group is affected

Ex: lactose intolerance

2. Complex malabsorption syndrome - all nutrient groups are affected

Ex: Enteritis

The causes for malabsorption?

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

- Ischemia

- Mesenteric congestion

Primary mucosal damage

- Inflammation

- Celiac disease

- Salmonella

Endocrine/metabolic disorders

- Diabetes

- Hypothyroidism

- Addisons disease

Types of substrate-specific malabsorption?

- Carbohydrate malabsorption

- Protein malabsorption

- Fat malabsorption

Carbohydrate malabsorption?

Occurs when enzymes needed for absorption of carbohydrates are deficient

- Oligosaccharides may deficient genetically (primary)

or

(secondary) to IBD, gastroenteritis & other diseases affecting the brush border

What happens when carbohydrates go undigested through the GI tract?

Bacteria will digest them

- Hydrogen gas is produced

- Osmotic diarrhea

Protein malabsoprtion?

Protein-specific malabsorption is rare

- Enteropeptidase deficiency exists - in chronic pancreatitis there is a maldigestion of proteins, HOWEVER; it is not a protein-specific malabsorption

What happens to the undigested protein in the GI-tract? (protein malabsorption)

Bacteria feed on them

- Produce a putrid diarrhea

- Hypoproteinemia will occur

- Methane gas is produced

Causes of fat malabsorption

Pancreatic lipase deficiency:

• Leads to impaired absorption of long-chain fatty acids

• Leads to steatorrhea

• There is normal absorption of cholesterol, bile acids & fat soluble vitamins

Bile deficiency:

• Decreases absorption of fat and all fat-soluble substances

• Remaining fat in GI will form soap w/ Ca2+ = increasing the absorption of oxalate

Why does malabsorption cause consequences like diarrhea and abdominal distention?

Either due to:

• Due to decreased absorption of substrate

• The presence of unabsorbed substrate in the colon

As bacteria will react differently to the different types of substrate.

Common features:

• Osmotically active particles are produced by the bacteria → retaining water & cause osmotic diarrhea

• Gas are produced → causing abdominal distention

General consequences of malabsorption?

- Protein deficiency

- Weight loss

- Hypovitaminosis

- Anemia & bleeding tendencies

- Loss of water, salt

- Hypokalaemia

- Hypocalcemia