6. Utilization of nutrients and its disorders. Maldigestions. Age-dependent features of nutrient utilization

Page created on February 10, 2019. Last updated on May 15, 2020 at 21:24

Summary

The most important disorders of digestions are lactose intolerance and chronic pancreatitis. In lactose intolerance the enzyme lactase is deficient. Lactose will instead be broken down by intestinal bacteria, causing diarrhoea and gas.

Pancreas produces most of the enzymes responsible for digestion, so chronic pancreatitis leads to maldigestion.

Pancreatic secretion

Digestion doesn’t depend too much on the gastric juice and oral amylase, but it definitely depends on the pancreatic juice. The exocrine pancreas produces the fundamental enzymes of digestion. It produces 1.5 – 2 litres of juice every day which contains bicarbonate to neutralize the stomach acid. It also contains various electrolytes.

The pancreatic enzymes digest the food but not to such small pieces that they can be absorbed as they are. Cell -bound enzymes must further digest the particles until they become small enough to be absorbed.

Digestion of lipids depend on the presence of bile acids that can emulsify the lipids. Without this emulsification is absorption of both lipids and lipid-soluble vitamins impossible.

Pathophysiology of pancreatic secretion

Pancreatic secretion is deficient acutely in acute pancreatitis and febrile infections.

Chronic deficiency of pancreatic secretions is seen in:

• Sclerosis of pancreatic vessels
• Protein deficiency
• Cachexia
• Toxic injury of the pancreas
• Chronic pancreatitis
• Cystic fibrosis
• Decreased secretin or cholecystokinin levels (somatostatinoma, too low duodenal pH)

Low duodenal pH doesn’t just decrease the pancreatic secretion, it impairs the function of the pancreatic enzymes as well.

No matter the cause will a pancreatic enzyme deficiency lead to generalized malabsorption of fats, carbohydrates and proteins.

Pancreatic hypersecretion occurs in:

• Chronic renal failure (due to decreased clearance of CCK, gastrin, GIP and glucagon)
• VIPoma
• Cirrhosis (secretin clearance is decreased)

Hypersecretion may cause obstruction of the pancreatic ducts as protein precipitate inside. This may cause obstruction with resulting chronic pancreatitis.

Intestinal secretion and digestion

The small intestine also produces juice, 2-3 litres every day. This juice has high K⁺ and bicarbonate content. Its composition is determined by GI hormones.

The small intestine has cell-bound enzymes like oligosaccharidases and oligopeptidases that are located inside the brush border. These enzymes are essential to digest the food particles into small enough particles that can be absorbed.

Lactose intolerance is a good example of what can occur when digestion doesn’t work properly. The disease is characterised by deficiency of lactase. When affected persons consume lactose the intestine will be unable to absorb the lactose, which will instead travel to the large intestine. There will bacteria ferment the lactose, producing particles like lactate, pyruvate, CO₂, and other osmotically active molecules. These molecules cause water to enter the lumen from the interstitium, causing diarrhoea. Lactose intolerance occurs more frequently in older people.

Transient deficiency of surface enzymes is also seen in infections of the intestine, like viral enteritis. This deficiency may contribute to the diarrhoea often seen in these cases.

The large intestine produces nearly no secretion. Secretion may be pathologically large if a large amount of bile acids reaches the colon. This causes diarrhoea even without food intake.

Decreased utilization of nutrients

In some pathological cases can we ingest a nutrient, but it will not be digested and therefore absorbed. We already saw that in pancreatic deficiency will the digestion of all nutrients be impaired.

In gastrectomy will there be no production of intrinsic factor, which makes it impossible to absorb vitamin B₁₂. With gastrectomy does also achlorhydria follow, which impairs iron absorption as the stomach acid normally converts the unabsorbable Fe³⁺ (ferric) iron to the absorbable Fe²⁺ (ferrous) iron.

Bile is essential for digestion and absorption of fat and lipid-soluble vitamins. In hepatobiliary problems where the bile excretion is reduced will there be a fat-malabsorption and malabsorption of lipid-soluble vitamins.