Lipid Digestion

Triglycerides with C16 and C18 fatty acids are of utmost quantitative importance for human nutrition. Whether fat is consumed in isolation or in emulsions, fat particles, regardless of their size, always contain additional lipophilic substances: phospholipids, cholesterol and cholesterol esters, fat-soluble vitamins, and more. The subsequent digestion process takes up to 24 hours, due to limiting reactions.

After ingestion, lingual lipase is mixed with the chyme (A). The enzyme is active at low pH values and breaks down short-chain fatty acids from milk fat triglycerides preferentially. The literature also describes a gastric lipase which may be specific to short- and medium-chain fatty acids from milk fat. Gastric motility ensures thorough mixing with the enzymes and a breakdown of fat into smaller particles. Even initially large proportions of fat are, therefore, emulsified by the end of their passage through the stomach. Short-chain fatty acids freed up in the stomach by hydrolysis at C1 or C3 can be directly absorbed into venous blood through the stomach wall. Opinions on the significance of this gastric fat digestion vary. In infants pancreatic function is not fully developed, so that the activities of lingual and gastric lipase may contribute to fat digestion in an important way. In adults, these gastric processes are of minor importance.

glycerides. Pancreatic lipase, which is inhibited by bile acids, now binds to the colipase, and hydrolysis of triglycerides occurs in positions 1 and 3 at the oil/ water interface.

A multitude of additional pancreatic enzymes operate according to the same principle. A cholesterol esterase hydro-lyzes cholesterol ester. The same can be achieved by a carboxylesterase. Phos-pholipases Aj and A2 hydrolyze phos-pholipids in positions 1 and 2, respectively. Phospholipase A2 with its end product lysophospholipid is of greater significance of the two. The size of the fat particles decreases with advancing hydrolysis. Together with bile acids, the resulting lypolysis products assemble spontaneously into negatively charged particles once they exceed critical concentrations. Ca2+ enhances this process. Since all lipophilic particles are included in this spontaneous aggregation, the aggregates are termed mixed micelles.

This digestive process produces, by hydrolysis, absorbable molecules, on the one hand; on the other hand, particle sizes are reduced by a factor of 100 from fat emulsion to micelle. The resulting maximized surface area is crucial for the subsequent contact with the intestinal mucosa.

Controlled by the gastric pylorus, the emulsified lipids are released into the duodenum where pancreatic juices and bile are added. Bile acids attach to the fat particles, causing their surface to become negatively charged, which allows colipase to attach to the tri-

Lipid Digestion

A. Digestion of Non-Water-Soluble Substances -

Lipophilic substances Cholesterol ester Phospholipids

Lingual lipase

Fat particles

Lipophilic substances Cholesterol ester Phospholipids

Fat particles

Emulsion Lipid Colipase

Emulsion

Micelle

Emulsion

Micelle

Particle size

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Responses

  • valentino
    Where this hydrolysis of triglycerides occurs?
    5 years ago
  • Girmay
    How lipase and bile break down fats during emulsification?
    5 years ago
  • robert
    How lipids are digested?
    5 years ago
  • andrea gottlieb
    How to measure lipid digestion?
    5 years ago
  • mollie
    Where is ffat digested?
    4 years ago
  • fastolph
    Can fat soluble vitamins be absorbed by fat ingested 24 hours earlier?
    4 years ago

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