Metabolism
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Children with fatty diarrhoea
Key points from this exercise:
After a moderately fatty meal the plasma appears milky, due to the presence of chylomicrons. Chylomicrons are relatively large plasma lipoproteins - droplets of triacylglycerol emulsified by surface proteins, unesterified cholesterol and phospholipids. They have a diameter of 0.1 - 1 µm, which is large enough to scatter light.
Over a period of 2 - 3 hours the plasma becomes clear again, as chylomicrons are cleared from the circulation.
Pancreatic lipase hydrolyses the fatty acids esterified to carbons 1 and 3 of glycerol in triacylglycerols, resulting in the formation of non-esterified fatty acids and 2-monoacylglycerol.
Non-esterified fatty acids, monoacylglycerol, phospholipids and bile salts emulsify the dietary lipid into micelles - droplets 4 - 6 nm in diameter that are small enough to be taken up into intestinal mucosal cells.
In the intestinal mucosal cell monoacylglycerol is re-esterified with long-chain fatty acids to form triacylglycerol, which is packaged into chylomicrons that enter the lymphatic circulation, and then enter the bloodstream at the thoracic duct.
This means that the products of lipid digestion and absorption are available to all tissues in the body before the liver clears the remnants. This is in contrast to the water-soluble products of digestion (monosaccharides and amino acids), which are absorbed through the hepatic portal vein, so that the liver controls what enters the circulation.
Medium-chain fatty acids are not esterified in the intestinal mucosa, but are absorbed into the hepatic portal vein.
A rare genetic condition results in impairment of chylomicron assembly, and accumulation of lipid droplets in the intestinal mucosal cells. This leads to fatty diarrhoea (steattorhoea) when the mucosal cells are shed into the intestinal lumen. Intestinal mucosal cells proliferate in the crypt, migrate up the villus, and undergo apoptosis and are shed into the intestinal lumen about 48 hours after proliferation.
Chylomicrons are cleared by the action of lipoprotein lipase in adipose tissue and muscle. This is an extracellular enzyme with a short half-life.Its synthesis is upregulated by insulin.