Digestion Absorption Of Lipids

The major lipids in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and must be hydrolyzed and emulsified to very small droplets (micelles) before they can be absorbed. The fat-soluble vitamins—A, D, E, and K— and a variety of other lipids (including cholesterol) are absorbed dissolved in the lipid micelles. Absorption of the fat-soluble vitamins is impaired on a very low fat diet.

Hydrolysis of triacylglycerols is initiated by lingual and gastric lipases that attack the sn-3 ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emul-sification. Pancreatic lipase is secreted into the small intestine and requires a further pancreatic protein, coli-pase, for activity. It is specific for the primary ester links—ie, positions 1 and 3 in triacylglycerols—result-ing in 2-monoacylglycerols and free fatty acids as the major end-products of luminal triacylglycerol digestion. Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids, so that less than 25% of ingested triacylglycerol is completely hydrolyzed to glyc-erol and fatty acids (Figure 44-2). Bile salts, formed in the liver and secreted in the bile, enable emulsification

Glucose

Galactose

Fructose

GLUT 5

Glucose

Galactose

Fructose

GLUT 5

SGLT 1

transporter protein

Brush border

Intestinal epithelium

To capillaries

GLUT 2

Figure 44-1. Transport of glucose, fructose, and galactose across the intestinal epithelium. The SGLT 1 transporter is coupled to the Na+-K+ pump, allowing glucose and galactose to be transported against their concentration gradients. The GLUT 5 Na+-independent facilitative transporter allows fructose as well as glucose and galactose to be transported with their concentration gradients. Exit from the cell for all the sugars is via the GLUT 2 facilitative transporter.

SGLT 1

transporter protein

Brush border

Intestinal epithelium

To capillaries

GLUT 2

Figure 44-1. Transport of glucose, fructose, and galactose across the intestinal epithelium. The SGLT 1 transporter is coupled to the Na+-K+ pump, allowing glucose and galactose to be transported against their concentration gradients. The GLUT 5 Na+-independent facilitative transporter allows fructose as well as glucose and galactose to be transported with their concentration gradients. Exit from the cell for all the sugars is via the GLUT 2 facilitative transporter.

of the products of lipid digestion into micelles and liposomes together with phospholipids and cholesterol from the bile. Because the micelles are soluble, they allow the products of digestion, including the fat-soluble vitamins, to be transported through the aqueous environment of the intestinal lumen and permit close contact with the brush border of the mucosal cells, allowing uptake into the epithelium, mainly of the jejunum. The bile salts pass on to the ileum, where most are absorbed into the enterohepatic circulation (Chapter 26). Within the intestinal epithelium, 1-monoacylglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are re-acylated to tri-acylglycerols via the monoacylglycerol pathway. Glyc-erol released in the intestinal lumen is not reutilized but passes into the portal vein; glycerol released within the

INTESTINAL LUMEN

LYMPHATIC VESSELS (LACTEALS)

Acyl Acyl Acyl

Triacylglycerol, 100%

INTESTINAL LUMEN

LYMPHATIC VESSELS (LACTEALS)

Acyl Acyl Acyl

Triacylglycerol, 100%

Figure 44-2. Digestion and absorption of triacylglycerols. The values given for percentage uptake may vary widely but indicate the relative importance of the three routes shown.

epithelium is reutilized for triacylglycerol synthesis via the normal phosphatidic acid pathway (Chapter 24). All long-chain fatty acids absorbed are converted to tri-acylglycerol in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomi-crons into the lymphatics, entering the blood stream via the thoracic duct (Chapter 25).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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