Food Chemistry and Fish in Nutrition

4.1.4.Metabolism

Metabolism of Triglycerides

Triglycerides are first converted to fatty acids and glycerol mostly in adipose tissue. The fatty acids are released into the plasma where they combine with serum albumin. Long chain fatty acids are oxidized in liver, heart, kidney, muscle, lung, brain and adipose tissue. Glycerol is utilized by liver, kidney, intestine and lactating mammary gland where the activating enzyme glycerokinase is present.

b) Metabolism of fatty acids

The fatty acids components of the lipids entering the liver also have several different pathways

1. Oxidation to CO₂ with ATP production: Free fatty acids may be activated and oxidized to yield acetyl-CoA and ATP. The acetyl-CoA is oxidized via the citric acid cycle to yield ATP by oxidative phosphorylation. Fatty acids are the major oxidative fuel in the liver.

2. Synthesis of fatty acids: There are three types of fatty acid synthesis. (1) Elongation of existing short chain fatty acid in the mitochondria (2) Microsomal system of chain elongation and (3) The cytoplasmic synthesis of fatty acid from acetyl CoA.

3. Biosynthesis of cholesterol: Some of the acetyl-CoA derived from fatty acids (and from glucose) will be used as the major precursor for the biosynthesis of cholesterol, which in turn is the precursor of the bile acids and bile salts, which are essential for the digestion and absorption of lipids.

4. Biosynthesis of lipids of plasma lipoproteins( Triglyceride and phospholipids): Fatty acids are also used as precursors for the synthesis of the lipid portion (triglycerides and phospholipids) of the plasma lipoproteins, which carry lipids to adipose or fat tissue for storage as triglycerides.

5. Fomation of ketone bodies: Excess acetyl-CoA released on oxidation of fatty acids and not required by the liver is converted into the ketone bodies, acetoactate and D-β-hydroxy butyrate, which are circulated via the blood to peripheral tissues, to be used as fuel for the citric acid cycle. The ketone bodies may be regarded as a transport form of acetyl groups. They can supply significant fraction of the energy to some peripheral tissues, up to one-third in the case of the heart