DIGESTION OF LIPIDS

 

INTRODUCTION: The major dietary lipids are triacyl glycerol, cholesterol and phospholipids. The average normal diet contains about 20-30 g of lipids per day. Western diet generally contains two or three times more than this quantity.

DIGESTION IN STOMACH: The lingual lipase from the mouth enters stomach along with the food. It has an optimum pH of 2.5-5. The enzyme, therefore, continues to be active in the stomach. It acts on short chain triglycerides (SCT). SCTs are present in milk, butter and ghee. The action of lingual lipase is observed to be more significant in the newborn infants. Gastric lipase is acid stable, with an optimum pH around 5.4. It is secreted by chief cells, the secretion is stimulated by gastrin. Up to 30% digestion of triglycerides occurs in stomach.

DIGESTION IN INTESTINES: Emulsification is a prerequisite for digestion of lipids. The lipids are dispersed into smaller droplets; surface tension is reduced; and surface area of droplets is increased. This process is favored by:

1. Bile salts also known as detergent action

2. Peristalsis which provides mechanical mixing

3. Phospholipids

ENZYMES IN INTESTINES:

1. Pancreatic lipase with Co-lipase

2. Cholesterol esterase

3.Phospholipase A2.

The bile (pH 7.7) entering the duodenum serves to neutralise the acid chyme from the stomach and provides a pH favorable for the action of pancreatic enzymes.

DIGESTION OF TRIGLYCERIDES: 1. Pancreatic lipase can easily hydrolyse the fatty acids esterified to the 1^st and 3^rd carbon atoms of glycerol forming 2-monoacylglycerol and two molecules of fatty acid.

2. Then an isomerase shifts the ester bond from position 2 to 1. The bond in the 1st position is then hydrolysed by the lipase to form free glycerol and fatty acid.

3. The major end products of the digestion of TAG are 2-MAG (78%), 1-MAG (6%), glycerol and fatty acids (14%). Thus digestion of TAG is partial (incomplete).

4. Cholesterol ester may be hydrolysed to free cholesterol and fatty acid. The action of phospholipase A2 produces lysophospholipid and a fatty acid.

Co-lipase: The binding of co-lipase to the triacyl glycerol molecules at the oil water interface is obligatory for the action of lipase. The co-lipase is secreted by the pancreas as an inactive zymogen. It is activated by trypsin.

RELATED;

1.  PHASES OF METABOLISM

2.  METABOLISM OF PROTEINS

3.  METABOLISM AND METABOLIC DISORDERS

REFERENCES