Lipids Lipids are heterogeneous group of water insoluble organic molecules. An adult ingests about 60 to 150 g of lipids per day. Dietary fat Composition  More than 90% are triglycerides, the other are  Cholesterol,  Cholesteryl esters,  Phospholipids and  Unesterified fatty acids.

Animal Sources Dairy products- butter, ghee Meat, Fish, Pork and eggs Vegetable Sources Cooking oils- Sun flower oil, Mustard oil, Ground nut oil Fats from other vegetable sources

? Challenge Lipids are insoluble or sparingly soluble in aqueous solution. The digestive enzymes, however, are present in aqueous medium. Fortunately, the digestive tract possesses specialized machinery to 1. Increase the surface area of lipids for digestion; 2. Solubilize the digested products for absorption.

Digestion in Stomach LINGUAL LIPASE The lingual lipase from the mouth enters stomach along with the food. It has an optimum pH of 2.5 – 5. (Acid stable lipase) 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 Gastric lipase is acid stable, with an optimum pH about 5.4. It is secreted by chief cells, the secretion is stimulated by gastrin. Upto 30% digestion of triglycerides occurs in stomach. (SCT and MCT)

Digestion in Intestine Emulsification: It is a pre requisite for digestion of lipids. (LCFA) The lipids are dispersed into smaller droplets, surface tension is reduced, and surface area of droplets is increased. Emulsification is important since the lipolytic enzymes can act only on the interfacial area between the aqueous and lipid phase. Emulsification is favored by: Bile salts (detergent action) Peristalysis (mechanical mixing) Phospholipids (surfactant action)

Bile salts Bile salts interacts with the dietary lipid particles and the aqueous duodenal contents, thereby stabilizing the particles as they become smaller from peristalsis and preventing them from coalescing. There by increasing the surface area of the particles for enhanced activity of enzymes.

DIGESTION IN INTESTINE

Co- Lipase The co-lipase is secreted by the pancreas as an inactive zymogen. It is activated by trypsin. The binding of co-lipase to the TAG molecule at the oil water interface is obligatory for the action of lipase. Co-lipase helps in anchoring and stabilizing lipase.

Control of Lipid Digestion by intestinal hormones Cholecystokinin – small peptide hormone. Released from mucosal cells of lower duodenum and jejunum. Decreases gastric motility. Causes contraction of gall bladder, releasing bile and mixture of bile salts. Secretin – small peptide hormone Released from intestinal cells in response to the low pH of the chyme entering the intestine. Increases bicarbonate secretion from pancreas, helps to neutralise the pH of intestinal contents and provides a pH favorable for the action of pancreatic enzymes.

PHYSIOLOGICALLY IMPORTANT LIPASES S.NOLIPASESITE OF ACTION PREFERRED SUBSTRATE PRODUCTS 1Lingual lipaseStomachSCTFFA + DAG 2Gastric LipaseStomachSCT & MCTFFA+DAG 3Pancreatic lipaseSmall intestineLCTFFA + 2DAG 4Phospholipase A2Small intestinePhospholipidsLysolecithin 5Lipoprotein lipaseCapillary wallTAG in chylomicron and VLDL FFA + Glycerol 6Hormone sensitive lipase AdipocyteTAG stored in adipose tissue FFA + DAG

ABSORPTION OF LIPIDS Mixed micelle formation The products of digestion, namely 2MAG, FFA, Cholesterol, Phospholipids, fat soluble vitamins and bile salts are incorporated into aggregates to form mixed micelles. The micelles are spherical particles with a hydrophilic exterior and hydrophobic interior core. Due to their amphipathic nature, the bile salts help to form micellar aggregates.

Micellar formation is essential for the absorption of lipid and fat soluble vitamins such as Vitamin A,D,E & K. The micelles are aligned at the microvillous surface of the jejunal mucosa. FA, 2MAG and other digested products passively diffuse into the intestinal mucosal cell. The bile salts are left behind which are mostly reabsorbed from the ileum and returned to the liver to be re-excreted (enterohepatic circulation).

Re- esterification inside the mucosal cell

Chylomicrons The TAG, cholesterol ester and phospholipid molecule along with apoproteins B48 and apoA are incorporated into chylomicrons. The chyle (milky fluid) from the intestinal mucosal cells loaded with chylomicrons are transported through lacteals into the thoracic duct and then emptyed into systemic circulation. The serum may appear milky after a high fat meal(post prandial lipemia) due to the presence of chylomicrons in circulation. Normally the lipemia clears with in a few hours by the uptake of chylomicrons by peripheral tissue and liver.

SCFA absorption is different SCFA (milk, butter, ghee) and MCFA (coconut oil and mothers milk) do not need re-esterification. They can directly enter into blood vessels, then to portal vein and finally to liver where they are immediately utilized for energy. Their absorption is rapid. They are better absorbed than LCFA.

Summary

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