Lipids of food, their importance, digestion, absorption. Micelles and chylomicrons. The role of intestinal wall, liver, lungs and adipose tissue in lipid metabolism

The importance of Lipids

80-100g of Lipids should be supplied through the diet pro day. The main Lipids of food stuffs are TAG.

TAG make up 99% of food Lipids:

1. Energetical material – oxidation of 1g of TAG is accompanied by discharge of 38,9kj (9,3kkal) of energy

2. TAG are sources of endogenic water – oxidation of 100g of TAG gives 107 ml of water

3. Mechanical, or protective function – organs occurs in adipose capsules

4. Structural role – TAG enter the membranes

5. TAG take part in thermoregulation. They are bad conductors of heat

6. Dissolving function. The majority of fat soluble compounds (particularly Fat soluble vitamins (FSV)) are absorbed by TAG and other lipids. Therefore the absorption of FSV is destroyed owing to derangement of absorption of lipids. It results in fat soluble vitamins deficiency.

Phospholipids There are Phosphoglycerols and Sphingolipids:

1. structural role – More than 50% of membrane's lipids are phospholipids which form belayer

2. they enter the transport forms of lipids – micelles, chylomicrons, lipoproteins and take part in emulgation, digestion, absorption and transport of lipids in the body

3. some phospholipids take part in blood clotting, for example Phosphatidylethanolamine and Phosphatidylserine

4. lipotropic function – phospholipids prevent liver from distrophy

5. phospholipids and their metabolites play an important role in transduction of signals into cells carrying out messenger's role

Cholesterol and its esters (Sterides)

1. structural role – approximately 30% of membrane's lipids are cholesterol. The role of its as structural material consists in: a) amortizator of membranes, i.e. it promotes aggregate's state of membranes; b) it prevents phospholipids of membranes from lipid peroxidation; c) dielectrical role, i.e. it promotes one-directional conduction of nervous impulse and impedes its dispersion

2. structural role, i.e. cholesterol is a precursor of glucocorticoids, mineralocorticoids, sexual hormones, vitamin D3 and bile acids

3. antihemolytic action which is due to OH-group of cholesterol. Saponins are bound with cholesterol therefore hemolysis is stopped

4. antitoxic role, i.e. toxins bind with cholesterol

5. negative role of cholesterol, i.e in derangement of cholesterol metabolism the following diseases are developed: atherosclerosis, bile stone disease, cancer

Enumerated lipids can be synthesized in the body, but PUFA (essential fatty acids) need for the lipid synthesis. The majority of these acids aren't synthesized in the body and need to be supplied through the diet. Therefore they are also called as vitamin F (from fat – lipid).

Following acids belong to PUFA: linoleic acid (C17H31COOH); linolenic acid (C17H29COOH); arachidonic acid (C19H31COOH)

The importance of PUFA:

1. structural role: PUFA enter the phospholipids of membranes

2. antiatherogenic role: a) PUFA binds with cholesterol and cholesterol esters are formed. They aren't stored in vessels because cholesterol becomes more mobile (more soluble)

3. arachidonic acid is a precursor of prostanoids (PGE2, TXA2, LTRB4). These prostanoids are inflammatory mediators

4. PUFA increase the resistance to infections

5. PUFA are lipotropic factors, i.e. they prevent liver from distrophy

The digestion of Lipids

Saliva doesn’t contain lipases. There is a gastric lipase which has got optimal pH near 3-5. Such pH is characteristicall for children therefore gastric lipase is acitive in childhood. Firstly lipids undergo the emulagation. This process occurs in lumen of small intestine. There are emulagators – compounds which decrease superficial tension of lipid’s drops (proteins, phospholipids, bile acids and soap). Under influence of peristalsis of intestine the lipid’s drop breaks up to smallest droplets. On these droplets bile acids and salts are absorbed and droplets acquire negative charge. Hence they repulse from each other. Some of droplets (30%) are absorbed into lymphatic vessels. But majority of them (70%) undergoes the digestion.

Lipase of pancreas is secreted in inactive form and it is activated under influence of salts of bile acids. Lipase acts on TAG and DAG. Lipase of intestinal juice is secreted in active form and acts on beta-MAG.

However, lipase of intestinal juice is not enough secreted therefore beta-MAG are main products of digestion of TAG.

Phospholipids are hydrolyzed under influence of phospholipase.

Cholesterolesterase is secreted by pancreas in inactive form. It is activated under influence of bile acids. This enzyme hydrolyzes esters of cholesterol.

The absorption of products of Lipids digestion

Glycerol, H3PO4, nitrogen-containing substances, FFA with quantity of carbon atoms less than 10 are absorbed passively in portal vein. Other products of lipids digestion form micelle.

Micelles enter the intestianal wall by pinocytosis.

The role of intestinal wall, lungs, liver and adipose tissue in Lipid Metabolism

The role of intestinal wall:

1. cleavage of Micelles

2. beta-MAG are used for lipogenesis (resynthesis of TAG by monoglycerol’s pathway)

3. the formation of chylomicrons

4. liponeogenesis

5. the formation of phospholipids

1 – Micelle breaks up and it results in release of bile acids and its components. Bile acids return into lumen of small intestine by 2 pathways: 1-st one is through pores of intestinal wall (1-2%); 2-nd one is entherohepatic circulation (98-99%). Entherohepatic circulation has an important role because it stipulates economy of bile acids, i.e. the same bile acids can be used some times (until 20).

2 – Lipogenesis is formation of lipids. This process occurs on monoglycerol’s pathway in intestinal wall:

3 – Chylomicrons are particles which transport TAG from intestine to another organs and tissues. They are foremed in intestinal wall and consist of 80% of TAG, 11% of Cholesterol, 7% of Phospholipids, 2% of Proteins. Phospholipids and Proteins are located in cover of chylomicron whereas TAG and Cholesterol inside of its.Chylomicrons enter the lymphatic vessels. Lymph is collected in thoracic ductus which enters the upper cava vein, heart and lungs.

The role of lungs in Lipid Metabolism

1. Lungs are barrier for Chylomicrons. It is a site of storage of Chylomicrons

2. TAG of Chylomicrons are hydrolyzed and products of hydrolysis (Glycerol and FFA) are used for energy production. Energy formed in oxidation of Glycerol and FFA is used for action of lungs and heating of inspired air

3. The synthesis of FFA is possible in lungs.

Chylomicrons pass lungs and enter the blood circulation. Mainly they are used by liver, adipose tissue and mammary glands. There is an enzyme Lipoprotein lipase (LPL) which hydrolyzes TAG of Chylomicrons and Lipoproteins to Glycerol and FFA. thIs enzyme is located on the wall of cappilaries. This process is named as intravessel's lipolysis. It results in formation of residual chylomicrons. They are used by liver and adipose tissue only.

The role of liver in Lipids Metabolism

90% of lipids and their products of digestion enter the liver on hepatic arteria. 10% of products of lipids digestion enter the liver through portal vein. tHe following processes occur in liver:

1. Anabolic processes: 1) lipogenesis (formation of TAG and PhL) on Phosphatidic pathway; 2) synthesis of Cholesterol and its esters; 3) storage of esters of cholesterol; 4) synthesis of ketone bodies; 5) formation of lipoproteins (LP); 6) synthesis of FFA; 7) liponeogenesis

2. Catabolic processes: 1) lipolysis; 2) beta-oxidation of FFA; 3) oxidation of Glycerol; 4) oxidation of cholesterol to bile acids; 5) lipid peroxidation

The role of adipose tissue in Lipid Metabolism:

1. lipogenesis on Phosphatidic pathway

2. storage of TAG

3. lipolysis

4. liponeogenesis

5. the formation of leptin. Leptin causes the sense of saturation by food. Its synthesis is destroyed in primary obesity that results in invert food behavior