Lipids in Diet .
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| Metabolism of Lipid . |
[1]. Lipids are mostly consumed in the form of neutral fats, which are also known as triglycerides.
[2]. Triglycerides are made up of glycerol nucleus and free fatty acids.
[3]. Triglycerides form the major constituent in foods of animal origin and much less in foods of plant origin.
[4]. Apart from triglycerides, usual diet also contains small quantities of cholesterol and cholesterol esters.
[5]. Dietary fats are classified into two types:
1. Saturated fats .
2. Unsaturated fats.
Saturated fats .
[1]. Saturated fats are the fats which contain triglycerides formed from only saturated fatty acids.
[2]. The fatty acids having maximum amount of hydrogen ions without any double bonds between carbon atoms are called saturated fatty acids.
Unsaturated Fats .
[1]. Fats containing unsaturated fatty acids are known as unsaturated fats.
[2]. Unsaturated fatty acids are fatty acids formed by dehydrogenation of saturated fatty acids.
[3]. Unsaturated fats are classified into three types:
1. Monounsaturated fats
2. Polyunsaturated fats
3. Trans fats.
1. Monounsaturated Fats .
Unsaturated fats which contain one double bond between the carbon atoms are called monounsaturated fats.
2. Polyunsaturated Fats .
[1]. Unsaturated fats with more than one double bond between the carbon atoms are called polyunsaturated fats.
[2].Polyunsaturated fats belong to the family of essential fatty acids (fatty acids required in diet).
Polyunsaturated fats are of two types .
1. Omega-3 fats or omega3 fatty acids having double bond in the third space from the end of the carbon chain
2. Omega-6 fats or omega6 fatty acids having double bond in the sixth space from the end of the carbon chain.
[3]. Both omega-3 and omega-6 fatty acids are beneficial to the body.
[4]. However, consuming too much of omega6 fatty acids results in hazards than benefits.
[5]. So, the diet containing 3 : 1 ratio of omega-6 to omega-3 fatty acids is often recommended by experts.
3. Trans Fats .
Trans fats or trans fatty acids are unsaturated fatty acids, with molecules containing trans (across or opposite side) double bonds between carbon atoms.
Sources and the functions of the different types of dietary fats .
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| Sources and functions of dietary fats . |
Digestion of Lipids .
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| Digestion of lipids . |
Lipids are digested by lipolytic enzymes.
In the Mouth .
[1]. Saliva contains lingual lipase.
[2]. This enzyme is secreted by lingual glands of mouth and swallowed along with saliva. So, the lipid digestion does not commence in the mouth .
In the Stomach .
Gastric lipase or tributyrase is the lipolytic enzyme present in gastric juice
In the Intestine .
Almost all the lipids are digested in the small intestine because of the availability of bile salts, pancreatic lipolytic enzymes and intestinal lipase.
Role of Bile Salts .
Bile salts play an important role in the digestion of lipids .
Lipolytic Enzymes in Pancreatic Juice .
[1]. Pancreatic lipase is the most important enzyme for the digestion of fats.
[2]. Other lipolytic enzymes of pancreatic juice are cholesterol ester hydrolase, phospholipase A and phospholipase B .
Lipolytic Enzyme in Succus Entericus .
Intestinal lipase is the only lipolytic enzyme present in succus entericus .
Final Products of Fat Digestion .
Fatty acids, cholesterol and monoglycerides are the final products of lipid digestion.
Absorption of Lipids .
[1]. Monoglycerides, cholesterol and fatty acids from the micelles enter the cells of intestinal mucosa by simple diffusion.
[2]. In the mucosal cells, most of the monoglycerides are converted into triglycerides. Triglycerides are also formed by re-esterification of fatty acids with more than 10 to 12 carbon atoms.
[3]. Some of the cholesterol is also esterified. Triglycerides and cholesterol esters are coated with a layer of protein, cholesterol and phospholipids to form the particles called chylomicrons.
[4]. Chylomicrons cannot pass through the membrane of the blood capillaries because of the larger size. So, these lipid particles enter the lymph vessels and then are transferred into blood from lymph.
[5]. Fatty acids containing less than 10 to 12 carbon atoms enter the portal blood from mucosal cells and are transported as free fatty acids or unesterified fatty acids.
[6]. Most of the fats are absorbed in the upper part of small intestine.
[7]. Presence of bile is essential for fat absorption.
Storage of Lipids .
[1]. Lipids are stored in adipose tissue and liver. Fat stored in adipose tissue is called neutral fat or tissue fat.
[2]. When chylomicrons are traveling through capillaries of adipose tissue or liver, the enzyme called lipoprotein lipase present in the capillary endothelium hydrolyzes triglycerides of chylomicrons into free fatty acids (FFA) and glycerol.
[3]. FFA and glycerol enter the fat cells (adipocytes or lipocytes) of the adipose tissue or liver cells. Then, the FFA and glycerol are again converted into triglycerides and stored in these cells.
[4]. Other contents of chylomicrons such as cholesterol and phospholipids, which are released into the blood combine with proteins to form lipoproteins.
[5]. When other tissues of the body need energy, triglycerides stored in adipose tissue is hydrolyzed into FFA and glycerol. FFA is transported to the body tissues through blood.
Transport of Lipids in the Blood – Lipoproteins .
[1]. Free fatty acids are transported in the blood in combination with albumin.
[2]. Other lipids are transported in the blood, in the form of lipoproteins.
Lipoproteins .
[1]. Lipoproteins are the small particles in the blood which contain cholesterol, phospholipids, triglycerides and proteins.
[2]. Proteins are beta-globulins called apoproteins.
Classification of Lipoproteins .
Lipoproteins are classified into four types on the basis of their density:
1. Very-low-density lipoproteins (VLDL) .
It Contain high concentration of triglycerides (formed from FFA and glycerol) and moderate concentration of cholesterol and phospholipids
2. Intermediate-density lipoproteins (IDL) .
[1]. It is Formed by the removal of large portion of triglycerides from VLDL by lipoprotein lipase.
[2]. Concentration of cholesterol and phospholipids increases because of removal of triglycerides
3. Low-density lipoproteins (LDL) .
[1]. It is Formed from IDL by the complete removal of triglycerides.
[2]. These lipoproteins contain only cholesterol and phospholipids
4. High-density lipoproteins (HDL) .
[1]. It Contain high concentrations of proteins with low concentration of cholesterol and phospholipids.
[2]. All the lipoproteins are synthesized in liver. HDL is synthesized in intestine also.
Functions of Lipoproteins .
Primary function of lipoproteins is to transport the lipids via blood to and from the tissues.
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| Functions of each lipoproteins . |
Importance of Lipoproteins .
High-density lipoprotein .
[1]. High density lipoprotein (HDL) is referred as the ‘good cholesterol’ because it carries cholesterol and phospholipids from tissues and organs back to the liver for degradation and elimination.
[2]. It prevents the deposition of cholesterol on the walls of arteries, by carrying cholesterol away from arteries to the liver.
[3]. High level of HDL is a good indicator of a healthy heart, because it reduces the blood cholesterol level.
[4]. HDL also helps in the normal functioning of some hormones and certain tissues of the body. It is also used for the formation of bile in liver.
Low-density lipoprotein .
[1]. Low density lipoprotein (LDL) is considered as the ‘bad cholesterol’ because it carries cholesterol and phospholipids from the liver to different areas of the body, viz. muscles, other tissues and organs such as heart.
[2]. It is responsible for deposition of cholesterol on walls of arteries causing atherosclerosis (blockage and hardening of the arteries). High level of LDL increases the risk of heart disease.
Very-low-density lipoprotein .
[1]. Very low density lipoprotein (VLDL) carries cholesterol from liver to organs and tissues in the body.
[2]. It is also associated with atherosclerosis and heart disease.
Adipose Tissue .
[1]. Adipose tissue or fat is a loose connective tissue that forms the storage site of fat in the form of triglycerides.
[2]. It is composed of adipocytes, which are also called fat cells or lipocytes.
[3]. Obesity does not depend on the body weight, but on the amount of body fat, specifically adipose tissue.
[4]. Adipose tissue is of two types, white adipose tissue and brown adipose tissue.
White Adipose Tissue or White Fat .
[1]. White adipose tissue is distributed through the body beneath the skin, forming subcutaneous fat.
[2]. It also surrounds the internal organs. This adipose tissue is formed by fat cells which are unilocular, i.e. these cells contain one large vacuole filled with fat.
Functions of White Adipose Tissue .
White adipose tissue has three functions .
1. Storage of energy .
[1]. Main function of white adipose tissue is the storage of lipids.
[2]. Utilization or storage of fat is regulated by hormones, particularly insulin, depending upon the blood glucose level.
[3]. If the blood glucose level increases, insulin stimulates synthesis and storage of fat in white adipose tissue .
[4]. On the other hand, if blood glucose level decreases insulin causes release of fat from adipose tissue. Released fat is utilized for energy
2. Heat insulation .
Insulation function is due to the presence of adipose tissue beneath the skin (subcutaneous adipose tissue)
3. Protection of internal organs .
White adipose tissue protects the body and internal organs by surrounding them and by acting like a mechanical cushion.
Brown Adipose Tissue Or Brown Fat .
[1]. Brown adipose tissue is a specialized form of adipose tissue, having the function opposite to that of white adipose tissue.
[2]. It is present only in certain areas of the body such as back of neck and intrascapular region.
[3]. It is abundant in infants forming about 5% of total adipose tissue. After infancy, brown adipose tissue disappears gradually and forms only about 1% of total adipose tissue in adults.
[4]. It is formed by fat cells which are multilocular, i.e. these cells contain many small vacuoles filled with fat.
[5]. The coloration of this adipose tissue is due to high vascularization and large number of iron-rich mitochondria.
Functions of Brown Adipose Tissue .
[1]. Brown adipose tissue does not store lipids but gene rates heat by burning lipids.
[2]. In infants and hibernating animals, brown adipose tissue plays an import ant role in regulating body temperature via non-shivering thermogenesis.
[3]. Heat production in brown fat is very essential for survival of infants and small animals in cold environment. It is because, the lipid in this tissue releases energy directly as heat.
[4]. The mitochondria found in brown adipose tissue contain a unique uncoupling protein called mitochondrial uncoupling protein 1 (UCP1). Also called thermogenin, this protein allows the controlled entry of protons without adenosine triphosphate (ATP) synthesis, in order to generate heat.
Metabolism of lipids .
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| Metabolism of Lipid . |
Lipid Profile .
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| Lipid Profile Value . |
[1]. Lipid profile is a group of blood tests which are carried out to determine the risk of coronary artery diseases (CAD).
[2]. Results of lipid profile are considered as good indicators of whether someone is prone to develop stroke or heart attack, caused by atherosclerosis.
[3]. In order to plan the course of treatment, the results of the lipid profile are correlated with age, sex and other risk factors of heart disease.
[4]. Tests included in lipid profile are total cholesterol, triglyceride, HDL, LDL, VLDL and total cholesterol – HDL ratio.
[5]. Total cholesterol to HDL ratio is helpful in predicting atherosclerosis and CAD. It is obtained by dividing total cholesterol by HDL.
[6]. High total cholesterol and low HDL increases the ratio. The increase in the ratio is undesirable. Conversely, high HDL and low total cholesterol lowers the ratio and the decrease in the ratio is desirable .
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