Digestion Absorption and Metabolism of Lipids Notes

Lipids In Diet

Lipids are mostly consumed in the form of neutral fats, which are also known as triglycerides. The triglycerides are made up of a glycerol nucleus and free fatty acids. Triglycerides form the major constituent in food of animal origin and much less in food of plant origin. Apart from triglycerides, the usual diet also contains small quantities of cholesterol and cholesterol esters.

Dietary fats are classified into two types:

1. Saturated fats
2. Unsaturated fats.

Read And Learn More: Medical Physiology Notes

1. Saturated Fats: Saturated fats are the fats which contain triglycerides formed from only saturated fatty acids. The fatty acids having the maximum amount of hydrogen ions without any double bonds between carbon atoms are called saturated fatty acids.

2. Unsaturated Fats: Fats containing unsaturated fatty acids are known as unsaturated fats. Unsaturated fatty acids are the fatty acids formed by the dehydrogenation of saturated fatty acids. Unsaturated fats are classified into three types:

1. Monounsaturated fats
2. Polyunsaturated fats
3. Trans fats.

1. Monounsaturated Fats: UnsV’Urotect fats which contain one double bond between the Carbon atoms are called monounsaturated fats.

2 Polyunsaturated Fats: Unsaturated fats with more than one double bond between the carbon atoms are called polyunsaturated fats. Polyunsaturated fats belong to the family of essential fatty acids (fatty acids required in diet). These fats are of two types:

1. Omega-3 fats or omega-3 fatty acids having a double bond in the third space from the end of the carbon chain
2. Omega-6 fats or omega-6 fatty acids having a double bond in the sixth space from the end of the carbon chain

Both omega-3 and omega-6 fatty acids are beneficial to the body. However, consuming too much of omega-6 fatty acids results in more hazards than benefits. So, a diet containing a 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. The sources and the functions of the different types of dietary fats are listed in.

Digestion Of Lipids

Lipids are digested by lipolytic enzymes.

• In The Mouth: Saliva contains lingual lipase. This enzyme is secreted by the lingual glands of the mouth and swallowed along with saliva. So, lipid digestion does not commence in the mouth.
• In The Stomach: Gastric lipase or tributaries 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: Pancreatic lipase is the most important enzyme for the digestion of fats. 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: The fatty acids, cholesterol, and monoglycerides are the final products of lipid digestion.

Absorption Of Lipids

Monoglycerides, cholesterol, and fatty acids from the micelles enter the cells of the intestinal mucosa by simple diffusion. From here, further transport occurs as follows:

1. In the mucosal cells, most of the monoglycerides are converted into triglycerides. The triglycerides are also formed by the re-esterification of fatty acids with more than 10-12 carbon atoms.

• Some of the cholesterol is also esterified. The triglycerides and cholesterol esters are ‘ with a layer of protein, cholesterol, and phospholipid:-; ‘.o form the particles called chylomicrons.
• Chylomicrons cannot pass through the membrane of the blood capillaries because of their larger size. So, these lipid particles enter the lymph vessels; and then are transferred into the blood from the lymph.

2. Fatty acids containing less than 10-12 carbon atoms enter the portal blood from mucosal cells and are transported as free fatty acids or unesterified fatty acids. Most of the fats are absorbed in the upper part of the small intestine. The presence of bile is essential for fat absorption.

Storage Of Lipids

• The lipids are stored in adipose tissue and the liver. The fat stored in adipose tissue is called neutral fat or tissue fat. When the chylomicrons are traveling through capillaries of adipose tissue or the liver, the enzyme called lipoprotein lipase present in capillary endothelium hydrolyzes triglycerides of chylomicrons into free fatty acids (FFA) and glycerol.
• 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. Other contents of chylomicrons such as cholesterol and phospholipids which are released into the blood combine with proteins to form lipoproteins.
• When other tissues of the body need energy, the triglycerides stored in adipose tissue is hydrolyzed into FFA and glycerol. The FFA is transported to the body tissues through the blood.

Transport Of Lipids In Blood – Lipoproteins

Free fatty acids are transported in the blood in combination with albumin. Other lipids are transported in the blood in the form of lipoproteins.

1. Lipoproteins: Lipoproteins are the small particles in the blood which contain cholesterol, phospholipids, triglycerides, and proteins. The proteins are beta globulins called apo-proteins. Lipoproteins are classified into four types on the basis of their density:

1. Very low-density lipoproteins (VLDL): Contain a high concentration of triglycerides (formed from FFA and glycerol) and a moderate concentration of cholesterol and phospholipids.
2. Intermediate density lipoproteins (IDL): Formed by the removal of large portion of triglycerides from VLDL by lipoprotein lipase. The concentration of cholesterol and phospholipids increases because of the removal of triglycerides.
3. Low-density lipoproteins (LDL.): Formed from IDL by the complete removal of triglycerides. These lipoproteins contain only cholesterol and phospholipids.
4. High-density lipoproteins (HDL): Contain high concentrations of proteins with low concentrations of cholesterol and phospholipids.

All the lipoproteins are synthesized in the liver. HDL is synthesized in the intestine also.

Functions of Lipoproteins: Primary function of lipoproteins is to transport lipids via blood to and from the tissues. The functions of each type of lipoprotein are given in.

Importance of Lipoproteins:

• High-density lipoprotein:
  • HDL is referred to as the “good cholesterol” because it carries cholesterol and phospholipids from tissues and organs back to the liver for degradation and elimination. It prevents the deposition of cholesterol on the walls of arteries by carrying cholesterol away from arteries to the liver.
  • A high level of HDL is a good indicator of a healthy heart because it reduces the blood cholesterol level. HDL also helps the normal functioning of some hormones and certain tissues of the body. It is also used for the formation of bile in the liver.
• 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 the heart.
  • It is responsible for the deposition of cholesterol on the walls of arteries causing atherosclerosis (blockage and hardening of the arteries). A high level of LDL increases the risk of heart disease.
• Very low-density lipoprotein: VLDL carries cholesterol from the liver to organs and tissues in the body. It is also associated with atherosclerosis and heart disease.

Metabolism Of Lipids

The metabolism of lipids is given in the form of a schematic diagram.

Lipid Profile

• The lipid profile is a group of blood tests which are carried out to determine the risk of coronary artery disease (CAD). The results of lipid profiles are considered as good indicators of whether someone is prone to develop stroke or heart attack caused by atherosclerosis.
• 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. The tests included in the lipid profile are total cholesterol, triglyceride, HDL, LDL, VLDL, and total cholesterol- HDL ratio.
• The total cholesterol to HDL ratio is helpful in predicting atherosclerosis and CAD. It is obtained by dividing the total cholesterol by HDL. High total cholesterol and low HDL increase the ratio. The increase in the ratio is undesirable.
• Conversely, high HDL and low total cholesterol lower the ratio, and a decrease in the ratio is desirable. The values of the lipid profile are given in.