Lipid Metabolism in Living Organisms | Organic Compounds | Biology

Although the knowledge of lipid metabolism which includes the metabolism of fats, phospholipids, sterol and glycolipids is incomplete but fat possesses its particular importance as well as higher calorigenic status that other foodstuffs, i.e., protein and carbohydrate.

Some of their characteristic importance is given below:

i. Much of the carbohydrate of the diet is converted to fat prior to its utilization for supply of energy.

ii. Some organs prefer fat as a fuel in preference to carbohydrate.

iii. As the calorific value of fat is over twice as great than carbohydrate and protein, i.e., 9.3 Cal/gm, so the fat is stored in the tissue (e.g., adipose tissue, subcutaneous fat, etc.) as principal form of energy.

iv. It supplies essential fatty acid (polyunsaturated fatty acid) and fat-soluble vitamins (viz., vit. A, D, E, K).

v. It is required in diet essentially for absorption of these essential substances.

vi. Although phospholipids, cholesterol and glycolipids are not stored in adipose tissue, but go to form essential structural constituent of various organs in the body.

vii. 10% of fats (the glycerol part) may be converted into sugar during the synthesis of carbohydrates.

viii. Amino acids can be synthesized from fatty acids and ammonia in the liver.

ix. Being a poor heat conductor the subcutaneous fat helps in heat regulation.

x. The depot fats act mechanically in protecting the vital organs and also act as cushions and packing tissues.

The lipids, after absorption, remain in the blood in the following forms:

Blood Fat:

(a) Neutral fats,

(b) Phospholipids (mainly lecithin and traces of other phospholipids),

(c) Cholesterol esters,

(d) Free cholesterol, and

(e) Free fatty acid.

Although the lipids are present in both cells and plasma of blood but the composition of lipids in plasma and cells widely vary. Since the composition of plasma lipids accurately reflects the actual state of lipid metabolism, so the composition of plasma lipids is generally studied. The composition of blood lipids is not a static one but variable and the picture of composition at a particular moment is the resultant of processes of addition of lipids to the blood and removal of lipids from blood.

i. Addition of Lipids may be due to:

a. Absorption from intestine.

b. Synthesis of fat and its mobilization.

ii. Removal of Lipids may be due to:

a. Deposition of fat in the depot.

b. Oxidation of fat in the tissue.

c. Utilisation for formation of tissue structural components.

Alimentary Lipaemia:

When a dog fed with a fatty meal, the concentration of its plasma lipids begins to rise within 1-2 hours and reaches maximum within 6 hours. This rise of plasma lipids after a fatty meal is termed as alimentary lipaemia.

The chief increase takes place in the neutral fat content. Phosphatides and cholesterol (both free and ester form) may also slightly increase, depending upon their amount in food and bile. In human subjects, an effect of a fatty meal on blood fat is not constant. In most cases a slight rise is seen. But active absorption of fat may occur without any appreciable rise in blood fat; even a fall has been noted after a fatty meal.

Element Constant and Element Variable:

The lipids in the body physiologically form two components:

(i) Element constant or structural lipids,

(ii) Element variable.

Although a sharp line of demarcation cannot be made between the two, it has been generally observed that the value of the former remains constant even under extremes of starvation whereas of the latter varies. Cytoplasm and cell membrance of all organs are composed of element constant, so that their fat content does not diminish in starvation. Element constant is composed chiefly of phospholipids. It remains an integral part of the cell protoplasm and is essential for its life.

Element variable is composed mainly of neutral fat. It is present mostly in the so-called fat deposits or adipose tissue, viz., fatty subcutaneous layer, mesentery, omentum, intramuscular connective tissue and con­nective tissue around organs, viz., heart and kidney, etc. It remains as free form and represents stored energy. It has been observed that depot fat is not static but in a continuous state of change due to its continuous syn­thesis and breakdown in the body.

A third type of fat, brown fat, which has a high metabolic rate, has been observed in infants but not in adults.

Role of Liver:

Liver has great metabolic activity in lipid metabolism. During starvation and certain other conditions when depot fats (element variable) mobilized, the liver fats increase. The liver acts as an intermediate station in the process of fat oxidation. Phospholipids are synthesized – in which form – they are discharged into the blood stream to be carried to the tissues for oxidation.

The synthesis and breakdown of fatty acid takes place with rapidity in liver. Liver can oxidize fatty acid to CO₂ and H₂O. Liver can lengthen, and shorten carbon chain as well as saturate and unsaturated them. Free cholesterol and cholesterol ester are synthesized in liver.

Fatty Degeneration:

In normal tissue, a portion of the lipid is present as lipid protein complex. This masked fat cannot be demon­strated by histological methods. Treatment with protein-splitting enzymes makes these lipoproteins ‘un­masked’ and can be stained by usual methods. Such unmasking, called fatty degeneration, also results from anaemia, infection, etc. That this fat is not transported from outside is proved by the fact that, the total fat content of the organ remains same.

Fatty infiltration is another condition where an organ becomes loaded with fat transported from outside. Such a condition results from poisoning of the liver with chloroform, carbon tetrachloride, etc. The total fat content of the organ increases.

Fatty Liver:

Excessive deposition of fat in the liver is due to the following causes:

i. Synthesis of fatty acids from carbohydrate and protein,

ii. Influx of dietary lipid,

iii. Mobilization of fatty acids from the depots to the liver.

These may be brought about by:

(a) Lack of insulin, e.g., in diabetes mellitus,

(b) Starvation,

(c) After injection of adrenocorticotrophic hormone (ACTH) and thyroid-stimulating hormone (TSH) in fasting animals,

(d) De­ficiency of choline in the diet,

(e) Poisoning by chloroform, phosphorus, etc., and

(f) Reduced atmospheric pressure as in high altitude.