Carbohydrates and proteins on being oxidized yield about four calories per gram where as fats about nine calories, it is evident that fats are more ideal for the storage of energy.
Fats have more energy it is because, that they are more reduced than carbohydrates and proteins, e.g., the ratio of hydrogen to carbon is greater in fats than in carbohydrates or proteins.
Fats in the body are stored under the skin in tissues called adipose tissue or depot fat. During extreme starvation considerable amounts of fat can be extracted from these adipose tissues where they are stored as energy fuel or energy source. When too much fat is deposited, obesity results.
In addition to being an energy storehouse, the fat tissue functions as an insulator and provides some physical support to internal organs.
There is a evidence that they also serve some important non-caloric metabolic functions. In this connection Burr and Bernes concluded that “there are ample reasons for recommending that the fats intake be not reduced much below the normal established by habit”.
This shows that the fat content of the diet may influence such diverse processes as the digestibility and absorbility of other food stuffs in the gastro-intestinal tract, and the rate of calcification of the bones.
Fats of the diet serve as a vehicle for the fat soluble vitamins, their presence in the protein free diet exerts a favourable effect on nitrogen retention, as well as en the course of pregnancy and lactation.
During the process of digestion the facts contained in the food are first simplified into substances such as glycerol, phosphoric acid and the fatty acids, under the influence of the enzymes present in pancreatic and intestinal juices (with the participation of bile).
The glycelrol and phosphoric acid (usually in the form of a salt) are soluble in water and easily absorbed.
The fatty acids containing about 10 carbon atoms or more, are not very much soluble, even as sodium salts.
The bile salts combine with the fatty acids (saponification) and thus render them soluble and suitable for absorption.
Various theories have been formulated about the nature of fat absorption. It is believed that the fatty acids are absorbed through the villi, and as they pass through the intestinal mucosa they are converted into phospholipids before resynthesized into simple glycerides.
Combination with phosphoric acid seems an essential step before many reactions can occur.
This phosphoric acid combination furnishes the necessary energy to make the reaction go.
The bulk of the fat of the food in the lymphatics appears in the form of colloidal particles with variable diameter called as chylomicrons.
A small portion of the absorbed fat, consisting mostly the chain fatty acids, is sent directly to the liver via the blood stream.
However, most of the absorbed fat passes into the lymphatics in the form of chylomicrons. The fat of lymphatics finally enters the blood circulation via the thoracic duct and may then make its way to the liver.
After absorption fat is either oxidized to release energy or stored in the body until required.
The extent to which fat is stored or oxidized is in some measure under the control of endocrine and nervous systems.
Considerable amounts of fat are stored in subcutaneous depots all over the body, these are known as adipose tissue or fat depots.
Fat storage is not only confined to the subcutaneous depots but it also occurs around the intestine, heart, kidneys and in between the muscle fibres. When the body feels the requirement of fat, the same is withdrawn from these depots.
All fat in the body does not come from dietary food but it is readily synthesized from carbohydrate and any excess of carbohydrate in the diet which is not immediately oxidized or stored as glycogen is converted to fat.
It has been shown that the fatty acids of the stored fat are in a constant state of flux. The body can somewhat modify dietary fat by converting the fatty acids into other fatty acids, but the animal fat tends to resemble-, to a great extent, the dietary fat.
According to Schoenhimer and Rbttenberg animals consume not only the food fat but also depot fat in equal amount, therefore, depot fat, is not a inert storage fat but it is constantly involved in metabolic processes. The saying mean is that depot fat very well resembles with dietary fat.
It has been shown by the experiments that even depot is undergoing a relatively rapid chemical interchange.
The changes usually involve a transformation of one fatty acid into another, even though the synthesized fatty acid may be abundantly supplied in the diet.
One of the most common of these transformations is the desaturation reaction which leads to the conversion of a saturated fatty acid into an unsaturated fatty acid.
These reactions usually take place in the presence of oxygen, which aids in the removal of the two hydrogens.
If oxygen is not available then conversion takes place through other way. It has been shown that in animal tissues, the unsaturated fatty acids such as oleic, linoleic and linolenic are not interconvertible.
This means that oleic acid cannot form linoleic acid and linoleic acid cannot form linolenic acid.
However, each particular unsaturated fatty acid can become more unsaturated within its own family of related acids.
In this latter sequence of reactions a polyunsaturated fatty acid of two additional form a specific polysaturated fatty acid with two more carbons and with two or more unconjugated double bonds, the additional double bonds appear toward the carboxyl group end of the molecule. However, a conversion of the one saturated fatty acid into another is quite common.