Rupjyoti Hazarika , Pradeepta Kumar Rath
Division of Poultry Science, Central Avian Research Institute,
Izatnagar, Bareilly-243122, Uttar Pradesh, India
The term, fats and oils, which previously were used for all materials those are ether extractable, are now commonly used only for the pure fatty acid esters of glycerol. Fats are those glycerol esters which are solids while oils are liquid in normal temperature. Fats and oils are derived from both plant and animal sources
Importance of fats in poultry nutrition
For these properties the energy value of lipids is far most important.
Relative energy value of fats
The empirical formula of a typical fat is C57H105O6. Compared to glucose fat contain many times more carbon and hydrogen atom in relation to its oxygen content. Thus, fat is capable of being burned to CO and H2O and the energy value of fat is considerably higher per unit weight than the energy value of glucose or other carbohydrates and protein.
From various experiment it has found that the gross energy value of pure fats and oils is about 9.4 kcal/gm. It is 2.25 times more than starch which has a gross energy value approximately 4.15 kcal/gm.
Digestion of fat
Digestion of an ingested triglyceride occurs in three steps:
1. Hydrolysis of the bonds between glycerol and the two fatty acids by lipase, leaving two free fatty acids and a monoglyceride.
2. Micelle formation of fatty acids and monoglycerides in the small intestine occurs, facilitated by bile salts.
3. The micelles move towards the intestinal wall, mostly in the jejunum, at which an exchange occurs and fatty acids and monoglycerides are absorbed. Re-esterification occurs in the enterocyte, chylomicrons are formed and drained from the intestinal wall mainly into portal vein.
Energy from fat metabolism:
Chicken stores energy in the body as neutral fats mainly in the adipose tissue. Chicken stores small amount of glycogen as energy source in the liver and muscles. The animal tissues obtained its lipid store from the lipids of the diet plus the fats derived from the acetyl CoA obtained during lipogenesis from carbohydrate and certain amino acids. The composition of fatty acids obtained from the diet may vary considerably in regards to degree of unsaturation and chain length. The process of lipogenesis from carbohydrate and amino acids appear to favor formation of saturated over unsaturated fatty acids in most mammals. In the chicks however lipogenesis apparently favors the production of considerable amount of oleic acids as well as some saturated fatty acids, particularly palmitic and stearic. From the overall mixture of fatty acids available both from the diet and from lipogenesis, the liver produces s a composite fat which is quite characteristic of the species. This operation involves shortening or elongation of the carbon skeleton of some dietary fatty acids as well as the introduction of double bond in the synthesis of oleic acid. The chicken like other animals is capable of synthesizing linoliec acid. Arachidonic acid can be synthesized only from linoleic acid.
Energy from Glycerol metabolism -
Lipase help mobilize the body fat stores by catalyzing the production of glycerol and fatty acids. Glycerol is glycogenic. It is catabolized via the glycolytic pathway. The metabolism of glycerol is shown in the following reaction along with the production of energy in terms of ATP per molecule of glycerol. The glycerol moiety of fats can be converted by the body either to fructose and then to glucose, thereby serving as a source of blood sugar or it can be converted to pyruvic acid. Both products of glycerol metabolism therefore are important energy metabolites. Glycerol is the only portion of triglyceride that can be converted to glucose.
Β- Oxidation of fatty acids-
Beta oxidation is the process by which fatty acid molecules are broken down in the mitochondria to generate acetyl-coA, which enters the citric acid cycle, and NADH and FADH2, which are used by the electron transport chain.
When glucose levels are low during long periods between meals the enzyme glucagon secreted by the pancreas stimulates adipose lipase activity to release fatty acids from triglycerides. Epinephrine can stimulate the same activity to release energy during the fight or flight response. Once released, these fatty acids travel through the blood to other tissues such as muscle where they are oxidized to provide energy through the mitochondrial beta-oxidation pathway. Since fatty acid beta-oxidation occurs in the mitochondrial matrix, long-chain fatty acids must be actively transported from the cytoplasm into mitochondria by carnitine palmitoyl transferase.
The beta oxidation of fatty acids for production of energy is brought about by degradation of the fatty acids by a series of reaction in which two carbon fragments are removed beginning at the carboxyl end of the fatty acid chain. The first step in the reaction involves combination of the fatty acids with coenzyme A to form the fatty acyl CoA compound. After the fatty acid has reacted with coenzyme A, it undergoes unsaturation at a β position by a dehydogenage. In β-oxidation of stearic acid, for example water is then added to the molecule at this position by an enzyme, enoyl hydrase , yielding β-hydroxy stearly CoA. This is acted upon by another dehydrogenage to produce β-keto stearyl CoA where upon there is a cleavage at the α-β- position brought about by another molecule of reduced Co enzyme A and one molecule of acetyl CoA. The Palmityl CoA residue is thereby degraded two carbon at a time until the molecule is reduced finally the last molecule of acetyl CoA.
The acetyl CoA formed by β-oxidation reacts with oxaloacetic acid to form citric acid and then is oxidized to carbon dioxide and water in the citric acid cycle. The energy derived from fatty acids results in the formation of the high energy phosphates (ATP) which are synthesized in the course of the reactions involved in the citric acid cycle. Although the β-oxidation of fatty acid yields many acetyl CoA and many ATP molecules, the overall reaction can be written as-
C12H35COOH + 26O2 -> 18CO2+ 18H2O+ 2711.8 kcal
The overall energy obtained by the body from the β-oxidation of one gram molecular weight of stearic acid is 2712 kcal. Since the gram molecule weight of stearic acid is 287.5, this amount to 9.53 kcal /gm of stearic acid. The fatty acid contains slightly more energy than the triglycerides because of the greater excess of carbon and hydrogen over oxygen in fatty acids as compared with that in glycerol.
Early studies on fat supplementation of chick diet indicated that level of fat in excess of about 10% was not tolerated. More recent work has shown that the reason for the earlier poor results with fat was due to a failure to increase the protein and amino acids levels in proportion to the increase energy content, thereby allowing the chicks to obtain their energy requirement with so little total food that they were protein deficient. It has now been shown that chicks can grow and develop almost as well as carbohydrate-free diet containing up to 33.8% triglyceride as they do with normal diet containing carbohydrate, as long as the matabolizable energy:protein ratio (ME/P) is maintained at about 13.2 kcal/gm of balanced protein.
Attempt to use free fatty acid in place of the triglycerides in carbohydrate free diet have resulted in sever growth retardation, since the glycerol protein of the triglyceride is need to supplement glucogenegsis from amino acid for maintenance of blood glucose levels. A small amount of carbohydrate completely prevents hypoglycemia but does not restore normal growth.
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