INTRODUCTION:
The fat included in the diet increases the amount energy provided to the bird.
Besides providing energy to the bird, adding fat into poultry diets helps reduce dust, which may have adverse effects on bird health.
An antioxidant should be added to diets to prevent the fat in the feed from becoming rancid, especially during hot weather because high temperatures accelerate the oxidation of fat. The location of the fatty acid in the glycerol molecule and the proportion of free fatty acids affect its energy value. Saturated fatty acids are less absorbable and have a lower energy value than unsaturated fatty acids because unsaturated fatty acids are polar solutes and are therefore readily incorporated into micelles and absorbed .
Vegetable oils are highly digestible and are therefore the best source of energy for farm animals. Oils are expensive because of the competition with human food, which limits its use when formulating least cost rations. According to Rose (2001) , animal fat or vegetable oils may be used as a source of energy up to a maximum of 6%. Above this level it becomes difficult to maintain pellet quality or to mechanically move the sticky feed when it is not pelleted.
Soybean is the most widely used dietary fat source, contributing more than half the total dietary fat source usage, while palm kernels and copra are least used. This is probably because soybeans are also a major source of protein for humans and animals. Therefore, the usage of each source depends on various factors, such as, chemical composition, presence or level of antinutritional factors, its cost and the level of other nutrients that it can provide to the animal.
Other sources, such as sunseed, rapeseed and canola, cottonseed and groundnuts contribute significantly as dietary fat sources.
With tremendous increase in the genetic potential of broilers leading to rapid weight gain with maximum feed conversion, their nutrient needs have also increased. Nutrient specifications for today’sfast-growing broiler birds are comparatively higher than thoseof earlier strains. High energy diets of modern broilerstrains make it imperative for inclusion of fats and oils into their diet, which provide higher energy per unit than cereals and other feed ingredients. High dense feed formulations are virtually impossible without the inclusion of fats and oils which are having at least twice the energy value as those of carbohydrates and proteins.In addition, fats also help to reduce dustiness in feed, improve palatability and reduce wear and tear in feedmill equipment. Fats and oils are principally almost similar in their structure with differences in chemical properties and physical states. They are esters of three fatty acids with glycerol commonly called triglycerides, with fats being triglycerols insolid form and oils being triglycerols inliquid form at room temperatures. Digestion and assimilation of fats is quite different and complex than that of carbohydrates and proteins. This involves three critical steps -emulsification of ingested fats, their hydrolysis by digestive enzymes and finally micelle formation which leads to the absorption of digested fatty acids into the portal system.Emulsification is a critical step, as it is difficult for the large fat molecules to be miscible in the aqueous environment of the intestine.This step will determine to a large extent the activity of lipase in digesting the fats to mono and di glycerides, which is the second step. The hydrolysis by lipase enzyme involves the release of fatty acids from position one and three of the glycerol backbone in the triglyceride molecule. The ultimate step of this involves the association of bile salts, monoglycerides and free fatty acids to form micelles, which move through the unstirred water layer and thereafter thecomponents are absorbed by gut cells. Overall, the digestion of fats is a complex process requiring adequate amounts of bile salts, pancreatic lipase and colipase.The function of colipase, which is a co-factor present in pancreatic secretions, is to aid in maintaining the lipase in an active configuration at the lipid-water interface. Colipase binds to the surface of lipid droplets and acts as an anchor for lipase allowing pancreatic lipase to digest triglycerides. Lack of any one of these essentials will impair the digestion and absorption processes. Bile is the main source of endogenous fat and fatty acids which plays animportant role in digestion and absorption of fat. Bile works as an emulsifier in the digestion process and in formation of mixed micelles which are crucial for the absorption process. Various factors that affect the digestibility of fats are as follows:
- AGE OF BIRD:
New born chicks have minimal ability to digest and absorb fat. This can be mainly attributed to two reasons namely, inefficient secretion of bile salts and poor lipase activity. It has been reported that the secretion of endogenous enzymes including lipase, trypsin and amylase increased many folds (20-100) between 4 to 21 days post hatch. However, it was observed that lipase activity increased much slower than other enzymes. The ability to digest and absorb fat in birds develops rapidly after the first few days of life and increase with advancing age. Various workers described that absorption of fats improved significantly as the age of birds increased. Low fat digestibility of young birds not only leads to wastage of costly nutrient but also leads to low AME. In a study, AME of fats in broilers fed diets containing vegetable oil and increasing levels of tallow at 2nd, 4th, 6th and 8th weeks of agewas determined. Results revealed that the AME of both fat sources increased between 2nd to 4th weeks of age, thereafter no further increase was noticed. The increment in AME was greater in tallow than vegetable oil. The lower AME in young birds were attributed to poor emulsification rather than deficiency in lipase activity.
- DEGREE OF SATURATION OF FATTY ACIDS:
Ability of fats to yield energy depends upon its chemical structure. Carbon chain length, degree of saturation and the location of double bonds in fatty acids, all have an impact on the digestion and absorption of fats. Degree of saturation of fatty acids has a key influence on the AME of fats. Animal fats such as tallow usually contain very high amount of saturated fatty acids; especially palmitic and stearic acids whicharepoorly digested and absorbed by poultry. Saturated fatty acids need a much higher concentration of bile salts for their emulsification and micelle formation before they are absorbed. Impact of the absence of bile acids was higher in the solubilization and absorption of saturated fatty acids than that of unsaturated fatty acids. As a result, oils from plants sources such as soybean oil which comprises high amount of unsaturated fatty acids were digested and absorbed more than animal fats. The length of the carbon chain in fatty acids also affect their digestion and absorption, short chain fatty acids being more digestible. The ratio of unsaturated to saturated fats also influenced the AME of the oils with an increasing proportion of unsaturated fat in a fat blend associated with higher AME. Some studies also suggested that blending of saturated and unsaturated fats may improve fat absorption and there may be a synergistic response with such blends. Distribution and position of fatty acids in the glycerol molecule is an important factor that affects the fat digestibility. During fat digestion process, triglycerides are hydrolyzed by theaction of pancreatic lipase with end products being two free fatty acids and monoacylglycerol. The specific arrangement of saturated and unsaturated fatty acids on the glycerol moiety of a triglyceride molecule can contribute to the differences between digestion of fat types. Pancreatic lipase shows specificity for the fatty acids esterified to glycerol in the 1stand 3rd- positions and leaves the 2nd position monoglyceride intact to be absorbed as such. The fatty acids in the form of monoglycerides have greater solubility for micelle formation than the same fatty acids released from the 1st- or 3rdpositions, which were more nonpolar, insoluble and less digestible.
- FAT QUALITY:
Quality of fats also has a profound effect on their digestibility and energy value. In practice, fat quality is determined mainly based on the level of impurities, free fatty acids and the rancidity of fat. Impurities in fat were usually measured as moisture, impurities and unsaponifiables (MIU). The maximum acceptable value of moisture is 10 g/kg fat. Impurities are determined as the percentage of insoluble fraction of fats or oils in petroleum ether and should be less than 10g/kg fat. Other criteria were determination of unsaponifiables which include sterols, pigments, waxes etc. with maximum level of 10g/kg of fat. Another method that is often used as a good indicator of fat quality is the level of free fatty acids (FFA). When fatty acids are not bound to triglycerides, they exist in free form and are called as free fatty acids. It has been reported that digestibility and the AME of fats are depressed with increasing concentrations of FFA. Fats with lowest FFA values when incorporated in the diet of birds could provide highest AME values in both young and older birds. Another important factor which leads to appreciable loss in fat quality is oxidative rancidity. Oxidative rancidity is a process that results in the degradation of fatty acids and occurs in unsaturated fatty acids due to oxidation of the double bonds. It affects the odor, color, flavor and decreases the nutritive value of fat. Several methods are used to evaluate oxidative rancidity in fats and oils including peroxide value and active oxygen method (AOM). Peroxide value is the commonly used indicator of fat oxidation and expressed as milliequivalent (meq) of peroxide per kg fat. It was observed that a slight rancid odor was developed in animal fats when peroxide levels reach 20 meq/kg, whereas the rancid odor in vegetable oils started developing at around 80 meq/kg. Since the concentration of unsaturated fatty acids are very high in vegetable oils like soyabean oil, they tend to be more sensitive and prone to oxidation particularly when stored at high temperatures. Oxidation of fats has negative effects on poultry performance and the quality of meat. It has been reported by various workers that feeding a diet with oil supplemented with high degree of oxidation resulted in lower body weight gain and feed intake. The oxidised oil contained substrates such as aldehydes, ketones etc., which may result in the development of rancid flavors and odors, and reduced the palatability of feed.
- NON-STARCH POLYSACCHARIDES (NSP)
NSP’s exhibit an antinutritional effect in the diets and are primarily responsible for negatively affecting the digestibility of various nutrients including fats. NSPs such as arabinoxylans and â-glucans increase the digesta viscosity and reduce the digestibility of nutrients by impeding the diffusion of digestive enzymes and substrates, and the contact between nutrients and absorption sites on intestinal mucosa. The negative effect of NSPs on digestibility remains pronounced on fats than other nutrients. This can be attributed to high intestinal viscosity caused by soluble NSPs that slow down the gut motility and impair diffusion and convective transport of droplets of emulsified fat, fatty acids, mixed micelles, bile salts and lipase within intestinal contents. Impaired fat digestion resulting from the increased bacterial proliferation in intestinal tract may have a systemic effect on the gut secretions and intestinal morphology. The increase in bacterial activity may also increase the deconjugation of bile acids. Deconjugated bile cannot be reabsorbed and will be excreted. Poor digestion of fat may occur due to reduced recycling and the resultant low concentration of bile salts in birds fed with diet containing high levels of NSP’s.
- CALCIUM LEVEL IN FEED:
Dietary calcium level has a profound effect on utilization and fat digestion. Digestion of fats yield free fatty acids and monoglycerides. These free fatty acids can react with other nutrients in the feed to form soluble or insoluble soaps. If insoluble soaps are formed with minerals such as calcium, both the fatty acid and the mineral will be unavailable to bird. It has been suggested that phytate, as calcium-phytate, may be involved in the formation of insoluble metallic soaps in the gut. Some type of fatty acids and dietary calcium levels are reported to have a deep effect on calcium metabolism and soap formation in broilers. Higher calcium levels in the feed resulted in decreased retention of fat due to soap formation, with potential for soap formation greater in saturated fatty acids when compared to unsaturated fatty acids such as oleic acid. Higher concentrations of calcium in diets containing animal fats may further increase metallic soap formation and lower the energy derived from lipids.
Fats are anexpensive source of energy in poultry diets and its digestion and absorption is a complex process. Ability to digest and absorb fat was limited in young birds particularly in first few weeks. There is still ambiguity as to, if this poor digestion in the first week of chick life is due to deficiency of lipase or bile or both. But sufficient quantities of bile salts for proper emulsification of fats in the gut is necessary for its maximum digestion and utilization. So, a good strategy to improve fat digestion in young birds and throughout the lifecycle in general should consider the use of external supplementation of emulsifiers or most preferably natural bio-surfactants like lysophospholipids, lecithins, etc. This complement the bile salts in proper emulsification and the miscibility of fats or oils, especially of animal fats in the gut which ultimately leads to its efficient hydrolysis by lipase enzyme. There is a distinct advantage of lysophospholipids over other emulsifiers as they are more hydrophilic than phospholipids, have one fatty acid residue per molecule and can form spherical micelles in aqueous solutions leading to improved emulsification in the gastrointestinal tract. Lysophospholipids can also form small sized micelles and Uare more effective than bile and soy lecithin
