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Genetic selection for growth over the last 30 years has resulted in a considerable increase in average weight gain. Associated with this are, increase in fatness and loss of reproductive ability. Modern broilers contain 150 to 200 g fat per kg body weight, over 85% of which is physiologically inessential. Fatness in poultry has two major attributes:
1. It depresses feed efficiency; some adipose tissues are of little economic value, i.e., abdominal fat is removed by evisceration, thus decreasing processing yield
2. Increased fat content in the chicken meat is therefore undesirable economically.
Nutritional manipulations taken to counter excessive body fatness include :
BRANCHED CHAIN AMINO ACIDS Leucine, Isolencine and Valine
It is well known fact that the effects of branched -chain amino acids play an important role on the processes of protein synthesis and protein degradation. Also it is known that skeletal muscle in particular is affected by the supply of amino acids, especially the BCAAs which exert an anabolic effect on muscle protein.
It was confirmed by the scientises that the only amino acids that can regulate protein synthesis in muscle tissue were the BCAAs and they proposed the mode of action was through changes in the sensitivity of muscle protein synthesis to insulin.
BRANCHED CHAIN AMINO ACIDS (BCAA) AND ITS IMPACT IN LEAN TISSUE EFFICIENCY
BCAA's has been demonstrated to be effective in promoting protein synthesis. BCAA can regulate the rate of protein turnover in skeletal and heart muscles. In heart tissue, BCAA has been demonstrated to accelerate protein synthesis by around 50% and inhibit protein degradation by upto 30%.
These effects were reflected by a number of BCAA metabolites, including alpha-ketoisocaproate, isovalerate, acetoacetate, isobutyrate and acetate. Effects of BCAA on protein synthesis are exerted during mRNA translation and seem to involve altering the rate of peptide chain initiation. Many studies in vivo have shown that BCAAs reduce protein degradation by increasing the proportion of polysomes.
Insulin release is influenced by dietary trace minerals. Chromium (Cr) has been shown to be involved by normal glucose metabolism and is necessary for optimal insulin function and glucose uptake by insulin-sensitive cells. Chromium deficiency can lead to a diminished responsiveness of tissues to insulin. Animals deficient in chromium have impaired pancreatic beta-cell functions and an altered glucose tolerance curve is similar to that, seen in non insulin-dependent diabetes in humans. Other factors associated with insufficient chromium levels are increased levels of circulating insulin, impaired growth, higher percent body fat, lower lean body mass and increased mortality rate.
It is reported an increased sensitivity of skeletal muscle to insulin as a result of chromium supplementation, with the significant difference in both carcass composition and feed utilization. Similar observations have led to the conclusion that conventional animal diets have chromium levels far too low for optimal production, and this can be alleviated through the addition of chromim to the diet. In a later study chicken supplemented with chromium at 0.5 ppm resulted in a greater percentage of lean tissue and a higher feed conversion ratio, than birds fed control diets. It is known that dietary and environmental stresses (including temperature, humidity and pathogens) can alter the animal's requirements for nutrients such as chromium.
CHROMIUM SUPPLEMENTATION IN STRESSES ANIMALS
Chromium supplementation improves the performance of stressed animals. Chromium is usually sub optimal in the diet and an animal's requirement for chromium is strongly influenced by stress. Only trivalent chromium Cr (III) is biologically active. Metallic chromium cannot be absorbed and Cr (IV) is toxic. Adequated absorption of chromium occurs only when it is associated with a specific organic molecule such as chromium tripicolinate (CrPic) or as high-chromium yeast.
Cr (III) exerts its physiological effects through its presence in the multiunit complex, the Glucose Tolerance Factor (GTF). The GTF is a large molecule composed of the trivalent chromium linked with nicotinic acid and three amino acids - glycine, glutamic acid and cysteine. This chromium containing compound was first described in brewers yeast and is reported to enhance the hypoglycaemic action of insulin in chicken. The physiological mechanisms by which the GTF enhances glucose tolerance are not clear, but suggested action is through improving the binding affinity fo insulin to is specific receptors.
It is known that chromium positively effects protein synthesis. Chromium supplementation increases protein deposition and reduces that deposition in growing chicken. The anabolic effects of chromium picolinate in chicken was attributed to a potentiation of insulin function, measured by an increase in the concentration of insulin binding to RBC plasma membranes and isolated fat cells. It also postulates that chromium may have a regulatory role in the maintenance of normal beta-cell glucose sensitivity. The mechanism of action of chromium on glucose tolerance and the insulin sensitivity of cells remains unclear. Chromium supplementation is beginning to be used commercially around the world. Chromium as chromium yeast has been permitted in chicken diets in Switzerland since January 1994.
Chromium may become a common micro ingredient for animals in the future, especially during period of stress. Further research into the effect of chromium on animals under stressful and normal conditions is required before dietary guidelines can be suggested.
A high polyunsaturated fatty acid (PUFA) content in the diet reduces fat accretion in chickens Omega-3 fatty acids present in fish oils, namingly docasahexaenoic (DHA) and eicosapentanoic acids (EPA), reduce fat deposition by reducing the circulating - very low density-lipoprotein (VLDL) levels in the blood. Fish oil has been reported to improve feed efficiency in broilers but its impact on growth performance remains unclear. Some long chain PUFAs, including linoleic acid and is generally converted to arachiodonic acid, a long chain PUFA, which is a precursor for the eicosanoids. Eicosanoids are local messenger molecules, which regulate the rates of protein synthesis and degradation. This investigation was designed to test the effect of differing long chain polyunsaturated fatty acids on lean tissue deposition. Fat sources high in Omega-3 fatty acids (fish oil) and linoleic acid (safflower oil and lard) were tested against linseed oil to assess lean growth in broilers.
A high percentage of PUFAs in the diet (over 1.44g/100g of feed) inhibits lipogenesis and depresses fat deposition in broilers.
Conjugated Linoleic Acid : The major sources of Conjugated Linoleic Acid (CLA) are animal products, beef tallow having around 2.6% of its fat as CLA. Plants possess a far lower CLA content, their oils ranging from 0.1% CLA (coconut oil) to 0.7% (sunflower oil). The c-9, t-11, CLA isomer has been isolated as the active compound, it is this isomer which is incorporated into the phospholipid bilayer in animal tissues. In beef lard, 84% of the CLA occurs in this active form, whilst in plant the c-9, t-11 isomer represents less than 50% of the total CLA. CLA can improve feed efficiency and increases the body weight possibly through its effects on prostaglandin synthesis and signal transduction pathways. CLa has the ability to decrease the catabolic response generated by the immune system without altering the normal immune systems functioning. Reducing this response minimizes the breakdown of skeletal muscle and thus improves feed conversion and enhances lean growth. Thus, CLA at a level of 0.5% of diet would reduce body fat and help increasing lean tissue deposition.
Arachidonic Acid : Arachidonic Acid (AA) is an important biological compund present in most cells in the plasma membrane. AA is a precursor for the eicosanoids, a group of biologically potent messenger molecules, which affect tissues near the cells that produce them. AA is released from the plasma membrane in a reaction catalyzed by phospholipase A2 AA in the cell is converted to the eicosanoids by the enzymes of the smooth endoplasmic reticulum. Free AA is oxidized by two enzymes, cycloxygenase and lipoxygenase. The cycloxygenase pathway converts AA into prostaglandins and the throboxanes. The third group of eicosanoids, leukotrienes are the result of AA being metabolized through the liposygenase pathway. These different eicosanoid classes exhibit an effect on reproduction. Thromboxane induces blood vessel constriction and platelet aggregation, while the leukotrienes are involved in many inflammatory and immune hypersensitivity reactions.
AA also has a reported role in the regulation of ion channels, enzyme activity and growth. The exact mechanisms of action remain unclear. Changes in AA metabilism alter the rates of protein degradation. The prostaglandin PGF2 has been identified as a regulator of the rates of protein synthesis and degradation. Also the addition of prostaglandins alone could stimulate protein synthesis in skeletal muscle.
There are long chain Omega-3 (n-3) PUFA's namingly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These n-3 fatty acids have a pharmacological impact on the thrombic and inflammatory systems. EPA and DHA influence haemostatis and the vascular system, and behave as regulators of inflammation. Fish oils rich in n-3 fatty acids reduce the catabolic response induced by immune stimulation and effectively may promote growth. Omega-3 fatty acids also reduce the very low density lipoprotein (VLDL) levels in blood, acting to lower the circulating free low density lipoprotein (LDL) concentration. Omega-3 fatty acids lower the blood levels of free LDLs (which are normally delivered to tissues for fat storage or is deposited directly in the arteries) and reduce the rate of triglyceride synthesis in the liver. Diets fortified with fish oils may effectively of broiler diets, but it's effect on growth and body fat deposition is unclear. One consideration in the use of fish oil as the n-3 fatty acid source is it's off flavour in bird diets and the reduced shelf life of the chicken meat. A combination of preserving agents and antioxidants may be used to increase shelf life and conceal the distasteful flavours.
The following formula is used for the evaluation of fat deposition in the bird's body.
Fat content % = (Intial weight - Final weight / final weight ) x 100
