Protein ideal diet
Source: Kyriazakis and Emmans, 1992 The low protein deposition at high protein but low energy intake indicates that energy was the limiting factor and that the excess protein was used with less efficiency and provided less net energy. Include just sufficient protein with a good amino acid balance to support maximum protein deposition at the highest possible efficiency. This can be important for areas where abundant cheap supplies of a poor quality protein are available. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, and transporting molecules from one location to another. Growth under commercial conditions is often less than under good experimental conditions, reflecting challenges to the immune system. For protein as a nutrient, see Protein (nutrient). Relationship between increasing protein intake of constant amino acid composition and protein deposition in the carcass of pigs between 20 and 45 kg live weight. Surplus protein may increase protein deposition through enhanced protein turnover, reduced efficiency of retention, greater N excretion and pollution, but with reduced net energy, less fat deposition and improved carcass composition. If complementary proteins and synthetic amino acids are not economically available, then quantity can make up for quality. Towards the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red). Initially, practical trials were carried out to determine the requirement for each of the indispensable amino acids in turn. However, fitting different statistical models to the data can result in quite large differences in apparent requirement with consequences for diet costs. For every one unit of net accretion of protein about 5 units of protein are synthesised. Determine marginal response to amino acid supply to calculate target amino acid level in feed. Effect of increasing energy in the diet (by adding starch) on the protein retention per unit of protein intake. Here the ideal lysine is set much higher. This has been influenced by efforts to model the growth of pigs and also to reduce environmental pollution by reducing dietary nitrogen to a minimum. The independent x axis is shown as live weight, calculated as six times protein mass. Such protein feeds are likely to cost more than the widely used feeds. In addition, voluntary intake increases, so the increased amount of protein required meeting the increased daily protein need, can be accommodated within a lower protein concentration in the diet. Fish have lower energy requirements and require a greater protein: metabolizable energy (ME) ratio. At the low and medium levels of protein, providing extra energy had very little effect on increasing N (protein) retention. Because lysine is normally the first limiting amino acid in most practical diets and therefore the requirements for lysine were the most studied in empirical trials, lysine is used as the reference amino acid and all others are expressed as a ratio to lysine (Table 2). Accounting for the basal endogenous loss substantially increases the value for true digestibility over apparent digestibility. However, part of the residues are not of feed origin but are of endogenous origin such as shed mucosal cells, remains of digestive enzymes and secreted mucoproteins. In addition, protein tissues are constantly being turned over. The same feeds were fed at three fixed levels of feeding, low, moderate and high. If the proportions of the different proteins in the body during growth also remain reasonably constant, then the ratios of the amino acids in the total body proteins will remain constant. A further refinement is to determine the digestibility of each amino acid. Energy is the main driving force of metabolism. In older or less productive animals lower energy diets may be used to achieve maximum protein deposition or secretion without excess fat deposition. Some adjustment may be made using the determined crude protein content of the batch and published regression equations of amino acid content in relation to crude protein. The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. This will repeat until the genetic potential of the animal or some other factor limits further protein accretion. In this study enzymically-hydrolysed casein supplemented with amino acids to meet all amino acid needs was used to measure basal endogenous loss at the zero test protein level. They are proportional to dry matter intake and not necessarily related to protein intake. In Figure 8b, the target pattern for young pigs is compared with the same feed amino acids that were considered in Figure 8a. tryptophan and methionine are used for purposes other than protein synthesis, and others such as cystine and threonine have large losses in intestinal mucoproteins. Instead of determining the basal endogenous loss in every trial, a mean value can be determined and used to correct apparent digestibility established at a single level of inclusion of test protein. The effects of processing on protein quality for monogastrics and ruminants are summarised. While mild heating in the presence of reducing sugars can specifically reduce the availability of lysine, the major problem is a generalised reduction in the digestibility of the protein. Thus the protein percentage of the diet and protein: energy ratio declines. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. As the animal grows more energy is needed for maintenance of the bigger body and to support an increasing proportion of fat deposition in the body. In ruminants, the supplement should provide undegradable but intestinally digested amino acids to complement microbial protein. Figure 1a. Dietary proteins can both cause and affect an immune response. Figure 4 demonstrates the change in maximum potential protein deposition for different sexes of improved European pig breeds. Carnivores have no ability to digest fibrous feed and even a limited ability to digest starchy carbohydrates. However, the feed itself may increase the endogenous loss. Consequently, the diet has to contain more of both protein and fat, but the protein: energy ratio is not greatly increased compared with pigs and poultry. A first approximation to the ideal ratio is the amino acid composition of the whole body, or of the tissue protein gained during growth. TABLE 3 Calculation of the amino acid scores compared with the ideal pattern for chicks. Typical values for true and apparent ileal digestibility of protein in the pig for some protein concentrates are given in Table 4. This may be a result of a high fibre content causing additional mucosal losses, a high viscosity preventing reabsorption of secreted proteins or antinutritional factors in the feed, such as proteolytic inhibitors and lectins causing enhanced secretion of enzymes and increased mucosal cell turnover respectively. Also as different proteins turn over at different rates, the ideal pattern changes with change in proportions of the different proteins being synthesised at any one time. Moisture content during heating is critical in both losses of available lysine and of sulphydryl groups. This is illustrated in Figure 5, which gives the determined synthesis and degradation contributions to the net N retention. To a large extent this is not due to a greater need for protein but a smaller need for energy. Dietary protein is not used efficiently as a source of energy. Poikilothermic animals (fish, reptiles) do not need energy to maintain their body temperature, whereas homoiothermic animals (birds and mammals) expend a considerable amount of energy (partly reflected as basal metabolic rate and maintenance energy, and partly as shivering or panting) to maintain a constant body temperature different to the environmental temperature. Protein supply was the limiting factor and an increase in protein supply increased protein deposition. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. However, this value continually changes as the animal grows, so is not convenient to use (see Figure 4). This article is about a class of molecules. Endogenous losses can be measured directly using isotopic labelling of either the feed protein or of endogenous proteins. The next amino acid in surplus is estimated as valine (CS 112), followed by isoleucine and arginine. In this example the supply of lysine (CS 103) and threonine (CS 107) are also just met. Miller Nutrition Laboratory Department of Clinical Veterinary Medicine University of Cambridge - UK This review examines energy-protein interrelationships, protein requirements of poultry, pigs, fish and ruminants, including the need for indispensable amino acids measured as ileal true digestible amino acids. Consequently, they are not indispensable but a dietary supply spares the need for the indispensable parent amino acid. For mammals and birds, express amino acid requirements and feed values as true ileal digestibility. Response of chicks to increase in protein with a constant amino acid composition supplied from a poor quality source (cereal-groundnut meal), or the poor quality source supplemented with methionine and lysine, or from a good quality source (cereal-herring meal). Figure 1b. Correction for basal endogenous amino acids in the terminal ileum gives the true ileal digestibility. A first approximation to amino acid digestibility can be obtained from the product of N digestibility and amino acid content of the protein. The amount in excess will be deaminated and the carbon skeleton used as a source of energy. The amino acids that cannot be synthesized must be provided by the diet. The maximum potential protein deposition increases to a maximum at about puberty and then decreases as maturity is approached. Dietary proteins may need special processing to reduce antigenic factors. Adequate energy must be supplied by the diet to make efficient use of dietary protein. Figure 2b. , 1981 INDISPENSIBLE AMINO ACID REQUIREMENTS Monogastric animals do not have a requirement for protein as such, but they require nine to ten amino acids which the body cannot synthesize, together with a source of amino nitrogen which can be used for the synthesis of the remaining amino acids. This protein was the first to have its structure solved by X-ray crystallography. Figure 9. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. If an existing diet (e. Mild processing gives best digestibility for monogastric animals and is especially important for young mammals and fish. Choose protein supplements to provide amino acids that complement amino acids of basic (usually cereal) energy sources. Response of chicks to feeds of differing quality. ILEAL DIGESTIBLE AMINO ACIDS In the past, most diet formulation has been based on the use of the chemically determined amino acid content of feeds, usually book values representative of the class rather than analyses of the individual batch. The optimum energy density varies with species, digestive system, age and environment. The ME value of protein at zero N retention takes into account the loss of energy in the excreta, such that the ME of protein and carbohydrate are approximately similar. The amino acid with the lowest score below 100 is the limiting amino acid. A linear chain of amino acid residues is called a polypeptide. Figure 7a illustrates the response of chicks to an increase in the diet of: Nearly equal growth can be achieved with much greater use of the poor quality protein (Wethli et al. To shortcut the work needed to study all possible situations with each amino acid, the concept of the ideal protein was formulated. , 1965). The sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code. Formulating a diet with constraints to limit the excess of one or more amino acids has the advantage of lowering dietary protein and reducing pollution, but other proteins need to be brought in that have complementary patterns of amino acids, high in the limiting amino acids but low in those whose excesses are to be reduced. These losses from normal metabolism are referred to as the basal endogenous loss. g. Arginine is an indispensable amino acid for birds and fish but in mammals it is synthesized as part of the urea cycle. e. Figure 7a. Since each protein has a fixed and characteristic sequence of amino acids, it follows that the ratio of the amino acids to one another is constant in any one protein. These values are independent of the level of protein in the feed (see Figure 9) and true ileal digestible amino acid values are additive. Mild to moderate heating causes loss of sulphydryl groups, formation of disulphide cross links, racemisation of L to D-aspartic acid and reduced digestibility of all amino acids. Typical protein contents of diets, expressed in these three ways for various livestock classes, are shown in Table 1. There is no point in targeting maximum growth rate or production if the last increment is uneconomic. The energy cost of protein synthesis in protein turnover, just to maintain the existing protein, has been estimated to account for 15 to 33 percent of energy needed for maintenance.
Figure 6. Corrections based on this measurement have been termed real ileal digestibility. g corn-soya based) gives good production and the true ileal digestible amino acid supply will be in a similar proportion to the total supply as it is to the ileal requirement to total requirement, changing the basis of formulation will give no benefit. The presence of dietary fibre, phytic acid or tannins in protein feeds reduce amino acid digestibility, increase endogenous N loss and the energetic cost of intestinal protein synthesis with consequent reduction in growth rate. Protein synthesis in the body involves a considerable expenditure of energy to create the activated amino acids to be linked together. In addition, two amino acids, cysteine and tyrosine, can be synthesized in the body but only from indispensable amino acids methionine and phenylalanine respectively. An example of the use of the ideal protein pattern to calculate chemical score of feeds is given in Table 3. Source: Campbell et al. This then is the true ileal digestibility and includes within it any increase in endogenous loss which is proportional to the test feed. Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable. When the two are combined in a ratio to achieve the minimum crude protein needed by young chicks (currently estimated as 22. In the ruminant, sufficient nitrogen and rumen degradable protein must be supplied to maximise bacterial fermentation, energy digestibility and feed intake. Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes. , 1985 Figure 2a shows the effect of feeding pigs with three constant levels of protein each having an increasing ME supplied by extra starch. In diet formulation the aim is to meet the requirements for at least these first two limiting amino acids. ENERGY-PROTEIN INTERRELATIONSHIPS The utilization of dietary proteins must be put in the context of the available energy supply. With increasing protein in the diet there are frequently small improvements in carcase quality, measured as increased protein and decreased fat content. Differences between species in their digestive system also affect the required concentration of protein. A typical response curve to lysine supplementation of a deficient diet for chicks is shown in Figure 6. Fish appear to have much higher protein needs than mammals and aquaculture diets (a very important area in developing countries) are high in protein. The ME value for mammals and birds, however, does not take into account the energy costs of synthesising urea or uric acid and the cost of excretion in the kidney. Miller E. This makes the assumption that each absorbed indispensable amino acid is used with the same efficiency for protein synthesis. Feed efficiency ratio response to supplementing a lysine deficient diet with increments of lysine in male chicks from three to six weeks of age. having a higher score, can only be used in protein synthesis up to the level sustained by the limiting amino acid. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three-dimensional structure that determines its activity. Specific tests for available lysine, based on reaction of a free epsilon-amino group in lysine, with reagents such as fluorodinditrobenzene (FDNB-available lysine), demonstrated the importance of the concept of availability of amino acids and the effects of processing in reducing amino acid availability (Carpenter and Booth, 1973). Typical values for standardized true ileal digestibility of N and key amino acids are given in Table 5. Indeed some of the fastest tissue replacement, such as in the intestinal epithelium and liver, lead to little or no net accretion. A high energy: protein ratio is needed to make the most efficient use of dietary protein. , 1975 The growth response is plotted against the limiting amino acid score (using current estimates of amino acid requirements) in Figure 7b, confirming the over-riding factor is the supply of the limiting amino acid in the diet. Effect of increasing energy (by adding starch to a constant protein feed) at three levels of protein intake on protein retention of male pigs over a period of 6 weeks from 12 kg live weight. Relationship between increasing the intake of protein (having a constant amino acid composition) and fat deposition in the carcass of pigs between 20 and 45 kg live weight. A protein contains at least one long polypeptide. The data are interpreted as either a) an exponential response over the whole range or b) a linear response to the requirement breakpoint. Microbiological assays showed that heat processing also reduced the availability of other amino acids such as methionine, and even those without reactive groups such as leucine (Miller et al. Indeed so high that it would appear that only some of the animal proteins are rich enough in lysine to balance the cereals and achieve the ideal ratio. Endogenous losses measured with an N-free diet are too low. The same feeds were fed at three fixed levels of feeding, low, moderate and high. The problem that not all the chemically determined amino acids are available to the animal at tissue level, has been known for many years but the lack of techniques to routinely determine amino acid availability has held back progress. In these terms, young growing animals have greater requirements for protein than older animals. The endogenous loss is given by the intercept value, which is reinforced by direct measurement of ileal N on the enzymically-hydrolysed casein diet, assumed to be 100 percent truly digested. Protein requirements, expressed as a percentage of diet or as a protein-energy ratio, decline with age in growing animals. However, as most of the synthesized arginine is broken down to release urea, the amount available for protein synthesis may be inadequate and a dietary supply may promote growth in young animals. Figure 1a shows the response of growing pigs given diets in which the amount of protein, with a constant amino acid profile, was varied while maintaining a constant energy supply by replacing starch with protein. At the high level of protein, additional energy gave a marked increase in protein deposition. Consequently, the ideal pattern has evolved in recent years as some of these factors have been studied. Source: Wethli et al. Source: Campbell et al. In this form the ideal pattern can be used to estimate the biological value of feed proteins through the calculation of a chemical score (CS) based on the proportion of the amino acid in the feed protein compared with that of ideal protein. They are termed indispensable or essential amino acids. This was determined from response curves to supplementation of deficient diets with the amino acid under consideration. Figure 7b. Protein requirements are reduced, with less pollution, by selecting proteins and amino acid supplements to approach the ideal protein pattern, but specifying maximum levels for excess amino acids will increase cost. Similarly glycine and serine may not be synthesised in sufficient quantities in certain situations, such as in young animals and rapidly growing chicks and so are termed conditionally indispensable. Protein nutrition requirements of farmed livestock and dietary supply - E. If the test protein is assayed at several levels of dietary inclusion, then the basal endogenous loss can be determined by extrapolation to zero inclusion and use of the regression coefficient of the increase in ileal N or individual amino acid against N or amino acid intake. Further reduction in the crude protein and soya should be possible, but only with more supplementation with methionine, lysine and threonine and also with valine, isoleucine and arginine, all of which are closely similar in CS. Source: Reeds et al. L. For example, as the proportion of protein involved in maintenance of the body compared with accretion of new tissue changes with age, so the ideal pattern will change to reflect the different proteins involved. More attention is now being paid to the response curve such that the economically optimum level of each amino acid can be targeted. Consequently, the score for the limiting amino acid becomes the chemical score for the protein. , 1985 Increasing the protein level also reduced the fat deposition in the carcass, indicating less net energy, even though metabolizable energy (ME) was maintained constant (Figure 1b). When additional protein is supplied at constant energy, there is an increase in both protein synthesis and in protein degradation, resulting in a smaller net increment in protein retention. , 1975). Some tissues are turning over faster than others. Researchers and National Committees have attempted to give a single value as the requirement that feed compounders could use as the target in least cost diet formulation. For fish, faecal values suffice or can be estimated using faecal digestibility in mink or ileal digestibility in chicks. Phase feeding multiple diets with decreasing protein content reduces environmental pollution. Source: Han and Baker, 1994 Consequently, if the requirement for one amino acid is determined by empirical trial in one situation, the requirements for all the others can be estimated by applying the ratio as determined for the ideal protein. Giving more feed increased the energy supply and allowed the response to dietary protein to continue until the new energy level again became limiting. This is readily accomplished by complementing the lysine deficient cereals with a lysine rich protein such as soya. Three levels of protein were fed to male pigs over a period of 6 weeks from 12 kg live weight. In general, the true digestibility of methionine is greater than that of N, while that of cystine is lower, but for the majority of indispensable amino acids, the ileal true digestibility differs very little from that of N. Their contribution to the total ileal N will be greater when low protein feeds such as cereals are fed. Extensive microbial fermentation in the hind gut (caeca or colon) ferments amino acid residues from undigested feed and replaces them with bacterial protein with a different amino acid profile. For maize, lysine is the first limiting amino acid. Increasing protein from low and limiting levels at constant energy increased protein deposition in the carcase until energy limited the response. In addition, the diets were given at three levels of feeding which increased both the protein and energy supply in a fixed ratio. Differences in the size, maintenance requirement and potential growth rate of individual chicks result in the flock response tailing off as the asymptote is reached. Analysis of amino acid residues reaching the terminal ileum enables the calculation of apparent ileal digestibility. Every time a new constraint is added to a best-cost diet matrix the cost of the diet invariably increases and can never decrease. For young, fast growing animals and high yielding lactating animals, aim to feed high-energy diets ad libitum to maximise production potential of animal protein. The effects of dietary proteins on the immune response, as sources of nutrients other than amino acids and of anti-nutritional factors are also considered. It is not necessary to meet the ideal balance for all amino acids. The difference in value compared with the chick diet is not real but more a reflection of the greater amount of research that has been put into the ideal protein pattern for pigs. Traditional methods of measuring digestibility by analysis of faecal residues is inappropriate for most mammalian and avian species. Amino acids present in a greater amount relative to the ideal protein than the limiting amino acid, i. The response is linear up to the maximum potential protein deposition and then reaches a plateau. Animal proteins are usually of good quality with very high levels of lysine. Source: NRC, 1998 Figure 4 Prediction of the maximum rate of protein retention in male, female and castrate pigs of an improved breed type at different stages of growth. Figure 2a. The practical trials indicated the requirements varied with many different conditions, such as sex, genetic strain, environmental temperature, growth rate and energy supply, in the same way as requirements for crude protein. 4 percent of a corn-soya diet), only the sulphur amino acids remain limiting. When the data are scaled by the protein intake, the efficiency of use of the diet, N for N retention is seen to increase with the energy to protein ratio in the diet (Figure 2b). Such feed related losses are best regarded as a charge against the feed rather than an increase in the requirement of the animal. Digestion and absorption of amino acids is complete by the end of the small intestine. The failure to achieve the same maximum growth reflects the reduced net energy value of the diet. When additional energy is provided, there is an increase in protein synthesis and a decrease in protein degradation and these two effects combine to enhance net protein retention. If energy is limiting dietary protein will be used inefficiently as another source of energy instead of being converted into body protein. Figure 10. This is not true since some amino acids e. Ruminant feeds benefit from more severe heat treatment and special processing to reduce protein degradability when amino acid composition is well balanced. Mild heating in the presence of reducing sugars or aldehydes results in loss of available lysine with little change in digestibility. The protein requirements of animals are given in terms of an amount of protein and its constituent amino acids per unit of time - usually the amount to be fed each day. L. Effect of increasing the amount of a single feed protein in the test diet on true and apparent ileal digestibility of the protein. Consequently, the apparent ileal digestibility of low protein feeds will be low, and the apparent ileal digestible amino acids of feedstuffs are not additive and this is a property necessary for feed formulation. Nevertheless, the net energy value for protein is less than that of carbohydrate and fat, and when dietary protein is exchanged for carbohydrate at equal ME, the net energy decreases, protein deposition may be increased but fat deposition is decreased. Clearly there are substantial differences between proteins in their ileal true digestibility.
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