Most of the disparity between published estimates of the optimum essential to non-essential amino acid ratios is attributable to the different ways of expressing the relations between the two amino acid groups, different classifications of amino acids with regard to their essentiality, and different methodological approaches. These issues are briefly discussed below.
Expressing the relations between essential and non-essential amino acids
There are various ways of expressing the relationships between dietary essential and nonessential amino acids. Essential amino acids have been related to total amino acids (Ikemoto et al., 1989), non-essential amino acids (Mitchel et al., 1968; Wang and Fuller, 1989; Markert et al., 1993), total nitrogen (Young and Zamora, 1968) or total protein (Dove et al., 1977a; Bedford and Summers, 1985). However, the validity of expressing the amino acid content on a weight basis is questionable. Non-essential amino acids differ considerably in their N concentrations which range from 108 g kg-1 in glutamate to 245 g kg-1 in asparagine. Variation in the contents of individual non-essential amino acids in both practical and experimental diets, can result in their sum expressed on weight basis changing even though the total concentration of nonessential N remains the same (Boisen, 1997).
Since the main function of non-essential amino acids is to provide non-specific nitrogen, the proportions of amino acid groups should be expressed on a nitrogen basis, either as essential to non-essential N ratio (Young and Zamora, 1968; Lenis et al., 1999) or essential to total N ratio (Taylor et al., 1996; Heger et al., 1998). The latter value appears to be more acceptable as the ratio of essential to non-essential N equals infinity at zero concentration of non-essential amino acids.
When crystalline amino acids are a main or sole source of dietary nitrogen, both of the above ways of N expression are essentially equivalent since the content of non-amino acid N is low or close to zero; the essential to total N ratio can be easily converted into essential to non-essential N ratio and vice versa. Crude protein of natural feedstuffs, however, contains a considerable amount of non-amino acid nitrogen such as nucleic acids and their derivatives, amino sugars, nitrates and nitrites, creatine, porphyrins, etc., only part of which may be used for the synthesis of non-essential amino acids (ARC, 1981; AFRC, 1987). Hence the essential to total N ratio may differ depending upon the type of diet. To avoid uncertainties associated with the utilization of sources of non-specific nitrogen other than amino acids, it seems reasonable to take into account amino acid nitrogen only. Therefore, the ratio between the amino acid groups is defined in this chapter as the ratio of essential amino acid N to total amino acid N (E:T). Wherever possible, the literature data expressed in another way have been recalculated.
Another factor that may influence the E:T value is the classification of amino acids (Table 1.1). Difficulties arise particularly with semi-essential amino acids that have been classified alternatively as essential or non-essential by different authors. In rats, Stucki and Harper (1962) classified arginine as essential but cystine and tyrosine as non-essential amino acids. On the other hand, Ikemoto et al. (1989) considered cystine and tyrosine to be essential, with arginine and histidine grouped among the non-essential amino acids. Other authors who studied the optimum E:T ratio in rats classified arginine, cystine and tyrosine as essential amino acids (Adkins et al., 1967; Heger et al. 1987). Similarly in pigs, arginine was considered essential in some studies (Heger et al., 1998; Lenis et al., 1999), yet other authors classified arginine as non-essential (Markert et al., 1993; Gotterbarm et al., 1998). In studies with poultry, glycine was included into the essential group by Stucki and Harper (1961) whereas Deschepper and de Groote (1995) and Alleman et al. (2000) classified this amino acid as non-essential. In their studies on the optimum E:T ratio in turkeys, Bedford and Summers (1988) included glycine into both essential and nonessential groups. Proline was considered essential for chicks in experiments by Sugahara and Ariyoshi (1968).
It is evident that any change in amino acid classification brings about a change in N content of both amino acid groups and, consequently, a change in E:T ratio. Arginine, due to its high N concentration, exerts the most significant effect. Thus Roth et al. (1994b) who classified arginine as non-essential estimated the optimum E:T ratio for growing pigs to be 0.45. However, when the data by Roth et al. (1994b) were recalculated with arginine as essential, the optimum E:T ratio increased to 0.59. It is also evident that any estimates of optimum E:T ratio are only justified when the essential amino acid pattern is ideal relative to the requirement. Any departure from the ideal pattern results in the partial degradation of essential amino acids, with the released nitrogen being used for the synthesis of non-essential amino acids or excreted. It follows that the essential amino acids present in excess relative to the requirement should be regarded as sources of nonessential N (Moran et al., 1967; Bedford and Summers, 1986). This is illustrated by the results of Ikemoto et al. (1989) who estimated the optimum essential to total amino acid ratio in rats fed on amino acid diets simulating egg protein or wheat gluten. Although the optimum ratio for well-balanced egg protein was found to be 0.4, the optimum value for wheat gluten was 0.9 indicating that, because of the severe deficiency of lysine, a great part of essential amino acids in wheat gluten was degraded and used for the synthesis of nonessential amino acids.
In order to be able to compare E:T ratios reported in the literature as well as those found in rats, pigs and poultry, the same method of amino acid classification was used in this chapter for all species, being slightly different from that presented in Table 1.1. Arginine, cystine and tyrosine were classified as essential whereas proline and taurine were considered non-essential. If the proportions of sulphur or aromatic amino acids were higher than those in the ideal protein, the excessive parts of cystine or tyrosine were taken as sources of non-essential N. In those cases where the amino acid classification was based on a different principle and data on amino acid composition of diets were available, E:T ratios were recalculated by the above method.
There are two main approaches to estimating optimum E:T ratio: measuring the response to alterations in essental and non-essential N at constant concentration of total dietary N or studying the effects of altering the E:T ratio at constant concentration of essential N by altering the concentration of total N. Both methods have their limitations. In isonitrogenous diets, a low E:T ratio is inevitably associated with a low response due to the deficiency of essential amino acids, the degree of which being dependent on the level of total nitrogen selected. At low E:T ratios, a sufficient intake of essential N relative to the requirement can be attained only with high-protein diets. Total dietary N may thus considerably affect both the shape of the response curve and its maximum (or the breakpoint in a rectilinear model) and consequently the estimated optimum. It is evident that at higher concentrations of total dietary N the requirement for essential N will be met at lower E:T values and therefore lower estimates of the optimum can be expected than for diets with low concentrations of total N. On the other hand, the use of isonitrogenous diets allows the response to E:T ratios to be studied within the whole range from 0 to 1, and if the study is carried out at several levels of total dietary N, it provides a more comprehensive view than one based on a constant level of essential N.
Estimating the optimum E:T value at a constant level of essential N seems to be more suitable for practical applications. This approach gives an assessment of the minimum amounts of total dietary N , at a given level of essential amino acids, needed for various types of response such as growth rate, protein deposition, amino acid utilization, nitrogen excretion, etc. When essential amino acids are maintained constant at levels near the optimum requirement, it is difficult to achieve low E:T ratios since the total dietary N content increases exponentially as the E:T value decreases; however, this disadvantage is not of much importance from a practical point of view. It is worth mentioning that, in spite of its merit, the method based on constant concentration of essential N has been used in only a few experiments (Dove et al., 1977b; Roth et al., 1994b, 1999; Heger et al., 1998), and the majority of reported optimum E:T ratios have been estimated using isonitrogenous diets.
As with any empirical method of determining the dose-response relationship, several criteria must be fulfilled to obtain a reliable estimate of the optimum value. Firstly, a wide range of E:T values must be selected and a sufficient number of treatments must be used to fit a suitable function to experimental data. This precondition is particularly important when fitting polynomial models to data giving a flat response curve, as is usually the case when studying the response to changing E:T ratios. As shown by Morris (1989), the most often used quadratic curve is sensitive to the range of input values and tends to estimate the maximum (or minimum) in the middle of the interval tested. When the range of the independent variable is small or incorrectly chosen, the estimated optimum can be biased with a significant error. Secondly, the response to E:T ratio must not be influenced by any other dietary or environmental factor. This precondition poses a problem when using diets with a low E:T ratio and a high total N content where a part of energy is required for deamination and elimination of the surplus protein, thereby decreasing the amount of metabolizable energy which may thus become a limiting factor. Thirdly, in balance experiments with constant levels of essential nitrogen in which the total N concentration varies within a wide range, the adaptation period must be long enough to allow for the short-time changes in N deposition in internal organs resulting from adaptive responses to the changes in non-essential amino acid intake.
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