The relative quantitative importance of physiological processes underlying dietary amino acid utiliz

An important application of a model of amino acid metabolism is to provide a deeper quantitative understanding of the inherent metabolic and physiological processes. A model can be used to quantify the significance of different aspects of the growth process.

Having broadly described the processes in protein metabolism, a mechanistic model, describing amino acid flow in the 50 kg live weight pig and embodying the concepts discussed in this chapter, is now applied to give an overall appreciation of amino acid transactions. The deterministic model was based around that described by Moughan (1989) except that in the presently described model daily body protein deposition was predicted rather than given as a model input and daily food intake was a model input, with dietary energy partitioning being simulated. The model allows for specifi cation of total and chemically available dietary amino acid intakes and predicts absorbed amino acids based on true ileal digestibility coefficients. It describes cutaneous amino acid loss as a function of metabolic body weight and endogenous gut amino acid losses as a function of dry matter intake. The model includes a weighting factor for gut endogenous amino acid flow to allow expression of the effect of elevated amounts of antinutritional factors or dietary fibre. The fractional rate of whole body protein synthesis is given as a function of the mean daily protein deposition rate (Pd) over the 3 days of growth preceding the day of simulation (assumed in this exercise to equal Pd on the day of simulation). The loss of protein nitrogen in the urine at maintenance is a set proportion of whole body protein synthesis and amino acids are assumed to be catabolized in proportion to their occurrence in body protein. Some amino acids (e.g. lysine) are assumed to be retained in the cell following protein breakdown and their catabolisms are discounted. The rates of inevitable catabolism are described as curvilinear functions of the amounts of amino acids absorbed in relation to the potential amino acid depositions (based on the genetic upper limit to protein retention, Pd^ J. The model predicts the amount of each amino acid available for growth (after maintenance and inevitable catabolism costs have been met) and the pattern of amino acids available for growth is compared with body protein amino acid composition to identify the first-limiting amino acid and to determine the imbalanced amino acids. In the model, if balanced protein available for growth is greater than Pdmax then excess amino acids are catabolized.

Net energy yields from amino acid catabolism are predicted, and with the non-protein digested energy, give metabolizable energy (ME). The daily ME is partitioned, ultimately to daily protein and lipid, facing a required minimal ratio of total lipid: protein in the body, to model the process of preferential catabolism.

The simulated flow of lysine in the 50 kg live wight pig, given a commercial grower diet (Table 11.4), is given in Table 11.5. The simulation data allow consideration of the modelled effects of level of food intake and Pd^. In the model, feed intake does not influence the urinary loss of nitrogen at maintenance but Pdrnax has a small effect. Inevitable catabolic nitrogen loss in the urine increases with increasing food intake and generally declines at a given food intake with increasing Pdmax but remains constant at high food intakes. The loss of urine nitrogen from catabolism due to excess amino acid supply is quantitatively significant at high food intakes, with the opposite being true for preferential catabolic loss. Cutaneous amino acid loss is unaffected by feeding level or Pd^ and gut loss is influenced by feed intake but not Pd . The loss of lysine due to imbalance was max ^

zero or minimal for the examples shown in

Table 11.4. Ingredient composition of a commercial barley-based diet3 formulated for growing pigs.

Composition

(9 kg-1 air-dry

Ingredient

weight)

Barley

732.5

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165.0

Meat-and-bone meal

50.0

Fish meal

50.0

Vitamins, minerals

2.5

aCrude protein, 178 g kg 1 ; apparent digestible energy, 13.26 MJ kg 1; total lysine, 9.2 g kg 1.

aCrude protein, 178 g kg 1 ; apparent digestible energy, 13.26 MJ kg 1; total lysine, 9.2 g kg 1.

Table 11.5 as lysine was generally the first-limiting dietary amino acid.

The data in Table 11.5 demonstrate that, particularly at higher food intakes, the process of inevitable catabolism may have an important effect on the utilization of the first-limiting amino acid. Absorption and endogenous gut loss are also of importance, with body protein turnover being of lesser significance and cutaneous loss of only minor significance. Preferential catabolism may contribute relatively significantly to amino acid loss in situations where metabolizable energy limits protein deposition. Similarly, excess amino acid supply can make a major contribution to inefficiency.

The influence that the different processes have on overall protein metabolism will, of course, vary with the type of diet, age and weight of the animal. Nevertheless, application of the above simplified model gives a general view of amino acid dynamics and allows a ranking of the importance of the respective processes.

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