Zero nitrogen balance

In order to calculate ME at zero nitrogen balance, it is assumed that uric acid is the major constituent of excreted urinary nitrogen and that the quantity of energy corresponding to I g of excreted nitrogen in the form of uric acid is 34.4 kj (or 36.5 kj according to some authors). Therefore, ME (MJ/kg) at zero nitrogen balance (AMEn) can be calculated by ... 

The two energy values presented in the tables are ME values at zero nitrogen balance (AMEn). One is for the adult cockerel, which serves as a reference in many laboratories, and the other is for a 3 week-old chicken. Chicken and cockerel differ as lower digestibilities of fats and starch are observed in the younger animal. In order to obtain the AME values, it is necessary to know or calculate nitrogen retention, which can be as high as 40 %, and multiply it by the nitrogen concentration (g/kg) of the diet or feed material and then multiply it by 0.0344 MJ. The result obtained is added to the AMEn value. 

Carre and Rozo, 1990 Fisher and McNab, 1987) taking into account the average chemical composition given in the tables. They are metabolisable energy values at zero nitrogen balance. 

Apparent metabolisable energy corrected for zero nitrogen balance, in MJ/kg... 

The influence of cysteine on the methionine requirement is demonstrated by the following study. The subject was a student who was awarded a Ph.U. after conducting the study on himself. The subject consumed diets that were complete except that the concentration of methionine was varied (Table 8.12), The dietary amino acids were supplied in the form of pure amino acids, rather than as protein, to allow full control over the levels of amino adds supplied by the diet. Cysteine was supplied as cy.stine. Cystine is a dimer of cysteine, in which the two cysteine residues are connected via a disulfide bond (R——R). Cystine is readily converted to cysteine in the body. The methionine requirement was assessed by determining the conditions that supported a zero or slightly positive nitrogen balance. 

Protein requirements have been determined primarily by nitrogen balance smdies wherein calculation of intake versus excretion of nitrogen reflects either synthesis or catabolism. The Estimated Average Requirement (approx. 0.66 g/ kg/day) is derived from the amount of nitrogen necessary for nitrogen balance to be zero in balance studies. The RDA for protein is based on the EAR plus a safety factor resulting in a protein recommendation of 0.8 g/kg/day for adults [18, 31], No tolerable upper limits have been set for either protein or indispensable amino acids due to insufficient data. 

For radiocarbon, the standard ratio s is provided by the preindustrial atmosphere, for which 8 = 0. Cosmic rays interacting with atmospheric nitrogen were the main source of preindustrial radiocarbon. In the steady state, this source drsource is just large enough to generate an atmospheric delta value equal to zero. The source appears in equation 9 for atmospheric radiocarbon. Its value, specified in subroutine SPECS, I adjust to yield a steady-state atmospheric delta value of 0. The source balances the decay of radiocarbon in the atmosphere and in all of the oceanic reservoirs. Because radiocarbon has an overall source and sink—unlike the phosphorus, total carbon, 13C, and alkalinity in this simulation—the steady-state values of radiocarbon do not depend on the initial values. 

The mass balances of the species in the diffusion media can be deduced from eq 23. Furthermore, the fluxes of the various species are often already known at steady state. For example, any inert gases (e.g., nitrogen) have a zero flux, and the fluxes of reactant gases are related to the current density by Faraday s law (eq 24). Although water generation is given by Faraday s law, water can evaporate or condense in the diffusion media. These reactions are often modeled by an expression similar to... 

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