LINEAR-MODEL OF NITROGEN-BALANCE AND EXAMINATION OF THE NATURE OF TRUE METABOLIZABLE ENERGY AND ITS NITROGEN CORRECTED FORM

1. The nature of nitrogen (N) corrected true metabolisable energy (TME N) was derived using a linear model of N balance, constructed from the relationship between excreted and ingested N. 2. TME was described in terms of a regression line, formed from 'fed' points relating energy voided to energy ingested (GE), as GE -(a(fed) + bGE) + a(fast). On as signment of theoretical excreta and ingested energy components, a devi ation from conceptual metabolisable energy (MEc), equal to the differe nce between a(fed) and a(fast) was established and attributed to metab olic urinary energy (UmE). 3. The N balance model is based on the form of relationship between N excreted and N ingested (NI) that exhibits a linear deviation at 'initial' rates of N ingestion. The model postul ates the following: The deviation is the result of a sparing effect of ingested N on the N component of UmE, viz. metabolic urinary N (UmN); The magnitude of UmN, through 'initial' values of fed N, is described by an intercept component, a(Np), and a slope quantity, -(b(Nr) - b(N na)) NI, where b(Nna) and b(Nr) are respectively the slopes of N excre tion through 'initial' and 'subsequent' rates of ingested food N; The magnitude of the deviation from zero nitrogen balance (ZNB) through 'i nitial' and 'subsequent' rates of ingested N is the sum of the previou s terms and a(Nm) -(1 - b(Nr)) NI, where a(Nm) is the intercept compon ent representing maintenance losses of N at fasting and (1 - b(Nr)) NI is the quantity of fed N retained to replace maintenance N loss. 4. A pplication of the appropriate energetic forms of UmN and a(Nm), viz. E -t a(Np) - E-t (b(Nr) - b(Nna)) NI and E(u)a(Nm), to the expression fo r obtaining TME, demonstrated that TME exceeded MEc by the quantities E-t (b(Nr) - b(Nna)) NI and E-t a(Np), for test food intakes resulting in 'initial' and 'subsequent' rates of food N, respectively. 5. Appli cation of appropriate energetic components of the model to simulate co rrection of TME to ZNB, demonstrated TMEZNB to be a biased quantity, d eviating from MEc by the amount - E-u (1 - b(Nr)) NI or expressed as a n excreta energy slope component, b(Eu)(1 - b(Nr) NI/GE)(GE) where E-u is an appropriate energy coefficient. An alternative perspective is t hat ZNB correction removes the energetic form of UmN as a source of bi as, but introduces one related to E(u)a(Nm). Its nature may be perceiv ed by regarding TME as a function of a regression line relating energy excreted (EE) to energy ingested that has been corrected for UmN ener getic bias and is pivoting on a fulcrum vertically aligned with the po sition of ZNB on the GE (x) axis. The repression line rotates anti-clo ckwise in response to ZNB correction by an amount equal to the magnitu de of E(u)a(Nm) measured on the EE (y) axis from the point of intercep tion. 6. The study identified processes that may be employed to remove bias and improve precision of TME.