The proposed model may be considered as an advance upon previous models; giving a synthesis of the contemporary information base and having a structure with maximum flexibility and minimum usage of fixed coefficients. It is distinctive in that it includes: (1) linkage from dietary nutrients to tissue retention via algorithms for main-stream biochemical processes; (2) a unified driver allowing compatible calculation of (i) the nutrient costs of energy and of protein cycling, (ii) the nutrient costs of maintenance and protein retention, and (iii) the interactions between energy and protein usage; (3) prediction of the rate and composition of tissue retention during growth, and of the cost of thermoregulation; (4) reversal of the role of conventional model parameters such that they become out-turns, rather than inputs, of the model, including (i) the metabolisability of energy, (ii) the ideality of protein and the efficiency of use of ileal digested ideal protein, (iii) the efficiency of utilisation of energy (k) for maintenance (kM), for protein (kPr) and lipid (kLr) retention, which are accepted as variables derived from calculation of efficiencies of production of ATP.