[Determination of endogenous N-excretion and nitrogen balance in the range of low N-intake in rats].

The investigations were carried out in order to develop foundations for a complex growth model including the effect of N-efficiency mechanism first of all for the rat. Therefore N-balance trials with rats of different live weights were made using diets of equal protein quality. The curve of the regressively calculated N-efficiency function courses relatively linear in its lower part, i.e. for low N-intakes. The protein quality value "A.b" was proved to be constant for all investigated live weight ranges. In similar trials with different protein qualities was also found a linear course for low N-intakes. The regressively estimated N-maintenance requirement (for N-Intake = 0) related to metabolic live weight (NMR/LWkg0.67) amounts to 180 mg/d. In addition endogenous N-excretions were determined in 15N tracer experiments on animals of different live weight fed graduated amounts of a quality constant protein. The NMR-values (sum of endogenous excretions from faeces and urea) depend on the level of N-intake, i.e. they can not be considered as constant. The endogenous faecal N-loss related to metabolic live weight is relatively constant indeed, whereas the amount of endogenous urea N-loss is dependent on the level of N-intake. Therefore calculations based on the N-efficiency curve, e.g. amino acid requirement dependent on performance and prediction of N-deposition respectively, should not be carried out on the basis of "N-retention" (N-balance + NMR) but only on the basis of N-balance.