[A new method for testing the quality of food proteins for maintenance metabolism. 4. Testing of isolated proteins as well as various protein sources of plant and animal origin].

Male adult rats (370 g body weight) were fed on maintenance level (460 kJ ME/kg 0,75). In a 10 days preliminary period they received a casein/methionin diet (95,5 W/W) supplemented with 10 mg N-15-excess per 0,178 kg metabolic body weight in form of ammonium acetate. Thereafter the animals were changed on 8 isonitrogenous diets containing as protein sources casein, soya protein, gelatine, whole egg, fish meal, pea, wheat and yeast. The N-15-excretion via urine and faeces 6th and 10th day after the end of labelling period was used to evaluate the dietary proteins for maintenance. N-15 in urine was lowest in animals fed with the wheat diet and highest after feeding the whole egg diet. From these data a so called "N-15-excretions BV" was calculated, which indicated the highest quality for wheat and soy protein in meeting the needs of the intermediary metabolism in maintenance. On the other hand, dietary protein sources influence the loss of endogenous nitrogen as metabolic faecal nitrogen (MFN). It was found to be lowest in animals fed with diets containing isolated proteins (6 mg MFN/100 g body weight) and highest after feeding protein sources of plant origin with a high content in crude fibre (10 mg MFN/100 g). Both, losses of N-15 via urine and via feces were combined in a parameter called "total BV". According to this parameter the differences in quality for maintenance were only little between the protein sources testet (casein 100, soy protein 100, pea 99, wheat 99, whole egg 92, fish meal 89, gelatin 89). It was concluded that in maintenance the supply with indispensable amino acids is not critical and that the supply with dispensable amino acids (or nonspecific nitrogen) is of great importance.