[A new method for testing the quality of food proteins for maintenance metabolism. 3. Methodological studies of 15N-labeled adult rats].

Experiments were made in order to estimate the optimum amount of daily N-15-application and the optimum period for labelling adult rats in maintenance conditions with N-15-ammonium acetate. Labelling periods of 7 and 10 days were combined with 10 or 30 mg N-15-excess per day and 0,178 kg0,75. N-15-label in urine growth to a plateau within 3--4 days during N-15-feeding. After omitting N-15 from the diet the N-15-label declined rapidly and reached again a relative stable value within 3 to 4 days. The curve pattern for N-15-label in faeces was similar, however, with a time lag of about 24 hours. The ratio between N-15-label in faeces was similar, however, with a time lag of about 24 hours. The ratio between N-15-label in urine and faeces was constant from the 6th to 10th day after the labelling period. Therefore, this period should be used for the estimation of metabolic faecal nitrogen on the basis of N-15-label of nitrogen in faeces and N-15-label of endogenous nitrogen secreted into the gastrointestinal tract. Both, N-15-label of urine and N-15-label of TCA-soluble fraction of plasma can be used as a indicator of the N-15-label of endogenous faecal nitrogen. But, from methodical point of view urine seems to be more convenient. Among the variants of labelling tested the best was found to be a 10 days period with 10 mg N'--15/0,178 kg0,75 daily.