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Studies on nitrogen metabolism in rats of different age fed on a protein-free diet. 1. Changes of body mass
Abstract:
The changes of body mass of rats of different age fed on a protein-free diet (PFD) were investigated. An experiment was conducted on 350 male Wistar rats in 14 age groups ranging from 23 to 275 days and an initial individual shrunk body mass ranging from 37 to 470 grams. Each group of 25 rats of the same age was divided randomly into 5 subgroups corresponding to the PFD feeding periods of 3, 6, 9, 12 and 16 days. Before starting and after finishing each feeding period, the rats were fasted for 14 to 20 hours and weighed, thus determining the initial and the final shrunk body mass (SBi and SBf), respectively. SBf was closely correlated with SBi in all age groups (Table 3). SBf was corrected for SBi by linear covariance. Corrected SBf means are presented in Table 4. Logarithms of SBf means from this table were processed by linear regression method against log t (where t = 3, 6, 9, 12 and 16 days on PFD). Coefficients of regression between age groups (k) did not differ significantly and the common bc = -0.062 +/- 0.002 less than 0**. Changes of body mass of rats during 16 days of ad libitum feeding on PFD are described be the equation: Y = Ak X t-0.062 in which: Y = SBf (in grams) and the values of Ak correspond to SBi in age group k. This demonstrates that the changes of shrunk body mass did not significantly depend on the age but SBf was significantly related to SBi, the latter depending on the age of rats. SBf expressed as percentage of SBi (Figure 1) demonstrates that the losses of body mass were greatest during the first 6 days of feeding on PFD; during this period SBf decreased to about 91% and, during the subsequent 10 days, to about 84% of SBi. The results of this experiment suggest that the body mass of rats fed on PFD, for which the nitrogen balance was determined, should be expressed by the body mass actually linked to the endogenous nitrogen metabolism and not by the mean, or by the initial, body mass.