Age-related changes in nitrogen balance and related variables in later life of aging male Wistar rats

Age-related changes in nitrogen balance and related variables in later life of male Wistar rats fed 14.0 g/day (group C) or 8.7 g/day (group R) of 20% casein diet throughout life were examined at intervals of 50 days from day 650 of age. In both groups, body weight decreased from about day 600, and the body weight at the final examinations (on day 1,000 in group C and 1,100 in group R) were 18% and 13%, respectively, less than those on day 600, although the animals consumed more than 98% of their food throughout. Their nitrogen balance fluctuated throughout later life in both groups, but the overall changes did not show a consistent negative or positive pattern. Consequently, the mean nitrogen balance after 650 days of age was calculated as -11.5 +/- 45.3 mg N/day in group C and -1.3 +/- 11.6 mg N/day in group R. On the other hand, protein digestibility increased steadily after 650 days of age in both groups. As a whole, nitrogen metabolism at the whole body level did not change consistently even in later life.