Studies on growth, nitrogen and energy metabolism in rats

Feed intake, growth, nitrogen retention and energy metabolism were measured in 12 male Wistar rats fed ad lib. for 14 weeks with non-purified diets. The feed intake increased rapidly in 4 weeks time from 16 g/d to 25 g/d, and then it was constant in the following 10 weeks. In relation to metabolic live weight the energy intake decreased from about 1400 to 800 kJ GE/kg0.75. With the high intake of energy the growth curve obtained is assumed to be near maximum level. The curve can be transformed to a linear one based on log days expressed as y, LW, g = -836 + 594 X log days with CV = 7.5% and r2 = 0.968. The nitrogen retention increased from 200 to 300 mg N/d in period III when the rats 60 days old, and then it decreased in the following 10 weeks to about 100 mg N/d. In relation to metabolic live weight nitrogen retention is assumed to be near maximum level, decreasing from 1100 to 200 mg N/kg0.75. In the live weight range from 100 to 350 g the total energy retention (RE) increased from about 65 to 125 kJ/d, corresponding to 36% of ME. The energy retained in "protein" increased from 30 to 45 kJ/d, and then it decreased to about 25 kJ/d, while the energy retained in "fat" increased steadily from about 30 to 100 kJ/d in the live weight range from 100 to 350 g. Thereby the ratio between energy retained in "protein" or "fat" decreased from 1:1 to 1:4.