Utilization of energy for maintenance and for fat and lean gains by mice selected for rapid postweaning growth rate

The metabolizable energy intake (MEI) required for maintenance and the efficiency of utilization of metabolizable energy available for gain (MEA) were determined for a line of mice (rapid growth) selected for 41 generations for rapid postweaning weight gain and for a contemporarily mated line (control) that had been randomly selected. Feed intake of individually housed rapid growth and control males was restricted above maintenance or was ad libitum from 21 to 42 days of age. Regressions of change in body energy per unity metabolic body size on MEI per unit metablic body size showed that the maintenance requirement for each line of mice was 176 kcal per unit metabolic body size per day and that the rapid growth line was more efficient than the control line in utilizing MEA (50% vs. 23%) to promote an increase in body energy. Although the proportions of MEA used for fat (PF) and lean (PL) gains and the net efficiencies with which those proportions were utilized for fat (NF) and lean (NL) gains were unknown, the products of proportion and efficiency for fat gain (PF X NF or fat energy deposition coefficient) and for lean gain (PL X NL or lean energy deposition coefficient) were determined. The results demonstrate that 41 generations of selection for rapid postweaning weight gain did not change the lean energy deposition coefficient, but did alter the fat energy deposition coefficient. These data suggest that the two lines of mice use different proportions of MEA for fat gain and/or utilize MEA for fat gain at different efficiencies.