Heat loss and energy retention during growth in congenitally obese and lean rats.

I. Heat losses associated with the utilization of metabolizable energy for synthesis of protein and of fat during growth mere studied in Zuekcr rats selected for normal leanncss or congenital obesity. z. Measurements of energy and nitrogen balance were made on groups of four lean and four fat rats offered food ad lib. and kept at zzo. Balance trials were also conducted on groups of fat or lean rats offered restricted amounts of food at two levels and kept at 22' or 28'. The medium rations offered to fat and lean rats were the same. The energy and N contents of frlt and lean rats were determined by carcass analysis at 35 d and at about 90 d of age. 3 . At ad lib. intake, fat rats ate 38 74 more than lean rats. Heat losses and N balance were similar. When fat and lean rats were pair-fed, heat loss and N retention werc lower in fat rats. Absolute values and changes during growth of heat loss (kJ/rat per 24 h) were closely correlated with values obtained for S balance. 4. Carcass analysis showed that energy retention in protein was, on average, 75 "/b in lean rats but only 14 % in fat rats. Estimates of energy retention from slaughter experiments and balance trials agreed well, but marked discrepancies existed between the different estimates of N retention. 5 . The net efliciencies of utilization of metabolizable energy for growth in lean and fat rats were 0.485 and 0.614 respectively. The energetic efficiencies of net protein synthesis and net fat synthesis were estimated to be 43 and 65 6. The interactions between appetite, growth and activity in the development of obesity in the Zucker rat are discussed. respectively.