Energy metabolism in genetically fat and lean chickens: diet- and cold-induced thermogenesis.

Energy metabolism was investigated by using indirect calorimetry in growing chickens of similar body weight but genetically selected for high (fat line, FL) or low (lean line, LL) abdominal fat content. Twelve 5- and 7-wk-old male chickens of each line were exposed at two different ambient temperatures, 10 and 25 degrees C. Two birds from the same line were placed in each calorimeter chamber for a 6-d period (2 d fasting and 4 d feeding). Similar food intakes and metabolizabilities of diet corrected for nitrogen balance revealed that FL and LL chickens ingested the same amount of metabolizable energy (ME). Moreover diet-induced thermogenesis did not appear to be defective in FL birds, and the maintenance ME requirement was similar in both lines. However, the partition of retained energy between fat and protein did differ with increased protein deposition and conversely, decreased fat retention in LL chickens, irrespective of age or ambient temperature. FL birds catabolized a greater proportion of dietary amino acids as confirmed by the increase in uric acid excretion. Finally, energy expenditure was significantly and similarly enhanced in both genotypes by cold exposure.