Development of energy expenditure in the Zucker rat during the first week of life

The respiratory CO2 production and O2 consumption of Zucker fa/fa and Fa/fa rats 2, 5 and 7 days old were measured for two hours from 16.30 to 18.30 h. Metabolic studies were performed at five different temperatures: 26, 28, 30, 33 and 35 degrees C. At all the ages studied, maximal energy expenditure was observed at an ambient temperature of 30 degrees C. From the age of 2 days and for all the temperatures studied, except 35 degrees C (thermoneutral temperature), the level of gaseous exchange was statistically lower in fa/fa than in Fa/fa pups. The difference between the two genotypes increased with age. On the other hand, there was no difference in the level of the respiratory quotient between the two genotypes, whatever the age or the temperature. In conclusion, the deficit in energy expenditure of the obese pups, that seemed to be related to a defective thermoregulatory thermogenesis, began very early and was probably a primary manifestation of their genetic defect. It was more than enough to account for their excess fat storage.