The oxygen consumption and carbon dioxide production of Wistar and Zucker lean (Fa/?) and obese (fa/fa) rats was measured at 4, 10, 20 and 30 degrees C. 2. There was a net synthesis of lipid at the expense of carbohydrate in Wistar rats at 20 degrees C, with active lipid oxidation at 4 degrees C, and increasing heat production at lower temperature. Zucker lean rats also showed this trend. 3. Zucker fa/fa rats synthesized lipid at 4, 10 and 20 degrees C, showing a less marked increase in heat production with lowering temperature. 4. It is postulated that Zucker obese rats synthesize lipids as a way to obtain residual metabolic heat to maintain their body temperature. This is part of a process--fully functional in Wistar and Zucker lean rats, and truncated in Zucker obese rats--in which liver lipogenesis can combine with brown adipose tissue lipolysis to generate enough heat to maintain body functions under a cold environment.
