Brown adipose tissue, liver, and diet-induced thermogenesis in cafeteria diet-fed rats

Diet-induced thermogenesis (DIT) in young rats overeating a "cafeteria" (CAF) diet of palatable human foods is characterized by a chronic, propranolol-inhibitable elevation in resting metabolic rate (VO2) and is associated with various changes in brown adipose tissue (BAT) that have been taken as evidence for BAT as the effector of DIT. But direct evidence for participation of BAT in DIT has been lacking. By employing a nonocclusive cannula to sample the venous effluent of interscapular BAT (IBAT) for analysis of its O2 content and measuring tissue blood flow with microspheres, we accomplished direct determination (Fick principle) of the O2 consumption of BAT in conscious CAF rats. In comparison with normophagic controls fed chow, the CAF rats exhibited a 43% increase in metabolizable energy intake, reduced food efficiency, a 22% elevation in resting VO2 at 28 degrees C (thermoneutrality) or 24 degrees C (housing temperature), and characteristic changes in the properties of their BAT (e.g., increased mass, protein content and mitochondrial GDP binding). They also exhibited the greater metabolic response to exogenous noradrenaline characteristic of CAF rats and the near elimination by propranolol of their elevation in VO2. By the criterion of their elevated VO2, the CAF rats were exhibiting DIT at the time of the measurements of BAT blood flow and blood O2 levels. However, BAT O2 consumption was found to be no greater in the CAF rats than in the controls at either 28 or 24 degrees C. At 28 degrees C it accounted for less than 1% of whole body VO2; at 24 degrees C it increased to about 10% of overall VO2 in both diet groups.(ABSTRACT TRUNCATED AT 250 WORDS)