Modulation of maximum thermogenesis by feeding in the white rat.

The present study investigated the availability of substrates as a limiting factor in acute cold exposure. By use of a self-control experimental design, thermogenesis was measured in rats at -10 degrees C under 21% O2-79% He following different feeding regimes. Overnight fasting resulted in the lowest total heat production (9,949 +/- 189 cal/120 min, mean +/- SE, n = 11), maximum rate of thermogenesis (1,320 +/- 23 cal/15 min), and final body temperature (31.0 +/- 0.3 degrees C). The values were significantly lower than those found after overnight rationing (10,630 +/- 231, 1,386 +/- 29, and 32.8 +/- 0.5, respectively) or ad libitum feeding (10,821 +/- 224, 1,395 +/- 27, and 33.5 +/- 0.3). Rats fasted overnight but fed intragastrically a 5-ml substrate mixture (3 kcal/ml) 60 min prior to cold exposure resulted in the highest values for all parameters (11,553 +/- 207, 1,488 +/- 23, and 33.7 +/- 0.4). The increases were not due to the specific dynamic action of food. Since the levels of maximum thermogenesis and cold tolerance can be modulated by feeding, it is evident that substrate availability limits thermogenesis in severe cold independent of respiratory-cardiovascular functions and cellular oxidative capabilities.