Thermogenesis in response to various intakes of palatable food

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Abstract
Complete energy balance studies were made on groups of overfed (A) and underfed (B) Wister rats. In experiment A one group was fed cafeteria diet ad libitium (the intake was 29% larger than the control), two other groups were fed the same diet but in restricted quantitites (18 and 9% above control), and a fourth group, fed a stock diet, served as control. In experiment B, caloric intake was restricted by 12 and 31% in two groups fed cafeteria diet, and by 21 and 34% in two other groups fed stock diet. The experiments lasted 41 days and during that period the protein gain was comparable between the control and the cafeteria 29% group (643.4 .+-. 33.3 vs. 578 .+-. 25.0) but the fat gain was significantly different between the two groups (863.2 .+-. 81.6 vs. 1663.2 .+-. 99.8 kJ). When energy expenditure (EE) (metabolizable energy less storage added to the cost of storage) is expressed as a percentage of metabolizable energy (ME) intake no significant difference was found among the groups. The average value was .simeq. 75%. This finding would not support the presence of dietary-induced thermogenesis in animals overfed on the cafeteria diet. However, since the obligatory cost associated with storing energy would not explain the higher EE of the overfed groups, it is suggested that the level of ME intake exerts continuous proportional regulatory action on EE and, as a result, energy is spared by underfeeding and it is wasted by overfeeding. Rats fed the cafeteria diet, independent of whether they are overfed or restricted were shown to increase both brown adipose tissue (BAT) protein content and thermogenic capacity in response to norepinephrine. For that reason it is suggested that it is not caloric content of the diet which affects BAT but instead other factors such as fat content or palatability of the food. The results also suggest that BAT is not directly implicated in the regulatory responses associated with various intakes of food. Indeed in the underfed experiment, although the capacity of the BAT was enhanced in the cafeteria groups, the EE was not different from that of the animals fed the stock diet.