Effects of variable caloric restriction on utilization of ingested energy in rats.

Body weight reductions were produced in five groups of male Wistar rats (325 g) by starvation or by variable degrees of underfeeding. The resulting body weights were maintained, by adjusting daily food intake, at constant levels for at least 18 days. We found that 1) energy conservation (i.e., a reduction in the amount of energy required for maintenance) developed in proportion to reduction in body weight, 2) energy conservation increased even while reduced body weights were maintained at constant levels, 3) reduction in the energy required for maintenance was largely due to reduced resting metabolic rate, 4) a maximal ability to conserve energy seemed to occur in severely food-restricted rats, and 5) the magnitude of energy conservation appeared to be proportional to the absolute reduction in body weight and was unrelated (in the long run) to rapidity of weight loss. This study confirms that precise mechanisms of energy conservation are set in motion by food restriction and attempts to identify some components of this adaptive response.