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Homeostatic responses to caloric restriction: influence of background metabolic rate

Department of Biomedical Sciences, Florida State University, Tallahassee, Florida

Submitted 14 December 2004 ; accepted in final form 1 June 2005

The biological responses to caloric restriction (CR) are generally examined in rats with elevated metabolic rates due to being housed at ambient temperatures (T_a) below the zone of thermoneutrality. We determined the physiological and behavioral responses to 2 wk of 30–40% CR in male FBNF1 rats housed in cool (T_a = 12°C) or thermoneutral (TMN; T_a = 30°C) conditions. Rats were instrumented with telemetry devices and housed continuously in home-cage calorimeters for the entire experiment. At baseline, rats housed in cool T_a had reduced rate of weight gain; thus a mild CR (5%) group at thermoneutrality for weight maintenance was also studied. Rats housed in cool T_a exhibited elevated caloric intake (cool = 77 ± 1; TMN = 54 ± 2 kcal), oxygen consumption (O₂; cool = 9.9 ± 0.1; TMN = 5.5 ± 0.1 ml/min), mean arterial pressure (cool = 103 ± 1; TMN = 80 ± 2 mmHg), and heart rate (cool = 374 ± 3; TMN = 275 ± 4 beats/min). Cool-CR rats exhibited greater CR-induced weight loss (cool = –62 ± 3; TMN = –42 ± 3 g) and reductions in O₂ (cool = –2.6 ± 0.1; TMN = –1.5 ± 0.1 ml/min) but similar CR-induced reductions in heart rate (cool = –59 ± 1; TMN= –51 ± 7 beats/min). CR had no effect on arterial blood pressure or locomotor activity in either group. Unexpectedly, weight maintenance produced significant reductions in O₂ and heart rate. At thermoneutrality, a single day of refeeding effectively abolished CR-induced reductions in O₂ and heart rate. The results reveal that rats with low or high baseline metabolic rate exhibit comparable compensatory reductions in O₂ and heart rate and suggest that T_a can be used to modulate the metabolic background on which the more prolonged effects of CR can be studied.

thermoneutrality; energy homeostasis; heart rate; oxygen consumption; locomotor activity

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