Long‐term caloric restriction reduces metabolic rate in rats

Abstract

Caloric restriction (CR) acutely lowers metabolic rate (MR) in rodents, however, it is not clear if this response is preserved during long term CR. We examined the physiological responses to one year of CR in rats housed in cool (Ta=15° C) or thermoneutral (TN; Ta=30° C) conditions. Four-month old male FBNF1 rats were acclimated to either cool or TN for 1 month. Baseline caloric intake (cool=78 ± 1; TN=55 ± 2 kcal) and oxygen consumption (VO2; via indirect calorimetry; cool=9.5 ± 0.1; TMN=5.2 ± 0.2 ml/min) were determined prior to assignment to either ad lib (AL) or CR groups (30–40% CR) within each Ta (n=8/group). After 1 yr, rats were studied for 24-h indirect calorimetry and sacrificed to obtain weights of metabolically active organs. The results reveal expected effects of CR on body weight and absolute VO2: (TN-AL: BW=538 ± 3 g; VO2=6.4 ± 0.1 ml/min; TN-CR: BW=320 ± 1 g; VO2=4.0 ± 0.1 ml/min; cool-AL: BW=483 ± 5 g; VO2=10.6 ± 0.1 ml/min; cool-CR: BW=354 ± 2 g; VO2=7.7 ± 0.1 ml/min). When VO2 was normalized using the Kleiber method, CR still significantly reduced MR in TN (AL: 10.1 ± 0.1; CR: 9.5 ± 0.1 ml/min/kg0.75) and cool (AL: 18.3 ± 0.2; CR: 16.7 ± 0.2 ml/min/kg0.75). Evaluation of the regression relationship between metabolic organ mass (sum of liver, heart, brain, and kidney mass) and total energy expenditure revealed a clear shift induced by CR to reduced energy expenditure per unit metabolic mass in both cool and TN groups. These findings support the hypothesis that CR produces sustained reductions in MR in rats. Supported by NIA AG023837.