Long-term calorie restriction reduces energy expenditure in aging monkeys

Calorie restriction to produce stable long-term adult body weight for appro ximately 10 years prevents obesity and diabetes in middle-aged rhesus monke ys. To determine whether this dietary regimen also alters energy metabolism , the doubly labeled water method was used to measure total daily energy ex penditure. Six adult male rhesus monkeys, which had been calorie-restricted for more than 10 years, were compared to 8 control adult monkeys, which ha d been fed ad libitum for their entire lives. The calorie-restricted monkey s weighed less than the ad-libitum fed monkeys and had a lower lean body ma ss and lower fat mass. Total daily energy expenditure was lower in the calo rie-restricted than in the ad-libitum fed monkeys, even when corrected for differences in body size using body weight (563 +/- 64 vs 780 +/- 53 kcal/d ; p < .04) surface area (547 +/- 67 vs 793 +/- 56 kcal/d; p < .05), or lean body mass (535 +/- 66 vs 801 +/- 54 kcal/d; p < .04) as covariates. Thyrox ine (T-4) was reduced and the free thyroxine index was suggestively lower i n the calorie-restricted monkeys whereas triiodothyronine (T-3) was not sig nificantly different. Activity in calorie-restricted monkeys was similar to that of a weight-matched younger adult comparison group. We conclude that the process of preventing obesity by long-term caloric restriction causes a significant and sustained long-term reduction in energy expenditure, even when corrected for lean body mass.