restricted females. Ad libitum-fed males had significantly increased BW, total body fat tissue mass, total body bone mineral concentration, and abdominal fat tissue mass relative to diet-restricted males. The primary effect of dietary restriction in both sexes was on total body fat tissue mass.

The diet-restricted monkeys were restricted further after 18 months to re-establish a 30% difference in food intake between the two groups because the ad libitum-fed controls had voluntarily decreased their food intake. After 3 years of diet restriction (70% of the ME intake of controls), body fat mass and lean body mass were significantly (P < 0.05) lower than in the ad libitum-fed control group (Ramsey et al., 1997). A comparison of DEXA with traditional somatometric measures for determining body fat in adult male rhesus monkeys was made at various time points over a 4-year period (Colman et al., 1999). Additionally, the precision of these methods was assessed by repeated measures on the same individuals. DEXA estimates of body fat were positively correlated with body weights, body fat mass indices, body circumferences, and abdominal skinfold thicknesses. DEXA assessments of soft-tissue composition were precise, with low coefficients of variation. The majority of observed variability in somatometric measures was explained by subject variance rather than by inter- or intraobserver variability or observer experience level. These researchers concluded that noninvasive DEXA technology provides precise estimates of body composition that correlate well with the somatometric measures traditionally used in primate studies.

No significant differences in physical activity were apparent between diet-restricted and ad libitum-fed rhesus macaques during the first 30 months of the Wisconsin study (Weed et al., 1997). This was similar to the finding of DeLany et al. (1998), at the University of Maryland, who reported that physical activity was similar for ad libitum-fed and diet-restricted male rhesus macaques when matched for age and BW. Nevertheless, Weed et al. (1997) reported that there were clearly discernable differences in diurnal and circadian activity in diet-restricted rhesus macaques after 6 years on the Wisconsin study. Some diet-restricted individuals exhibited increased pacing and grooming behaviors. These changes in activity were not, however, related to measured alterations in 24-hour energy balance.

After 4.5 years, body composition and energy balance of 30 male rhesus in the NIA study were measured. The data were grouped by primate age: juveniles (6.5-7 years old), adults (8.5-10 years old), and old (over 24 years old). Both diet-restricted and ad libitum-fed monkeys were represented in the juvenile and adult groups, but all the old monkeys were ad libitum-fed (Lane et al., 1995a). Absolute body fat was not significantly altered by diet restriction, but the percentage of lean body mass decreased with age as the percentage of body fat increased. Despite substantial differences in food intake, the percentage of dietary energy that was apparently digestible (83%) was similar in all groups.

The anti-aging effects of diet restriction are believed to be associated with changes in energy metabolism. Rectal body temperature decreased progressively with age from 2 to 30 years in rhesus macaques fed ad libitum but was about 0.5°C lower in age-matched monkeys subjected to 6 years of diet restriction (Lane et al., 1996). During short-term diet restriction, 24-hour energy expenditure was reduced by about 24% (Lane et al., 1996). Absolute energy expenditures (as determined by the doubly labeled water method) over 24 hours were consistently lower in diet-restricted monkeys; but when expressed as a function of metabolic mass, 24-hour energy expenditures and energy balances were not different between long-term diet-restricted and ad libitum-fed monkeys (Lane et al., 1995a). DeLany et al. (1998) found, however, that energy expenditure (also determined by the doubly labeled water method) was lower in rhesus monkeys that were diet-restricted for more than 10 years than in ad libitum-fed controls, even with correction for differences in body size with BW, surface area, or lean body mass as a covariate. Weekly adjustments of energy intake to maintain a stable BW over the long term were shown to prevent obesity and the onset of type II diabetes, a disease that develops in many middle-aged rhesus monkeys (Hansen and Bodkin, 1993). Ramsey et al. (1996) reported that nighttime energy expenditures (determined by indirect calorimetry) were significantly (P < 0.001) lower in rhesus macaques at the 24- and 30-month assessments of diet restriction than in ad libitum-fed controls after adjustment for lean body mass. However, morning, afternoon, and total energy expenditures did not differ between groups.

Dietary intakes and morphologic measurements of the NIA rhesus macaques and squirrel monkeys were reported after 5 years on the study. The target diet restriction was to 70% of ad libitum intake; the average diet restriction for the two younger groups of rhesus macaques was to 67%, whereas the average diet restriction for the two younger groups of squirrel monkeys was to 78% (Weindruch et al., 1995). Nutritionally adequate restricted diets reduced BW and crown-rump length by 10-20% in rhesus monkeys. However, the influence of diet restriction on squirrel monkeys was less obvious, probably because the restriction did not reach the target for this species. Such health measures as body temperature, adiposity, blood pressure, and blood concentrations of glucose, insulin, and triglycerides were reduced, and there was a trend toward lower blood concentrations of glycosylated hemoglobin in the diet-restricted rhesus monkeys in the Wisconsin study after 5 years (Moon and Taylor, 1994; Weindruch, 1996). Insulin sensitivity, however, increased in the diet-restricted monkeys, and this was linked to changes in BW and abdominal girth.

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