216, 145, and 93 kcal·kg-1 of BW, respectively, and by diet-restricted juvenile, adult, and old rhesus macaques were 153, 102, and 66 kcal·kg-1 of BW. Daily GE intakes by juvenile, adult, and old squirrel monkeys fed ad libitum were 341, 264, and 229 kcal·kg-1of BW, respectively, and for diet-restricted squirrel monkeys 242, 188, and 160 kcal·kg-1 of BW, respectively. Those values were calculated on the basis of measured daily dietary intakes in grams, GE concentrations of the diets, and average BW of the animal groups (Ingram et al., 1990).

A second aging primate study, done at the University of Wisconsin Regional Primate Research Center, was initiated concurrently (Kemnitz et al., 1993; Weindruch, 1996; Ramsey et al., 1997, 2000). An original group of 30 adult male rhesus macaques (M. mulatta) 8-14 years old, divided into 15 fed ad libitum and 15 that were diet-restricted to 30% below ad libitum intake, was expanded to include 16 additional male and 30 female rhesus macaques (Moon and Taylor, 1994). The monkeys were individually caged to control access to diet and to allow accurate daily measurement of dietary intake and feed waste. All monkeys were fed a defined, pelleted diet containing (by weight) 15% lactalbumin, 10% corn oil, about 65% carbohydrate, and 5% cellulose (Kemnitz et al., 1993; Ramsey et al., 1997). Ad libitum-fed controls were given free access to this semipurified diet for 6-8 h·d-1 while diet-restricted monkeys were fed the diet at 70% of their baseline intake, predetermined individually. A piece of fresh fruit was provided daily (kind and energy contribution not identified). The semipurified diet furnished ME at an estimated 3.98 kcal·g-1 using the percentages and ME values of individual ingredients in the formula as provided by Merrill and Watt (1955).

A third aging primate study, done at the University of Maryland, examined the effects of ME restriction and its relation to obesity and signs of diabetes in adult rhesus macaques (M. mulatta) (Hansen and Bodkin, 1993; Bodkin et al., 1995). After the macques reached full maturity, ME intake was restricted by weekly diet-intake adjustments to maintain a stable adult weight of 10-12 kg (Hansen and Bodkin, 1993). That method of caloric titration retarded middle-age-onset obesity (which is common in rhesus monkeys) and resulted in lower blood insulin concentrations and higher glucose tolerance in the diet-restricted animals (Hansen and Bodkin, 1993). After 9 years of diet restriction, the daily ME intake required to maintain a stable adult BW proved to be 40% less than the ME intake by ad libitum-fed controls.

Seven older (average, 20.7 years) male rhesus macaques (M. mulatta) were kept on a restricted diet for about 9 years. Seven male rhesus of similar age (average, 21 years), with no evidence of diabetes or impaired glucose tolerance served as ad libitum-fed controls (Bodkin et al., 1995). Four of the ad libitum-fed males were offered a standard commercial monkey diet with a composition (by weight) of 17% protein, 70% carbohydrate, 13% fat, and ME at 3.5 kcal·g-1. Three ad libitum-fed males were provided a complete liquid diet (Ensure®, Ross Laboratories, Columbus, OH) designed for human consumption, containing 14% protein, 55% carbohydrate, 31% fat, and ME at 4.9 kcal·g-1 of DM. In dilute form, this product provided ME at 1.0 kcal·ml-1. The diet-restricted monkeys were fed the commercial monkey diet three times per day. Restrictions in diet intake resulted in an average 35% reduction in ME intake compared with the ad libitum-fed controls, or ME at 582 and 894 kcal·d-1, respectively (Bodkin et al., 1995).

After 1 year of the NIA study, diet restriction appeared to have had a greater effect on BW gain among squirrel monkeys than among rhesus when absolute BW gain in diet-restricted animals was expressed as a percentage of that in controls (Ingram et al., 1990). When juvenile and adult rhesus macaques were diet restricted, absolute BW increases were 48% and 29% of those observed in ad libitum-fed controls, respectively. The absolute BW increases in the diet-restricted squirrel monkey juveniles and adults were only 35% and 24% of those in ad libitum-fed controls, respectively. When rates of BW gain in diet-restricted animals were expressed as a percentage of those in ad libitum-fed controls, relative gains were 46% and 49% for juvenile and adult rhesus macaques, respectively. For juvenile and adult squirrel monkeys, these estimates were 32% and 20%, respectively. Absolute diet consumption was reduced by 23% and 24% in diet-restricted juvenile and adult rhesus, respectively; the corresponding reductions in squirrel monkeys were 22% and 24%. Old monkeys of both species continued to gain BW when fed their respective diets.

After 1 year of diet restriction, the adult rhesus macaques in the Wisconsin study were in apparent good health and had no clinical evidence of detrimental effects. The adult ad libitum-fed controls had dietary intakes below National Research Council (1978) recommendations (Ingram et al., 1990), but average BW increased by 9% during the first year of the study (Kemnitz et al., 1993). The diet-restricted monkeys did not gain BW and had 33% less body fat than the controls, but there were no lean body mass differences until after 2 years (Ramsey et al., 1997). Dual-energy x-ray absorptiometry (DEXA) was used to measure the effect of 20-30% dietary restriction on body composition at baseline and after 6, 12, and 18 months (Colman et al., 1998). At baseline, males had significantly (P < 0.05) greater values than females for BW, body mass index, total body lean tissue mass, appendicular skeletal mass, and total body bone mineral concentration. When analyzed longitudinally through 18 months, ad libitum-fed females had significantly increased BW, total body fat tissue mass, total body percent fat tissue mass, total body lean tissue mass, appendicular skeletal muscle mass, total body bone mineral concentration, and abdominal fat tissue mass relative to diet-

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