by diet restriction in rats (Sell et al., 1996), and despite the lack of statistical significance, diet-restricted male Wistar rats and male rhesus monkeys generally exhibited a trend toward faster healing than their ad libitum-fed controls (Roth et al., 1997).

Atherosclerosis

Atherosclerosis remains one of the most important age-associated diseases in humans. Most studies that use nonhuman primates to examine the relation of diet to atherosclerotic risk include diets that are isocaloric but with modifications in concentrations of cholesterol, in fatty acid distribution, or in the relative proportions of energy from fat, carbohydrate, and protein (Verdery et al., 1997). Studies with the adult diet-restricted monkey model (intake reduced by 30%, 5% dietary fat, and cholesterol at 4.5 mg per 100 g) have produced decreased plasma concentrations of triglycerides and increased concentrations of HDL2b, the high-density lipoprotein subfraction associated with protection from atherosclerosis. Differences in plasma lipid and lipoprotein concentrations occurring with diet restriction could be accounted for, in part, by decreased BW and improved glucose regulation. The results suggest that diet restriction, as mediated by its beneficial effects on body composition and glucose metabolism, could affect human longevity by decreasing atherosclerotic incidence. Plasma concentrations of low-density lipoprotein (LDL) cholesterol were similar in ad libitum-fed and diet-restricted rhesus monkeys more than 5 years old (82 vs 72 mg·dl-1, respectively [Edwards et al., 1998]). However, LDL particles from diet-restricted animals had a significantly lower molecular weight (2.9 vs 3.2 g·µmol-1, respectively) and were depleted in triglyceride (249 vs 433 mol·particle-1, respectively) and phospholipid (686 vs 837 mol·particle-1, respectively). Thus, diet restriction might be an intervention that retards the consequences of aging, in part by altering factors that contribute to atherogenesis.

BODY COMPOSITION

Although it is well documented in the human-nutrition literature, relatively few studies have been conducted to determine the variability of body composition of nonhuman primates. Body composition is typically described in terms of body fat and lean body mass. Lean body mass (LBM) is defined as body weight minus ether-extractable fat and is thus synonymous with fat-free mass (Forbes, 1990). A number of factors influence body composition, including nutrient and energy intake, sex, age, and level of activity.

Total dissections of pygmy chimpanzees suggest that males have a higher proportion of muscle relative to body weight than females (McFarland and Zihlman, 1994). Young adult (6-9 years old) and middle-age (13-19 years old) male rhesus macaques had more lean soft tissue and less body fat than females in the same age classes (Hudson et al., 1996). The percentage of body fat was greatest during middle age in females and during older adulthood (20-36 years old) in males. There was progressive loss of weight and lean body mass during older adulthood in both sexes in the same animals (Kemnitz, 1994). In adult rhesus macaques the androgenic hormones, testosterone and dihydrotestosterone, promote increases in body mass, which is largely attributable to accretion of lean tissue (Kemnitz et al., 1988).

When body composition was measured in squirrel monkeys during growth, moisture and protein concentrations were found to be linearly related to body mass, but fat and ash were not (Russo et al., 1980). No sex differences were detected.

The effects of nutrient and caloric intakes on body mass and composition are of particular interest. The influence of moderate caloric restriction (to 70% of ad libitum intake) on body mass and composition have been evaluated in Macaca mulatta (Wolden-Hanson et al. 1992). After 12 months of caloric restriction, body weights of restricted animals were 89% of weights of controls; the difference was attributed to reductions in body fat (65% of that in controls). After 24 months, restricted animals weighed 75% as much as control animals, with body fat and LBM40% and 93% of those in controls, respectively. Similar differences in body weight and LBMwere observed in animals that were ad libitum-fed or calorie-restricted (to 70% of ad libitum) over a 4.5-year period (Baer et al., 1998). However, there were no statistically significant differences in body fat (Table 9-6).

Body composition was determined in lean (control) male squirrel monkeys, fatted controls, and obese monkeys. The mean body composition of lean animals, with body weights of 733-950 g, was 64.3% water, 21.7% protein, 7.0% fat, and 7.0% ash and miscellaneous.

The validity of body-composition data is strongly related to the methods used to obtain them. Advantages and disadvantages of the various techniques used in human studies have been reviewed (Forbes, 1990). The animal-care

TABLE 9-6 Physical Characteristics (Mean ± SD) of Control (Ad Libitum-Fed) and Diet-Restricted (30% Restriction) Macaca mulatta after 4.5 Years (Baer et al., 1998)

 

Control (n = 9)

Diet-Restricted (n = 10)

Body weight, kg

8.8 ± 0.3

7.4 ± 0.3a

Body mass index, kg·m-2

27.3 ± 0.8

22.6 ± 0.7a

Lean body mass, %

7.7 ± 0.3

6.3 ± 0.3a

Body fat, %

12.1 ± 2.2

14.2 ± 2.0

aMeans in the same row were different (P 0.05).



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