restrictions associated with use of nonhuman primates as study subjects limit the techniques that can be used. Meehan et al. (1989) evaluated deuterium oxide (D2O) dilution, bioelectric impedance (BIA), and skinfold thickness for assessing body composition in western lowland gorillas (Table 9-7).Body-composition estimates based on D2O dilution were not statistically different from those based on the BIA method. Skinfold measurements were highly variable and could not be correlated with either method.

OBESITY

Growth of primates includes changes in body composition (Alberts and Altmann, 2001). True growth can be defined as an increase in the size of muscles, bones, internal organs, and other associated parts of the body, as contrasted with fat deposition. After adult dimensions are reached, body remodeling continues; and during aging, the body tends to accumulate fat and lose lean. With a persistently positive energy balance, accumulations of adipose tissue cause body weights to increase, and this ultimately leads to obesity.

A natural tendency for captive rhesus monkeys to develop obesity was observed first by Hamilton et al. (1972) and later by Kemnitz and co-workers (Kemnitz et al., 1989; Schwartz et al., 1993; Wolden-Hanson, et al., 1993) and by Jen et al. (1985). The incidence of obesity in free-ranging, provisioned rhesus monkeys on the Puerto Rico island of Cayo Santiago was 7% (Schwartz et al., 1993), which was about 20% less than observed in laboratory rhesus monkeys (Jen et al., 1985; Kemnitz et al., 1989). The frequency of obesity in rhesus monkeys in the wild is unknown but is believed to be lower (Kemnitz et al., 1989).

Studies on development of spontaneous obesity in other macaque species have been reviewed (Kemnitz, 1984). The incidence and degree of obesity in bonnet macaques, stumptailed macaques, and pigtailed macaques appear to be similar to those in the much more extensively studied rhesus monkey. A rather high incidence (20-60%, depend

TABLE 9-7 Body Fat (%) Determined with Three Methods in Western Lowland Gorillas (Meehan et al., 1989)

Method

Mean ± SD

Min

Max

Female

D2O

29.5 ± 4.1a

6.7

44.5

BIA

35.4 ± 2.3a

27.0

50.4

Skinfold

20.3 ± 1.1b

16.8

25.9

Male

D2O

15.9 ± 2.2a

8.1

19.8

BIA

22.1 ± 3.1a

12.4

30.2

Skinfold

19.9 ± 0.7b

18.1

22.1

a,bStatistically significant difference among methods (P < 0.05, Tukey test).

ing on age) of spontaneous obesity has been noted in squirrel monkeys raised in the laboratory on semipurified liquid diets (Ausman et al., 1981). In contrast with squirrel monkeys, cebus monkeys (Cebus albifrons) did not exhibit a trend toward obesity before or after sexual maturation when maintained and fed similarly for a 7-year period (Ausman et al., 1981). In all settings, it is apparent that only some animals become obese. The data suggest there may be an individual genetic predisposition to obesity in the monkey, as in humans. Furthermore, in monkeys, the predisposition might be species-specific. When unlimited calories are available, only some animals—those with a genetic predisposition—develop obesity. The genetic components of this phenomenon are not understood.

The rhesus monkey has been used as a model for studies of the causes and effects of obesity in humans by both Wisconsin and Maryland groups. The measures used to describe obesity include a variety of combinations of somatometric, compositional, and body weight data. The definition of obesity in one study was based on body weight: obese monkeys were those which had body weights greater than 2 standard deviations (SD) above the mean for their sex (Kemnitz et al., 1989). A remarkably high correlation (r = 0.978) has been found between body mass index (body mass [or weight] in kilograms divided by the square of crown-rump length in meters) and body fat mass in a group of seven obese and seven normal-weight males and females. Body fat was estimated with tritiated water (for method, see Kemnitz and Francken, 1986). Jen et al. (1985) developed a similar measure, termed the obesity index Rh (body mass [or weight] in kilograms divided by the square of crown-rump length in centimeters), to characterize the fatness of individual rhesus macaques. This measurement was chosen as an appropriate descriptor of obesity based on its high correlation with body weight and blood concentrations of insulin and glucose and its lack of correlation with height. When fat constituted over 25% of body mass, Jen et al. (1985) defined the monkeys as obese. All monkeys had body weights over 13 kg. Monkeys weighing 13-15 kg varied in fatness, but in all monkeys weighing over 15 kg, more than 25% of body mass was fat.

The obese rhesus monkeys in most studies have been fed ad libitum. There was a natural tendency for such monkeys to gradually fatten so that by the age of about 9 years some were obese (Kemnitz, 1984). In the studies of Hansen and colleagues (Hansen and Bodkin, 1993; Hansen et al., 1995) with nonobese animals in the 9-year age range, two groups of animals were selected for longitudinal study. One group of six monkeys had their weights measured and kept constant by food restriction in what was termed “a body weight clamp”; food intake was measured at the start of the study, and then only the amount of food required to maintain constant body weights was fed for the rest of the study. A comparison group of six age- and sex-matched



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