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IA: SQ, Intra-abdominal to subcutaneous abdominal fat ratio as measured using computed tomography.

axis, and the sympathetic nervous system were characterized in female cynomolgus monkeys. Abdominal fat mass was characterized by computed tomography as previously described (49,55). The monkeys lived in their social groups for 2^ years, and social behavior was recorded throughout this time period. Females above the mean of the ratio of IA:SQ abdominal fat mass were compared to females below the mean.

Females with high IA:SQ abdominal fat ratios spent less time in affiliative social interaction, were more frequently the victims of aggression, and were socially subordinate compared to females with low IA: SQ abdominal fat ratios (Table 15.1).

There was also a modest correlation between behavioral depression and the IA: SQ ratio (Spearman's rho = 0.26, P = 0.05, 1-tailed test), suggesting that females with relatively greater amounts of intra-abdominal fat were more likely to display behavioral depression. Heart rate, a non-invasive indicator of sympathetic nervous system activity, was measured while the animals were in their social groups, using a telemetry system, from 15:00 h to 8:00 h the following day for three consecutive days. Heart rates of these animals are generally lowest at night, increase during the time of day when there is the most activity in their building, and decrease in the afternoon after the activity level in the building declines. Two months following the formation of social groups, there were no differences in high versus low IA:SQ females. However, by 24 months, differences between these groups had emerged. The

Figure 15.4 Association between abdominal fat distribution and heart rate in female monkeys. (Heart rates were recorded 2 months and 24 months following social group formation.) IA: SQ, Intra-abdominal to subcutaneous abdominal fat ratio as measured using computed tomography; HR, heart rate in beats per minute (bpm). □ Low IA: SQ; • high IA: SQ

Figure 15.4 Association between abdominal fat distribution and heart rate in female monkeys. (Heart rates were recorded 2 months and 24 months following social group formation.) IA: SQ, Intra-abdominal to subcutaneous abdominal fat ratio as measured using computed tomography; HR, heart rate in beats per minute (bpm). □ Low IA: SQ; • high IA: SQ

Figure 15.5 Dexamethasone suppression test. Relationship between insensitivity to Cortisol negative feedback and abdominal fat distribution. Females with low IA:SQ intra-abdominal to subcutaneous fat reduce Cortisol secretion by 79%, whereas those with high IA: SQ abdominal fat reduce cortisol secretion by 54% (<0.05). IA: SQ, intra-abdominal to subcutaneous abdominal fat ratio as measured using computed tomography. White line, low IA: SQ; solid line, high IA: SQ

Figure 15.5 Dexamethasone suppression test. Relationship between insensitivity to Cortisol negative feedback and abdominal fat distribution. Females with low IA:SQ intra-abdominal to subcutaneous fat reduce Cortisol secretion by 79%, whereas those with high IA: SQ abdominal fat reduce cortisol secretion by 54% (<0.05). IA: SQ, intra-abdominal to subcutaneous abdominal fat ratio as measured using computed tomography. White line, low IA: SQ; solid line, high IA: SQ

heart rates of all females were similarly elevated during the day; however, in the afternoon and night, heart rates of the females in the high IA:SQ abdominal fat group were higher than those in the low group (P = < 0.05; Figure 15.4).

Hypothalamic-pituitary-adrenal (HPA) function was assessed using a dexamethasone suppression test. Suppression of serum cortisol in response to dexamethasone was greater in females in the lower half of the distribution of IA: SQ abdominal fat mass (P < 0.05; Figure 15.5). This observation suggests that the central regulatory areas of the HPA axis of females with a relatively low IA: SQ ratio are more sensitive to circulatory cortisol concentrations than those with relatively high IA: SQ. Taken together, these data suggest that females with relatively greater amounts of visceral fat are also characterized by behavioral and physiological attributes indicative of chronic stress. Furthermore, the sympathetic nervous system and the HPA axis may mediate the relationship between social stress and regional adiposity. Our findings in cynomolgus monkeys support the hypothesis proposed by some that stress and a hypersensitive HPA axis are central abnormalities in abdominal obesity of human beings (51).

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