This study has shown that an approximately 30% CR can be safely initiated and maintained long-term in adult male rhesus macaques. These animals are now entering old age, an exciting time for an aging intervention study. Most of the results presented here represent the impact of changing body weight on the skeleton but may also represent the metabolic effects of CR. The reported DXA results add understanding of the skeletal response to CR in a nonhuman primate. Unfortunately, the results from our biochemical markers were highly variable and not particularly informative. Perhaps, the use of more specific markers in the future will prove to be more revealing.
A growing body of evidence demonstrates a positive association between bone mineral density and body mass. A study of moderate CR in women showed a significant relationship between reduction in body mass and reduction in bone mass (Ramsdale and Bassey 1994
). This is consistent with the theory that the most important influence on bone mass is the load it is required to bear (Frost 1993
; Lanyon et al. 1986
). Similarly, studies in both healthy men (Nuti et al. 1995
) and older women (Blain et al. 2001
) have found BMD to be highly related to lean body mass. In female rhesus macaque vertabrae, a weak positive correlation between bone density and weight has been reported (Grynpas et al. 1989
). Contrary to this, DeRousseau (1985
) found a statistically significant inverse relationship between body weight and bone mass in female rhesus macaques, possibly due to the effects of osteoarthritis (OA).
As further evidence of the effect of body weight on bone mass, decreased load on bone, or unloading, has been proven in numerous studies to cause accelerated bone loss among mammals (Wheadon and Heaney 1993
; Amin 2010
; Vernikos and Schneider 2010
). Additionally, prolonged inactivity directly leads to bone loss at weight bearing sites (Qin et al. 2010
; Habold et al. 2011
). Complimentary to this, regular physical activity, resulting in increased loading, increases both the mineral content and the density of bone as well as reducing the rate of demineralization (DiGiovanna 1994
; Wheadon and Heaney 1993
). Furthermore, athletes have been shown to have BMD values greater than nonathletes (Eisman et al. 1991
; Maimoun et al. 2011
; Markou et al. 2010
), and an even more dramatic example, tennis players have higher BMD in their dominant compared to their nondominant arm (Calbet et al. 1998
; Haapasalo et al. 1998
). Specifically regarding these mechanical-loading factors, the skeleton likely responds to mechanical stress such as increased body weight with a stimulation of osteoblast activity (Wardlaw 1996
; Turner and Pavalko 1998
; Mullender et al. 2004
). However, in this group of animals, osteocalcin, a markers of osteoblast activity was not different between the diet groups.
A study in humans found increased BMD of the distal forearm in conjunction with obesity (Hyldstrup et al. 1993
). These obesity-related alterations in BMD have also been found to a greater degree in the vertebral column. Since Hyldstrup et al. (1993
) found this relationship in a peripheral bone, they argue that the effect of obesity on the skeleton is not entirely due to increased mechanical loading. Instead, they suggest that the increased fat tissue mass in obesity leads to increased estradiol levels, due to aromatization of estrogen precursors in adipocytes.
Decreased levels of estrogen and testosterone are linked to loss of bone mass as is the case in postmenopausal women and hypoandrogenic males, respectively. Since estrogen and other highly lipid soluble substances can be stored and subsequently released by body fat, it is believed that increased amounts of fat can result in higher maintenance levels of estrogen and therefore a reduction in estrogen-depletion bone loss (DeRousseau 1985
). Heavier body weights are also associated with increased serum concentrations of testosterone (Edelstein and Barrett-Connor 1993
). Increased fat mass is also likely to support greater conversion of androgens to estrogens. Furthermore, lower concentrations of sex hormone-binding globulin have been noted in obese individuals (Haffner and Bauer 1992
) effectively increasing circulating levels of bioactive estrogen and testosterone. Although the mechanism is still unclear, a reduction in estrogen levels leads to increased skeletal remodeling perhaps via cytokine dysregulation in the bone microenvironment (Horowitz 1993
More recently, the hormonal activity of adipose tissue has gained appreciation. Several adipose-derived hormones (adipokines) are now known to directly modulate the activity of bone cells as well as indirectly influence the skeleton through their actions on the central nervous system. The best known of these adipokines is leptin, a hormone we have previously shown to correlate directly with body fat mass in these animals (Colman et al. 1998
). Locally, leptin is responsible for skeletal preservation by increasing osteoblast proliferation and differentiation (Thomas et al. 1999
) and inhibiting osteoclastogenesis (Holloway et al. 2002
). Indirectly, leptin may act on the skeleton through stimulation of growth hormone secretion and its effects on the hypothalamic–pituitary axis (Carro et al. 1997
). Centrally, leptin has been shown to inhibit serotonin synthesis thereby inhibiting bone mass accrual (Karsenty and Oury 2010
OA, specifically the osteophytes associated with this pathology, can lead to an increase in the amount of mineral detected by DXA technology. For this reason, it is important to account for the presence of OA in a study of DXA-measured BMC and BMD, particularly given the prevalence of OA in aging populations. We have previously shown that OA increased with age and body mass in these animals (Duncan et al. 2011
). In order to account for the presence of OA, we performed lumbar spine scans in two projections, PA and lateral with the understanding that lateral scans would reduce the impact of OA on results. This effect of OA on the interpretation of PA lumbar spine scans may explain the loss of bone mass in R animals on lateral scans, while no significant loss was evident on PA scans.
In summary, male rhesus macaques subjected to long-term CR tend to have lower bone mass than age- and sex-matched C monkeys. However, bone turnover, as measured by biochemical markers is not altered by this intervention. It is believed that the lower bone mass in the R animals reflects the smaller body size of the R monkeys and not a pathological osteopenic condition.