of work: Caloric restriction (CR) remains the only intervention that reproducibly extends lifespan and retards aging processes in mammals. ALthough the effects of CR on aging have been widely studied in rodent models and other short-lived species, the relevance of this intervention to human aging is not known. In 1987, the NIA began the first controlled trial of CR in a primate species. Results from these studies have thus far demonstrated that it is possible to initiate long-term CR in primates and that physiological responses to the diet are similar to those reported in rodents. In addition, CR reduced risk factors for age-related disease such as cardiovascular disease and diabetes. Summarized in the following paragraphs are the most recent findings related to these ongoing studies. In the past year several studies have been completed relating to the physiology of aging in primates and effects of CR and other interventions. For example, we reported that, consistent with human data, rhesus monkeys on CR exhibit a circadian pattern of melatonin secretion with peak levels occurring between midnight and 2 AM. Similar to humans the peak melatonin level declined significantly in older rhesus monkeys. This age-related decline was prevented in monkeys on CR. Previous studies have shown that CR slowed the age-related decline in blood levels of dehydroepiandrosterone-sulfate (DHEAS), a hormone of adrenal origin. These findings suggest that CR may alter postmaturational aging and may contribute to retardation of changes in endocrine function that occur with age. Another set of studies published this year examined the effects of CR on skeletal health and metabolism in both male and female rhesus monkeys. We reported that male monkeys on CR exhibit slight, but significant reductions in bone mineral density at the mid and distal radius. Female monkeys on the diet generally have lower bone mass, compared to controls, but this difference was not statistically significant. In these studies we also examined several markers of skeletal metabolism, calcium homeostasis and reproductive function. Several markers examined showed the expected age-related changes. However, there were no effects of CR on markers of bone turnover (osteocalcin, urinary cross-links), calcium homeostasis (parathyroid hormone, serum calcium levels, and vitamin D), or reproductive function (estrogen, progesterone, follicle stimulating hormone and lutenizing hormone). In addition, CR did not disrupt normal menstrual cycling in females. These data suggested that CR had no adverse effects on normal skeletal physiology except that bone mass was reduced. In depth analyses of the relationship between CR-induced changes in body composition revealed that the reduced bone mineral density observed in CR monkeys was entirely explained (statistically) by diet-induced reductions in weight and lean mass. Since it is not possible to conduct controlled studies of bone fracture threshold in the monkeys, we assessed the effects of CR on bone mass and strength in rodents. We observed that under identical dietary conditions Fischer344 rats on CR lost weight, lean mass, and bone density. However, fracture threshold was not altered in bones from CR rats. These findings suggest that bone density is reduced by CR, but this does not lead to significant reductions in bone strength. It is possible that the observed reductions in bone density represent skeletal remodeling in response to the diet and that CR will not result in increased fracture occurrence. In collaboration with the Laboratory of Cardiovascular Sciences, we evaluated the effects of a compound designed to break protein cross-links on indices of vascular stiffness, in monkeys. We reported that ALT-711, a selective cross-link breaker, improved indices of arterial stiffness such as aortic pulse wave velocity and augmentation index. Preliminary studies also suggested that CR improved these same markers of cardiovascular function. Studies are underway presently to more carefully investigate the effects of CR, with and without a high salt diet, on cardiovascular measures related to arterial stiffness. Finally, we have continued to closely monitor morbidity and mortality in our longitudinal studies of CR in rhesus and squirrel monkeys. The numbers of deaths and diagnosed disease remain too low for appropriate statistical analyses; however, the preliminary data are consistent with a reduction of morbidity and mortality by CR in both species.
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