Caloric restriction (CR: reducing caloric intake 30-40 percent below ad libitum levels) remains the only intervention that reproducibly extends lifespan, reduces the incidence and delays the onset of age-related disease, enhances stress protection, and attenuates functional decline 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 has not been established. 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 (30 percent reduction from control levels) in primates and that physiological responses to the diet are similar to those reported in rodents. In addition, CR reduces risk factors for age-related disease, such as cardiovascular disease and diabetes. In the past year several studies have been completed relating to the physiology of aging in primates and rodents and effects of CR and other interventions. Analysis of motor performance in rhesus monkeys has demonstrated significant age-decline in reaction time required in a fine motor task, and monkeys on CR are showing enhanced performance in the most complex aspect of the task. We have also continued to monitor morbidity and mortality in our longitudinal studies of CR in rhesus and squirrel monkeys. The numbers of deaths and diagnosed diseases remain too low for conclusive statistical analyses; however, the emerging data continue to show lower morbidity and mortality in the CR monkeys in both species. In collaboration with the Laboratory of Cardiovascular Sciences, studies are underway to investigate the effects of CR on cardiovascular measures related to arterial stiffness in monkeys on diets with low and high salt content. Thus far, we have found that higher salt content increases arterial stiffness, but that this response depends on the ability of the monkeys to excrete salt in their urine. We have also begun a study in rhesus monkeys to evaluate the effect of a high cholesterol diet on arterial stiffness. A major objective of our research program is to develop a high throughput assay system to examine effects of CR and CR mimetics. We have now demonstrated the further utility of an in vivo serum assay to move this objective along. Previously we had shown that cells grown in serum obtained from CR rats or monkeys exhibited greater resistance to stress. More recently, we found that serum from CR rats upregulates a gene (Sir2) that has been implicated in longevity of lower organisms. This upregulation results in a sequestration of the proapoptotic factor Bax away from mitochondria, thereby inhibiting stress-induced apoptotic cell death. Thus, CR could extend life-span by inducing SIRT1 expression and promoting the long-term survival of irreplaceable cells.
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