A 25-year study published recently in Nature Communications has demonstrated that caloric restriction in rhesus monkeys bestowed numerous health benefits by reducing both mortality and age-associated diseases. These results contradict an earlier study which reported no differences in survival rates, but the scientists also believe they have an explanation for this.
Restricted calorie intake whilst maintaining a continued supply of essential nutrients has been demonstrated previously to increase the longevity of several different organisms, from unicellular yeast to mice and fruit flies. Alongside a longer lifespan, this caloric restriction (CR) also delayed the onset of multiple age-related diseases in rodents. In order to gain more translatable information which could infer the effects of CR in humans, researchers turned to monkeys. This is because non-human primates display physiological, anatomical and behavioral similarities to humans. Rhesus macaques, which were used in this study, also display similar aging characteristics to humans such as greying of the hair and loss of muscle tone. Certain diseases which are associated with increased age in humans such as diabetes and bone loss also become more prevalent with aging in these animals.
Researchers from the University of Wisconsin, Madison (UW) used 76 rhesus monkeys for the study which were divided into two groups. In the control group the monkeys were fed ad libitum and received as much food as they wanted, whereas the other (CR) group received a diet containing 30% less calories than what they had been previously eating. The monkeys within the control group had a 2.9-fold increased risk of disease and a 3-fold increased risk of death when compared with the CR group. This is in stark contrast to a previous influential study carried out by the National Institute on Aging (NIA). They also carried out a long-term study on 120 monkeys and found no significant differences between the CR group and the control group.
The scientists from UW pondered why this could be, and have offered some possible explanations. According to Ricki Colman from UW, her group began working with adult monkeys which meant that they already knew how much food the monkeys wanted to eat; they therefore took this amount as the normal calorie intake and reduced the calories by 30% from this. The NIA, however, based their feeding amounts on a standardized chart issued by the National Academy of Science. The scientists from UW concluded that this resulted in the monkeys in the control group also receiving a reduction in calories, which could explain why no differences were seen. Colman also found that the control monkeys at NIA weighed less at all times during the study when compared to the monkeys used in the UW study.
Although encouraging and informative, these data should be interpreted with care. One of the authors of the study, Rozalyn Anderson, pointed out that it is important to realize that the results should not be used as a lifestyle recommendation; few people can cope with a constant 30% reduction in calories. "We are not studying it so people can go out and do it, but to delve into the underlying causes of age-related disease susceptibility," she adds. According to Anderson, in the species that have been investigated so far CR causes metabolism to be reprogrammed and affects the ability of the organism to respond to environmental changes.
Although similar data are available from human studies, they are more clinically restricted than monkey studies. Rodent studies previously carried out are also limited in what they can inform us of what is happening in humans. The authors propose that the benefits of CR on aging are conserved among primates, and suggest that the advances from this study will translate directly to human aging and health.