An important collaboration between 2 research teams has revealed that calorie restriction has a mixed, but positive effect on rhesus monkeys
Calorie restriction (CR) has shown remarkable effects on many lab animals, and to date is one of the most reliable ways of extending healthspan and lifespan in most species. However, in a few species it doesn't work very well and a big questions remains - what about humans? Primates, being our closest cousins, are usually an excellent indication of effects in humans, so 2 important studies previously sought to test calorie restriction on rhesus monkeys in an attempt to unravel whether such a diet could improve human health too.
Reviewing a complex picture
Both studies, one from the University of Wisconsin-Madison and another from the National Institute on Aging, have already revealed contrasting data. The UW study suggested that CR extended survival in monkeys and reduced rates of cardiovascular disease, cancer and insulin resistance. The NIA study found some health improvements but little in the way of overall survival extension. A new collaboration between both teams has therefore sought to unravel why the studies found different results.
"These conflicting outcomes had cast a shadow of doubt on the translatability of the caloric-restriction paradigm as a means to understand aging and what creates age-related disease vulnerability"
After comparison of data involving over 200 monkey subjects, the first finding is intriguingly that CR may actually harm younger monkeys if began early on. It only appeared to have benefits when applied in adulthood, in contrast to some other rodent studies in which earlier intervention produced stronger results.
Diet and sex matters
The second conclusion was that control groups in the studies eat different amounts. The NIA study control group was fed less than the UW group, which suggests that even a small difference may have some beneficial effects. Furthermore, the diet of the UW group involved far greater sugar intake and as a result has higher adiposity (a higher body fat percentage). The NIA monkeys ate an arguably 'better' diet, and as a result that was less difference between the control group and CR group.
The last inference was that sex appeared to effect insulin sensitivity and adiposity. Females did not respond as negatively to higher body fat content, and retained higher insulin sensitivity. Males on the other hand, seemed more acutely vulnerable.
Of course, CR in the best case scenario is still relatively weak; we desperately need to develop more potent technologies to improve human health. However, this sorely needed review reinstates that CR likely does have beneficial effects in primates. This means humans may have similar responses too. The picture is obviously complex though, and for those already eating a balanced, reasonable diet, extra restriction may yield very small rewards. Additional factors such as age and gender may also play a big factor in response.
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