The jury is in: restricting calories does extend longevity, though how the effect is achieved differs from species to species, it turns out. That helps explain the mixed results science has obtained in different species, and even within the same one.
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With rats, for instance, restricting diet at a young age produced healthier, longer-lived rodents. Fruit flies, a favorite among geneticists because they proliferate so wildly, were even used to probe the genetics of the phenomenon. In the rhesus macaque, eating less proved to help adults live longer and better, but calorie restriction is not beneficial for younger monkeys, explain the scientists in their paper in Nature Communications.
In fact the generally beneficial effect of calorie restriction on longevity in mammals had been realized around 70 years ago, and proved to be one of the more highly conserved characteristics in life. In 2002 the same thing was proven in the humble budding yeast, for instance, where lifespan was extended by limiting glucose (or by dampening the activity of glucose-sensing enzymes).
But although the principle seemed to apply to a vast number of species, how it works was another matter. Nor is it clear how it will apply to humans.
What you eat is how long you live
The latest study was done on rhesus monkeys, as a collaboration by two normally competing teams, University of Wisconsin-Madison and from the National Institute on Aging, that worked together to resolve the confusing results that had been achieved so far.
In 2009, one team (UWM) reported significant longevity extension and better health for monkeys that ate less than their peers.
Three years later, the other team, the NIA, reported no significant improvement in survival, though it confirmed improved health. The next year they reported that they had found an improvement in longevity after all.
The teams were baffled as to the differing outcomes. Finally they sat down together and analyzed data collected over years on almost 200 beasts, and say they have figured out the key differences between the sets of rhesus experiments.
One: The two groups restricted the monkeys' diet at different ages. Giving adults less to eat improves their health and extends lifespan; this is not true of younger monkeys.
Two: In the older group of monkeys at NIA, the control monkeys ate less than the Wisconsin control group. The NIA control monkeys lived longer on average compared with the UWM control monkeys. The conclusion was that small differences in food intake in primates can significantly affect aging and wellbeing.
Third, the two groups fed their monkeys very differently. The NIA monkeys supped on naturally sourced foods. The UWM primates were given processed food with higher sugar content and were fatter than the NIA control monkeys at NIA.
That indicates, say the scientists, "that at non-restricted levels of food intake, what is eaten can make a big difference for fat mass and body composition."
And one more phenomenon noticed in the monkeys, which is likely to translate to people: gender differences in the relationship between diet, fat, and insulin sensitivity. Males are more vulnerable to the adverse effects of fatness than females.
Previous studies even indicated that motor function in aged rhesus monkeys was also improved by restricting their calorie intake.
It bears saying that "calorie restriction" in these studies is never to the point of malnutrition. Starving your parents in the name of devotion isn't the idea.
Importantly, the scientists stress that their study does not provide conclusive evidence as to whether caloric restriction slows down the monkeys' biological ageing. The maner in which calorie restriction extends lifespan in monkeys, as opposed to fruit flies, remains to be studied. Also, exactly to what degree caloric restriction needs to be constrained to confer maximal benefit on health and survival remains to be established, in the rhesus, let alone humans.
In fact, though work is being done on the topic, and the internet brims with fake news on it too, conclusions for humans remain to be proven. But based on the rhesus, it seems that restricting calorie intake and preferring naturally sourced foods, rather than processed, from adulthood could be beneficial. Of course, at the individual level, we can never know if we could have lived longer or not, but it is food for thought.