Calorie restriction, or CR, is big news in the anti-aging world, but how effective is it really? The evidence might be more mixed than you think
What is calorie restriction?
It's simple. CR limits daily calories but maintains optimal nutrition. Drastic restriction without nutrition is obviously starvation, but maintaining a lower calorie intake could be beneficial.
What's so great about it?
In a world of expensive supplements and snake oil, it offers appealing simplicity. In theory it's cheap and virtually everyone can do it, if they have the willpower. It's also had a fair amount of success in the lab.
Results are still mixed, and extrapolating animal models to humans often fails.
Hang on a second, CR can be harmful?
CR has positive effects on some animals, but a recent study has thrown a spanner in the works by showing CR actually reduces lifespan in some mice. In this study, researchers analyzed the effect of calorie restriction on 41 different strains of mice, both male and female. They found that calorie restriction extends lifespan in a subset of strains, but actually shortens lifespan in an even larger subset (see graph below). Additionally, male and female mice of the same strain reacted differently to CR.
This is important because it shows calorie restriction isn't universally beneficial. In the wild food intake is often unreliable, and one theory is that life evolved to deal with this by putting the body into starvation mode and recycling components; focusing on repair instead of growth. This may work in some creatures, but different organisms have undergone different evolutionary pressures. There's no guarantee CR is good for all, and this eye opening study shows it may even be harmful.
Before we go onto humans in a second article, let's take an overview of CR in animals:
The story begins in the 1930s, when researchers found that certain rats lived 40% longer under a calorie restricted diet. More research has since confirmed similar effects in other organisms, particularly in model organisms like C.elegans.
What about our primate cousins?
Success or failure in worms and rodents is one thing, but what about monkeys? Primates are usually a better predictor of success in humans.
The National Institute on Aging has been studying the effects of CR on rhesus monkeys since 1987, but 2 studies have different results:
In 2012 one study showed that CR has some beneficial health effects including a more youthful appearance, but doesn't increase lifespan.
In 2014 different results were published. Restricting calories was shown to provide multiple health benefits and lower risk of age-related disease and mortality. It helped more monkeys live longer, but again there wasn't an increase in actual maximal lifespan.
Can we explain the differences?
The 2012 study fed the control group a diet low in sugar, including fish oil and antioxidants. Some scientists have suggested that because the control group had a moderate diet anyway, they may have also been slightly calorie restricted. Overall health in both groups was good, and even the control group lived longer on average than expected.
The 2014 team was less fussy with the control group's diet, giving them more sugar and a higher calorie intake. Some people argue they only found more encouraging results because the control group was eating more of a 'Western' diet and were less healthy in comparison.
“When we began these studies, the dogma was that a calorie is a calorie. I think it’s clear that the types of calories the monkeys ate made a profound difference”
A summary of calorie restriction in animals
CR works for some animals, but not all; if you're a mouse you should be extremely cautious. In monkeys it doesn't seem to cause any health problems, but whether it's more effective than a moderate diet remains to be seen. There are hints of positive effects, especially in contrast to more unhealthy diets, but too many questions remain to make any solid conclusions. It might help primates live longer on average and reduce some disease risk, but doesn't extend maximum lifespan.
That's great for a worm but what about me?!
In the coming part 2 we discuss what we know about calorie restriction in humans