When it comes to halting ageing, we’ve tried everything under the Sun. But until there’s an anti-ageing pill breakthrough, scientists are trying to make headway with changes we can make in our everyday lives – including what we eat.
Now, a major new study claims to finally provide some answers to how your diet could help you live longer, but also what the downsides are.
The short answer? They say that dietary restriction (reducing your overall daily calories) has a bigger impact on reducing your lifespan than doing an intermittent fasting diet.
Published in the journal Nature, the team tracked the health of 960 mice on different diets. They made sure there was a good range of genetic diversity across the group of mice to mimic the human population – though experts say we can’t assume the results apply to us.
“This is a fascinating and counterintuitive result, but mice aren't just tiny people so we need to be very careful extrapolating these results to humans,” longevity expert Dr Andrew Steele, who was not involved in the study, told BBC Science Focus.
Nevertheless, he added: “One reason these findings are relevant is because we can't wait the necessary decades to perform a dietary restriction or fasting experiment in humans to see if it makes us live longer, so scientists have been looking for proxy measures to see if people get healthier in the short term.”
During the much shorter lifespans of the mice, the researchers performed regular blood tests and other health evaluations. The mice were assigned one of five diets:
- Eating freely
- Only given 60 per cent of their baseline calories a day
- Only given 80 per cent of their baseline calories
- No food for one day per week (but eating freely the rest of the week)
- The same as group 4, but fasting for two consecutive days a week
The researchers discovered that the free-eating mice (group 1) lived an average of 25 months, while those on the intermittent fasting diets (groups 4 and 5) lived about 28 months. But those in the low-calorie groups lived, on average, even longer: 30 months for the mice eating 80 per cent of their baseline calories, and a whopping 34 months for those on 60 per cent.
In fact, the very-low-calorie diets consistently extended the mice’s lifespans regardless of their body fat and glucose levels – which are usually considered good indicators of health and ageing.
Surprisingly, the mice who lived the longest lost the least weight overall. On the other side, the mice that lost the most weight had little energy, compromised immune and reproductive systems, and shorter lives.
“Our study really points to the importance of resilience,” said Prof Gary Churchill, of The Jackson Laboratory, who led the study. “The most robust animals keep their weight on even in the face of stress and caloric restriction, and they are the ones that live the longest.”
So what does this mean? Essentially, if you’re wanting to extend your lifespan, you may want to hold back on extreme levels of weight loss to get there. According to Churchill, a more moderate level of calorie restriction may be a better approach to your long-term health.
Your genetics may get in the way
As always, it’s not as simple as it sounds. Firstly, there was huge variation between individuals: even in the calorie-restriction groups the mice’s lifespans ranged from a few months to four and a half years.
Secondly, the resilience to weight loss changed across individuals. In every group, the mice that lived the longest were the ones who were able to keep their immune cell health during periods of food stress, as well as those who didn’t lose body fat later in life.
The researchers think that genetic factors in your DNA that we are yet to identify could be behind the different impacts of diets on different people.
“If you want to live a long time, there are things you can control within your lifetime such as diet, but really what you want is a very old grandmother,” Churchill said.
Nevertheless, the study reveals that metabolic markers like weight, body fat and glucose are perhaps not as useful as we thought when it comes to measuring whether calorie restriction is working in people – even if they remain very important indicators of health overall.
“If you need to lose weight and you find fasting works better for you than dietary restriction, then go for it! We know that being overweight is bad for our healthy life expectancy,” Steele said.
“Whether people whose weight is in the healthy range should be cutting back further, or fasting one or two days a week, is less clear – and, while fascinating, I don't think this study is going to put that incredibly long-lived debate to rest just yet.”
About our expert
Dr Andrew Steele is a scientist, writer and presenter. He is the author of Ageless: The New Science of Getting Older Without Getting Old. After completing a PhD in Physics, Steele decided to make the unusual leap to biology and has used computers to decode our DNA at the Francis Crick Institute in London.
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