About 90 years ago, Cornell University scientists put a group of rats on a very low calorie diet.

Ordinarily, these rats lived for about three years – but when their calories were heavily restricted, they lived for four.

In other words, their lives were extended by a third.

They also remained youthful-looking longer, and suffered less chronic disease.

Here it was – the fountain of youth

Since then, unsurprisingly, scientists have been trying to replicate these results in monkeys – with the results often described as “promising”.

The results of short-term studies in people have tended to be “suggestive”.

Away from the rigours of the lab, the research has led to blanket claims (particularly in the context of intermittent fasting) that, yes, a low-calorie diet will add years to your life.

That has not been proven yet. And it’s not exactly what the research has been exploring.

What are the scientists up to?

The most useful definition of ageing I’ve read comes from a 2002 paper in The Milbank Quarterly, a journal of population health and policy.

It describes ageing as “a process that converts healthy young adults into less healthy older ones with an increasing risk of illness and death”.

We’ve previously reported on the Longevity Diet, a version of the Mediterranean diet that includes a regular period of low-calorie intake that mimics the effects of fasting – and which, according to its creator, prompts stem cells to regenerate the immune system.

So far it has shown to be effective at reducing the risk of age-related diseases, but a combination of healthy diet and exercise tends to do that.

And keep in mind that when you read that exercise or eating more salad will help you live longer, it means you’ll live longer than the burger and fries addict permanently plonked in front of a screen.

However, what scientists are beginning to show, at a molecular level, is that calorie restriction might work to slow down some of the biological processes associated with ageing.

None of this research has the drama of 90-year-olds walking around with the vitality of 50-year-olds. The gains are much more nuanced, if significant for one’s health, and for many people won’t be worth the sacrifice.

Slowing the clocks

Long-term studies involving people are rare.

So there’s quite a bit of interest in a new paper from the Columbia Ageing Centre at Columbia University Mailman School of Public Health.

This was a randomised controlled trial that looked at the effects of significant calorie reduction over a two-year period.

The 220 participants were healthy, non-obese adults, aged 21 to 50, split into two groups.

One group followed a regular diet, where the recommended daily calorie intake was followed.

A healthy diet for women involves eating about 2000 calories a day. For men it’s between 2500 and 3000 calories a day, depending on their build.

The experimental group consumed 25 per cent fewer calories than is normally recommended. The challenge was to ensure the participants had their nutritional requirements met and didn’t end up malnourished.

To measure biological ageing in the trial, the researchers analysed blood samples collected from participants at pre-intervention baseline and after 12 and 24 months of follow-up.

This is where it gets tricky

According to a statement from Columbia, the researchers analysed methylation marks on DNA extracted from white blood cells.

DNA methylation marks “are chemical tags on the DNA sequence that regulate the expression of genes and are known to change with ageing”.

DNA methylation biomarkers are known to determine biological age of any tissue across the entire human lifespan, even during development.

“Humans live a long time,” said senior author Dr Daniel Belsky, associate professor of epidemiology at Columbia Mailman School.

“So it isn’t practical to follow them until we see differences in ageing-related disease or survival. Instead, we rely on biomarkers developed to measure the pace and progress of biological ageing over the duration of the study.”

In the primary analysis, the scientists focused on three measurements of the DNA methylation data, sometimes known as ‘epigenetic clocks’.

These clocks are actually blood tests

Each of these ‘clocks’ is a particular blood test.

The first two, the PhenoAge and GrimAge clocks, estimate biological age. This is the chronological age at which a person’s biology would appear ‘normal’.

These measures “can be thought of as ‘odometers’. They provide a static measure of how much ageing a person has experienced”.

The third measure, a new test developed by Columbia, is called  DunedinPACE.

This estimates the pace of ageing, or the rate of biological deterioration over time. Think of DunedinPACE as the speedometer.

The results

In the participants who underwent calorie restriction, the DunedinPACE clock recorded a 2 to 3 per cent slowing in the pace of ageing.

That doesn’t sound like much but in other studies this would be read as a 10 to 15 per cent reduction in mortality risk.

This would be similar, in effect, to giving up smoking.

As a health benefit, that’s a highly significant result, but it’s a long way from looking prettier in the mirror.

And for many people, managing a diet that delivers 25 per cent less energy than is recommended would prove to be a fussy, spartan and joy-compromised way to live.