The idea that eating less might make us live longer has been around for thousands of years. Even Hippocrates, the Ancient Greek physician, argued that, "When a patient is fed too richly, the disease is fed as well. Any excess is against nature."
Scientists have now spent decades testing whether his advice holds true.
The first striking evidence came in the 1930s, when American nutritionist Dr Clive McCay found that rats fed a restricted diet lived almost twice as long as those who were allowed to eat what they liked.
And they weren’t struggling on, too hungry to muster the energy to die. These rats were, in fact, healthier in their old age, with better-looking lungs and kidneys and no cancer (until the rats’ food supply was increased again right at the end of the experiment).
In the intervening century or so, we’ve found that cutting back calories can have life- and health-extending effects across the tree of life: from single-celled fungus yeast, to nematode worms, flies, spiders, grasshoppers, guppies, trout, mice, hamsters and dogs.
Why? The theory is that reduced food intake might push a biological button inside our cells, telling them to hunker down.
If snacks are scarce, it makes little sense to burn calories by, for example, gearing up for reproduction. This is an energetically expensive process with the added disadvantage that your offspring would be born into a world without enough food.
Instead, evolution would prefer that an animal in this situation preserves its energy and reproduce another day. Instead, the body slows the ageing process, improving the odds that you’re still biologically young and fit enough to reproduce when food returns.
The anti-ageing effects of eating less food
While there’s a decent weight of evidence for calorie restriction in animals, data in humans are harder to come by.
Funders, ethics committees and participants are understandably reluctant to sign off on the forcible adoption of multi-decade meal plans.
The biggest trial to date was the so-called CALERIE trial (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy – see what they did there?), where participants aimed to cut back their intake by 25 per cent for two years.
(In practice, the average reduction was just 12 per cent – an indication of how hard diets like this are to maintain, even with a lot of help from scientists.)
Two years is obviously far too short a time to determine if it made people live longer, but they did lose an average of 8kg (17.6lb), and saw modest reductions in LDL cholesterol, blood pressure, blood sugar and markers of inflammation.
Trimming the protein from your diet
There’s another question that’s perhaps even more important if you want to put this idea into practice yourself: what exactly should you be cutting back on?
Recent research suggests that we actually need to eat less of the current health influencer darling: protein.
For instance, one study from The University of Sydney suggested mice on a low-protein diet lived about 30 per cent longer than those fed protein-rich chow.
It might even get more specific than that. Proteins are made up of 20 chemical building blocks called amino acids, and cutting back on one or more of them can be enough to increase lifespan.
Research has found that cutting back on ‘branched-chain’ amino acids (BCAAs, named after the shape of their chemical structure) can make male mice live 30 per cent longer. (Why it doesn’t work in female mice is unknown.)
Cutting back specifically on isoleucine, another amino acid, helped male mice live 33 per cent longer (but female mice just 7 per cent).
Research into other amino acids, however, shows that it’s a delicate balance. Take methionine, for instance.
Mice fed a diet containing 0.15 per cent of this amino acid lived a whole 10 per cent longer than mice eating a more typical 0.4 per cent methionine diet.
Mice in the same study were also fed 0.1 per cent methionine and died early, often with rectal prolapse – I think I’d rather die of old age than run the risk of that happening.
Recent work has switched from restricting dietary components to trying to optimise them instead. The challenge is that, with 20 amino acids, the number of combinations rapidly becomes overwhelming.
Even if you wanted to try just ‘high’ and ‘low’ amounts of each amino acid in combination, that would require over a million experiments.
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The amino acid diet based on your DNA
Scientists have tried to sidestep this problem by looking into our DNA, which contains the instructions for building proteins. The LEGO bricks that make up proteins are amino acids.
What if we provide an organism with a diet whose amino acid ratio directly mirrors the proportions found in the DNA?
An initial study in fruit flies found that those on the DNA-matched diet grew bigger, grew faster, laid more eggs and lived longer than the flies who were fed regular food.
Follow-up work has taken place in mice – when fed the ideal dietary amino acid ratio found in their DNA, they grew bigger faster, and the male mice had more muscle and better sperm production.
But we don’t yet know whether these mice will also live longer.
What might be happening biologically when protein is cut from a diet is unclear, but scientists are starting to pull back the curtain. Like straightforward calorie restriction, it seems to slow down the ageing process pretty broadly.
A new paper published in May 2025 even suggests that low-protein diets could reduce damage and mutations in DNA.
The suggestion isn’t that protein directly causes mutations, but that its effect on an organism’s metabolism results in the production of ‘free radicals’ that can cause damage to DNA and other molecules in our cells.
Mutations in DNA are known to cause cancer and have long been suspected of having a more general role in the ageing process.
The idea that dietary changes could indirectly affect something as fundamental as the rate of these chemical ‘typos’ appearing in our DNA is worth exploring.
Some people need more protein
So, should you start cutting back on protein? While the animal data is suggestive, results in humans are more mixed.
One 2014 paper found that people eating less protein lived longer than those with a high-protein diet: a 50-year-old eating less than 45g (1.6oz) of protein per day could expect to live around four years longer than one eating 90g (3.2oz) daily.
Giving general advice is tricky, though: in the over-65s, the same study found the reverse effect. One reason could be that we tend to lose muscle mass with age, and eating extra protein makes it easier to pile on the lean pounds.
The scientists also found that those eating more plant protein didn’t see this increased risk of death in mid-life.
This has led some to argue that the risk increase has more to do with eating excess meat – especially red and processed forms – rather than protein in general.
Another possibility is that plant proteins tend to be lower in certain amino acids, including methionine, so it might be that people who eat more veg are already enjoying a bit of methionine restriction as a result of their diet.
Unfortunately, there haven’t been any human studies on purposely restricting amino acids yet.
But it would be really interesting to try this in people – perhaps not with protein powders, but by finding some combination of dietary protein sources that match up to our DNA’s requirements, making it easier to implement in everyday life.
Finding this could help sidestep the side effects of dietary restriction: people cutting back on certain foods report hunger (unsurprisingly!), feeling cold, a lower sex drive, irritability and slower wound healing.
As the old longevity science joke goes, dietary restriction might not make you live longer, but it will certainly feel like longer.
A better pill than protein restriction
Perhaps the solution doesn’t lie in the fridge, but in the medicine cabinet instead. A drug called rapamycin, for instance, activates a cellular recycling pathway (one that also occurs during dietary restriction), making mice live up to 60 per cent longer.
Diabetes drugs that lower blood sugar are another way to emulate eating less and they also extend the lifespan of mice.
And, of course, GLP-1 agonists such as semaglutide (Ozempic) seem to prevent a whole range of conditions at once, in large part by directly reducing how much people eat.
Could one or more of these medicines help us all stay healthy without needing to observe a decades-long diet?
As someone who wants to live a long, healthy life but doesn’t want to be a ‘hangry’ 100-year-old, I hope the clinical trials start soon.
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