It's 6.30pm, and so far today Dave Fisher has eaten 150g of prawns, a skinless chicken leg, 50g of raw peas, and a few handfuls of blackberries, strawberries, blueberries and nuts. Dinner is yet to come, and will consist of a lean chicken breast and a small salad. In total, he will have ingested around 1,600 calories, perhaps a 1,000 fewer than a typical Western man of similar height and age.

Fisher, 51, is not trying to lose weight, though weight loss is the inevitable result of the dietary regime he began 20 years ago. At 5ft 10in and 10st 8lb he is lean rather than skinny and says he appreciates food far more than he did when he was eating a lot more of it. He eats so little every day because he wants to be healthy well into old age, and because he thinks it could be the key to unlocking 20 or 30 years of extra life.

"There are two goals to a calorie-restricted diet," says Fisher. "One is maximum lifetime expansion. The other is an improvement in health in the meantime, and reducing the chances of succumbing to the diseases of ageing – heart disease, cancer and so on. I think it's worth doing for either goal."

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Fisher is one of a growing number of calorie restrictors who believe, to put it simply, that less food can equal more life. Calorie restriction (or CR) may sound faddy and Californian but it has certainly piqued the interest of scientists. Last week, scientists announced the conclusion of a 20-year study by a team at the University of Wisconsin on rhesus monkeys. It is the best known of a plethora of animal experiments that are testing the claim that very low-calorie diets can improve health and extend life. And like most of them, it has produced striking results in their favour.

The Wisconsin study involved 76 monkeys split into two groups, one on a calorie-restricted diet and a control group that ate normally. It found that those on normal diets were three times as likely to develop age-related illnesses such as cancer and cardiovascular diseases. By the end of the study, half of the normal eaters had died, while only 20 per cent of the calorie-restricted ones had. What's more, calorie restriction appeared to help preserve the brain health of the primates, too, leading to better memory, motor skills and problem solving.

Time after time, in fact, researchers have found that animals which are forced to eat more sparingly seem to age more slowly. Experiments have suggested that CR can extend lifespans by as much as 30 per cent – at least in rodents. Similarly, the diet has boosted the longevity of worms, dogs and cows. For people like Brian Delaney, president of the Calorie Restriction Society and author, with Lisa Walford, of The Longevity Diet, published by Four Walls Eight Windows, the evidence is pretty irrefutable. "I adopted CR mostly because of overwhelming evidence in laboratory animals, combined with a bit of speculation on how evolution works," he says. "The variety of species in which it's been tested – and shown to work – is so broad that it wouldn't make sense for this 'longevity diet effect' to have disappeared from our genes."

That may be true, but, without evidence from human studies, the message has been easy to ignore. Humans are more difficult to study. They tend to sneak in a Mars bar when no one is looking, or fall off the CR wagon completely after a few months. Nevertheless, two human CR studies have recently been completed, and they hint, tantalisingly, at the possibility that restricting calories really can improve health in a way that holds out the promise of extended life.

There are provisos. The studies were small and in one case the results are only preliminary findings. But they're certainly exciting. Another study by researchers from the University of Washington showed that the hearts of CR dieters had the elasticity and efficiency of people 15 years younger. There was even evidence that a calorie restricted diet had reversed – as well as delayed – the normal decline of ageing. "We don't know how long each individual will end up living," says Dr John O Holloszy, one of the researchers, "but they certainly have a longer life expectancy than average because they're most likely not going to die from a heart attack, stroke or diabetes. And if in fact their hearts are ageing more slowly, it's conceivable they'll live for a very long time."

The study involved just 25 people, and reducing your chances of age-related illness isn't necessarily the same as increasing your maximum lifespan. Dave Fisher took part in the Washington study and admits that "definitive proof that CR increases maximum lifespan will only come when someone lives to 130".

But another study, at the University of Louisiana, may have fitted another piece into the CR jigsaw. This found that a strict CR diet reduced age-related DNA damage. Again, the study was small – 48 people – and only looked at changes over a six-month period. But microscopic damage to DNA is linked with cancer and other ailments of later life. The researchers say the results are "striking".

Nobody believes the apparent benefits of CR can be achieved by simply missing meals or discarding half of your hamburger. Calorie restrictors optimise nutrition. Without doing so, they're just starving themselves. And critics argue that cutting calories so dramatically while maintaining a healthy nutritional balance is extremely difficult to do. As a modern lifestyle choice, CR hardly seems compatible with busy lives and snatched lunch breaks, and missing out on vital nutrients would more than undermine any beneficial effects from restricting calories.

CR also seems to contradict our genetic impetus. In times of plenty, our genes urge us to store fat at every opportunity. So why would they reward us with longer life for doing the exact opposite? In fact, advocates of CR believe this apparent evolutionary contradiction is exactly why the longevity diet is biologically valid.

It's all down to a nifty piece of biological self-preservation, they argue. When food is scarce our body temperatures drop, energy is conserved and – at a microscopic level – we activate a survival response. It gives us a better chance of still being around when there's plenty to eat again, and when we can get the chance to fulfil our genetic purpose by producing children.

"There is evidence that DNA repair is more efficient on a CR diet," says Fisher. "The theory is that, in times of famine, the body puts more resources into keeping itself alive. When there's plenty of calories around, the body goes into breeding mode. It doesn't bother to repair itself, because, when you can reproduce and pass on your genes, you – as an individual – no longer matter that much."

So a CR diet tricks our genes into looking after the body they've got, rather than looking for immortality in future generations. At least, that's the theory. Not everyone is convinced, but as results begin to filter through from human studies, it's becoming more and more difficult to dismiss the claims of the longevity diet, whatever its evolutionary purpose. It's possible that lifespans of 120 years could be just one (radical) lifestyle choice away.