The no-food diet

Starvation, you'd think, is never a good idea, and subjecting yourself voluntarily to a period of fasting – starvation for a short time – is usually associated with religious practice. And within traditional Christian culture, the common view is that, when we fast, we punish ourselves and harm our bodies. But the latest research suggests the opposite, in fact, might be true.

Nobody is saying that we should starve ourselves for long periods. But a mounting body of evidence is hinting strongly at the therapeutic value of intermittent fasting – of taking the occasionally day – weekly or monthly, perhaps – off food altogether.

Much of that evidence comes from what Dr Mark Mattson, of the National Institute on Ageing, an American research body, says is a "quite large body of results" from animal studies. Mattson explains that in laboratory tests, rats and mice on fasting diets tend to live longer, develop fewer cancers and show reduced cognitive decline in ageing compared with animals with continuous access to food.

A few recent human studies seem to back up the view that intermittent fasting, and calorie restriction more generally, are fertile areas for healthcare research. Most intriguingly, quite a large study by researchers at the University of Utah found that fasting on the first Sunday of every month, as many Mormons do for religious reasons, was associated with a 40 per cent reduction in heart disease risk. Mormons who fast are also likely to be non-smoking teetotallers, but the researchers controlled for factors that might also offer protection against heart disease. Fasting was also linked to a lower incidence of diabetes, the study found.

A study conducted in 2007, meanwhile, found that asthma patients who regularly fasted had fewer symptoms and better airway function than those who didn't.

At the moment, the reasons for this are open to speculation. Some researchers say that fasting and calorie restriction change the way our bodies use food, and "turn down" all our major systems. According to Dr Marc Hellerstein, a professor of endocrinology, metabolism and nutrition at the University of California at Berkeley, our bodies are "brilliant" at reacting to not eating. "We're not good at responding to too many calories, but we are very good at responding to fasting. Fasting, in itself, is not an unhealthy process."

Studies on animals suggest this is a protective response. At a cellular level, the dearth of food prompts a mild stress reaction. "We've seen this with brain cells and also liver and heart cells," says Dr Mattson. "Mild stress prompts an increase in production of stress-resistant proteins."

Those proteins offer protection against greater stress. The body reacts to fasting in a way that is ultimately beneficial. And another theory is that restricting calories might help to protect against cancer by slowing the growth of cancerous cells.

Proving that fasting may have a protective effect is one thing. Recommending it as a lifestyle choice is entirely another. Though intermittent fasting for health is becoming popular, particularly in America, some experts think it might be counter-productive. Dr Joanne Lunn, a senior nutrition scientist at the British Nutrition Foundation, thinks that any faith in stark, black and white solutions such as fasting is misplaced.

"Fasting can leave the body short of the nutrients that it doesn't store and can leave you feeling fatigued," she says. "It's much better to look at what you're eating all the time. And if you take a study like the Mormon one, the fasting is part of the culture. It might be a much harder thing to do when friends and family aren't on board, when you have a full day at work to do, if you want to exercise, and so on. We know from studies in children and adults that only missing breakfast reduces both creativity and concentration levels."

But other experts think that intermittent fasting is a good deal easier than other forms of calorie restriction. Fasting for one day per month or fortnight isn't so hard, they say, when you know that you can go back to normal eating only a few hours later.

I'm with Dr Lunn. If you're not used to fasting, and you have a healthy appetite, going without food for 24 hours, and with just water to drink, is a major undertaking. At least, it was for me. The first night was fine, but within a couple of hours of missing breakfast the next day it felt like the cold was seeping into my bones and that full concentration was impossible. By 2pm, I would happily have broken my fast on the first bit of leftover lunch that caught my eye. This was the north of England during a February cold snap, and I'm well known among friends for both the size of my appetite and the haziness of my focus, but this was different. It felt, for a while, like a slow torture. As the afternoon progressed, I began to seriously slump.

And then a funny thing happened. About 4.30pm, I forgot how hungry I was. I did some solid work. I had a brisk walk. I spent a few hours feeling lean and mean and really quite well. It was only when the aroma of frying fat started wafting in from the takeaway across the road that I remembered that I was absolutely ravenous.

Dr Lunn had pointed out that one problem with fasting is that it would take an awful lot of self-control to avoid breaking it with a binge. I do not have that self-control and broke it with a Chinese takeaway. I'd gone 24 hours without food, but the chances of it becoming an integral part of any weekly or monthly routine are slim.

That might change once more human studies are carried out on its benefits, though. Thanks to almost universally encouraging results from animal studies, several are already under way or in the pipeline. It's not inconceivable that in the next few years doctors will be recommending intermittent fasting in the same breath as five-a-day and regular exercise.

Fast forward: How it works

Four to eight hours

Your body will have finished metabolising the glucose present in your bloodstream from your last meal and will begin to convert glycogen – the form in which glucose is stored in your body – into glucose. To begin with, the body uses glycogen in your liver. Those fasting will feel hunger pangs and a gurgling stomach.

Eight to 12 hours

Once liver stores are depleted, glycogen stored in your muscles is used instead. Note that this is not muscle tissue. If these reserves are depleted, fats are then converted to substances called ketones, which operate as a glucose substitute. Depending on the size of your most recent meal, and your body weight, craving for food will increase until it reaches a plateau.

12 hours+

You continue metabolising your body's fat. Some people who fast for long periods report a cessation in feelings of hunger. However, when fat levels get to around seven per cent of your body weight, your body will register an increased craving for food. This indicates the approach of starvation, the moment when your body runs out of fat, and begins to metabolise protein.

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