Youre probably familiar with the idea, not always concrete, that fasting precipitates good health. Maybe youve been tempted to give the

Game-changing research has proven that short diets that mimic the act of fasting can trigger powerful natural healing and regenerative processes lying dormant in our bodies. Processes that not only burn fat but also fight heart disease, slow ageing, lower cholesterol, reboot the immune system and extend lifespan.

It doesnt stop there: the most recent studies have shown that fasting can kill forms of human cancer in mice and enhance the effects of chemotherapy. In some trials, fasting itself proved as effective as chemotherapy. In short, science is telling us that going hungry could prevent and treat some of the biggest killers of men.

The man behind this research and the structured, prescriptive diet that Im following is Dr Valter Longo. A professor of gerontology at the University of Southern California, he embarked upon this line of inquiry when he first noticed that yeast cells simple organisms that predate humans by millions of years respond to periods of fasting by protecting cells and fighting toxins. Yeast that had been fed a low-calorie diet, Longo discovered, lived up to 10 times longer than the control group.

Longos latest findings show that humans have similar self-healing mechanisms buried deep in our DNA and that periodically fasting on low-calorie diets has a wholesale regenerative effect on the body. Some of the reasons behind this are understood; others remain a mystery. It is believed that fasting kills cancer cells by starving them of glycogen (on which they depend for fuel more than regular cells), thus tricking them into releasing damaging free radicals that spark cellular suicide.

But fastings more general healing powers derive from a different mechanism. Longo has shown that fasting triggers the creation of new stem cells, which repair the body by regenerating tissues, organs and blood, and re-booting the immune system. It sounds too good to be true, I tell Longo when I call him at his California lab. Well, the regeneration that fasting triggers is remarkable, he says. During abstention, organs like the liver, as well as the entire immune system, shrink because they are deemed less necessary. When the body rebuilds these systems, however, it activates the release of healthy stem cells which regenerate newer, younger, more functional versions.

Its akin to pressing the reset button on your bodys operating system, he says. The rebuilding is so massive its probably the closest humans can get to the original embryogenesis process the growth of an embryo during weeks one to eight of human development, he reveals. This is not just a proliferation of new cells but a very coordinated one. It is an evolved programme that has been around for billions of years because organisms starved all the time. For the earliest Homo sapiens, devoid of Smeg fridges and Tesco, periods of feast and famine were the norm. All we need to do, maintains Longo, is rediscover this primitive state of being.

Theres a catch. Its not merely a case of not eating for five days. You have to severely restrict calories, yes, but Longos research shows that those you do eat must have the right balance of nutrients to trigger a natural process called autophagy, which encourages your body to cannibalise waste cell debris. Think of it as your own natural recycling process, removing damaged cells that could cause disease or ill health. Crucially, autophagy also shuts down the enzyme PKA the absence of which has been linked to extended lifespan and the proliferation of healthy stem cells and decreases the hormone IGF-1, which is associated with ageing and cancer risk.

Longos findings could well enrich our lives, but not before they enrich the coffers of L-Nutra, a specially established spin-off company of the University of Southern California. Together with L-Nutra, Professor Longo has developed ProLon: the pre-packaged, off the (pharmacists) shelf Fasting Mimicking and Enhancing Diet (FMED). This five-day menu of all-natural soups, nut bars and snacks provides enough calories to minimise cravings (at least theoretically) without jeopardising the miracles apparently brought about by abstinence. ProLon works by cheating your body into thinking youre literally starving, even though youre just hungry.

Day one is 1090 calories (10 per cent protein, 56 per cent fat, 34 per cent carbs), with days two to five dropping to 725 calories (9 per cent protein, 44 per cent fat and 47 per cent carbs). The first priority when designing the food was that it had to mimic fasting, explains Longo. The food is 20 years of painstaking work: removing, adding, substituting. If I add that ingredient, will it interfere with the PKA effect? Will this one affect the IGF-1 changes? Hunger is also an issue so we tried to make people feel relatively full.

Human trials of ProLon are ongoing, but L-Nutra expects to be able to launch the FMED kits imminently, priced at around 150 for a five-day pack. The healthier among us will be advised to undergo a course of FMED every four months or so, while obese people might benefit from a monthly cycle.

In the name of science, journalism and a little bit of weight management thrown in, I ask Professor Longo to send me a five-day course of ProLon. A mere 48 hours later I am tucking into the contents of my first little white box. The packets within look like space food but taste better than expected. The minestrone soup is watery yet filling, and the choco-crisp bar a surprising treat. Another pack contains olives, which I hate, so I throw them away. I am evidently not starving.

Day two is a different matter. Hunger haunts me all day. I eat my soup with a teaspoon to make it feel more substantial. It doesnt work. I want to stop but Longos words echo around my head: The regeneration of cells and the autophagy only start in days three, four and five, he says. Thats when you push the system to an extreme and back.

My trip to the newsagent on day three almost derails my efforts entirely. When, by the evening, my symptoms have still not subsided, I decide to contact Longo again. What I am experiencing is not only natural but beneficial, I am assured. Our brains are energy-sapping organs that normally run on 100 per cent glucose. But while fasting, our livers produces the ketone body B-hydroxybutyrate, a kind of emergency energy source for the mind, saving energy derived from fat and muscle for other organs.

At this stage in the fast your brain switches to the ketone body utilisation mode, possibly for the first time in your life, says Longo. This can cause a mild headache. I mention my own brain feels like its at the business end of a pneumatic drill, but this is batted away with yet more good news: studies have shown that ketones fight inflammation associated with type 2 diabetes, MS and Alzheimers. To the converted, there is apparently no end to fastings list of fortunes.

Not everyone is similarly convinced, however. Professor Kieran Clarke, from University of Oxfords Department of Physiology, Anatomy and Genetics, thinks fasting could be a smokescreen for the real message. A lot of the benefits [of fasting] relate to extending life, and its possible, but we dont know if its the fasting per se that achieves that, he says. It could just be related to the benefits of helping people reach normal bodyweight. If you look at the records, healthy people invariably do not fast.

Dr Tim Woodman, medical director at Bupa UK, goes further. He believes that fasting could actually have reverse health effects. Losing weight too quickly means that you also tend to lose muscle and water, he says. As a result, your body starts to work more slowly and needs fewer calories to function. So when you resume eating normally, the extra calories will be stored as fat.

One of the most vocal opponents to fasting being seen as a magic bullet is Dr Joel Fuhrman, author of The End of Dieting and Super Immunity. Fuhrman is the man behind so-called nutritarianism, the theory that the key to longevity lies in maximising the number of micronutrients per calorie consumed. Fasting, he holds, is a mere distraction. The modern diet is so nutrient-poor that to discuss fasting without framing it within a normal diet is inappropriate. Our diets are too deficient in micronutrients, phytochemicals and antioxidants to maintain normal immune system function.

If you want to live longer and healthier, says Fuhrman, forget fasting: just eat raw greens; a 30g portion of walnuts; chia seeds and flax seeds; a dish containing mushrooms, beans and onions; plus three portions of fruit like cherries, plums and oranges every single day. A nutritarian diet is the most effective way to not get heart disease, cancer or dementia, and to push the envelope of human longevity closer to 100 years old, he says.

Oh, and ditch the meat. The real impediment to our lifespan, claims Fuhrman, is animal protein. It may be essential for building muscle, burning fat, maintaining a healthy metabolism and supplying a range of important vitamins, but in Fuhrmans eyes it ultimately does your immune system no favours. Huge studies following people for 15-24 years prove that eating more animal products causes cancer, heart disease and shorter lifespans, he says. There is no controversy here. All nutritional scientists agree.

Indeed, the World Health Organisation has recently added red and processed meats to its list of carcinogens. These studies are not new some date back to 2010 but such is our cultural fascination with increasing our protein intake that weve just been ignoring them.

I advocate up to 10 per cent of your diet from animal products, with the other 90 per cent from plants, says Fuhrman. A few days of fasting wont help unless you change you regular diet, too. Animal protein elevates IGF-1, which speeds up the ageing process and negatively affects lifespan. Yes, calorific restriction might briefly restrict IGF-1, but if you then eat a lot of animal products after, it wont actually drop. If you both restrict animal protein and eat a more vegan diet, however, you get the benefits of fasting without actually having to fast.

To eat or not to eat? It would have been naive to expect a clear-cut consensus on such a radical concept. But the arguments made by Fuhrman and Longo arent too dissimilar. Fuhrman acknowledges that fasting is effective; he just takes issue with best practice when temporary abstainers start eating again. And in spite of cases made by critics of fasting, the research into its potential is still compelling.

An extensive review in Research in Complementary Medicine concluded that there was large empirical and observational evidence that medically supervised modified fasting combats high blood pressure, diabetes and obesity. This is simply too significant to ignore.

Were never going to say that fasting is so effective that it means people can smoke and eat badly as soon as they stop, says Professor Longo. The fact is if somebody has a vegan, low-protein diet, a BMI of 23, and regularly exercises, then they will benefit from fasting, just not as much as someone who is unhealthy.

Following an undeniably difficult midpoint, day four of my own experiment is something of an epiphany. I experience a surge of mental energy owing, I am told, to an increase in the levels of noradrenaline and dopamine in my brain. On the first two to three days of your fast you tend to sleep a lot, explains Raimund Wilhelmi of the famous German fasting clinic Buchinger Wilhelmi. Then, like a phoenix, you rise, and after four or five days you feel very positive.

Unfortunately my final rise is less phoenix-like, more vodbull hangover. Day five is tough. With mixed feelings I note that I have lost 1.7kg in just five days not the main purpose of the fast, but also not insignificant. On day six, with the fast finally over, the Toffee Crisps Ive been fantasising about are back on the menu, but my cravings have faded. My appetite has changed, as has my appreciation of portions and flavours. What I can say with some conviction is that fasting will rewire your mind.

There is yet some way to go. Professor Longo believes we are only just beginning to understand the potential power of fasting. Think about this, he says. If you are a 50-year-old man, you have a new child, and that baby is born perfect, it proves your cells know exactly how to build perfection again. How do they know? And is it possible that this can be done within an organism itself, without having to generate a new one? Longo sounds excited, verging on giddy. That is what we are chasing here.

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Is Fasting the Secret to a Longer Life? - Men's Health UK