How to be healthier, live longer and lose weight, fast!

It's toxic overload time again. And guess what? The Americans have found a brand new solution to the eternal question of how to be thin and healthy: not eating at all. Mmm tasty, says Peta Bee gulping her water, but what does science have to say?

Check in at the New Life Health Center in Jamaica Plain, Massachusetts, and, before you are shown to your room, staff will perform a hair and urine analysis to check your toxic overload and prescribe a daily diet of, well, nothing, for the duration of your stay. If you can't stick to the Atkins, South Beach or any other diet, don't worry: simply undo the damage food has done by periodically consuming no calories at all. Fasting is the latest food (or should that be non-food?) fad and the ultimate way to experience it is at one of a growing number of dedicated non-food hotels.

Most residents - or patients, as staff prefer to call them - at New Life stay for a minimum of five days and pay upwards of £1,000 for a basic room and liquid diet which is touted as a treatment for everything from obesity to arthritis. A lucky few with minor complaints such as headaches are permitted herbal tea or other substances in minuscule quantities. For the rest, plain water must suffice until solids are slowly re-introduced as the fast comes to an end. It is a similar story at Japan's first detox hotel, Fasting Arena in the Nango Mountains, where guests pay £150 or more a night to be fed nothing but an enzyme-rich vegetable soup, fruit juice, herbal tea and mineral water. If you fancy it, then join the queue: there is a waiting list of several months to book one of the hotel's 35 rooms.

Eating nothing, you might think, is not the most nutritionally sound route to wellbeing. Yet emerging scientific research suggests otherwise. Dr Mark Mattson and his colleagues at America's National Institute of Ageing are currently carrying out a study to find out if regular day-long fasts have a positive effect on health after research with mice showed that fasting appeared to increase their lifespan. Reporting his initial findings in the Proceedings of the National Academy of Sciences last year, Dr Mattson says that mice that were fed only every other day appeared to benefit the most. Their longevity was greater not only than that of those mice allowed to eat when they wanted, but also those given 40 per cent fewer calories overall. Not only did the fasting mice in Dr Mattson's study live longer, they also recorded better scores in measures relating to diabetes.

At the end of the experiment, all mice were injected with a toxin that damages cells in the hippocampus, the part of the brain where cell damage causes Alzheimer's disease. Fasting mice were shown to be more resistant to the toxins. "We think what happens is that going without food imposes a mild stress on cells and that those cells respond by increasing their ability to cope with more severe stress," says Dr Mattson, who has skipped breakfast for the past 20 years and who, at 5ft 9in, weighs only 8st 7lb. "It's sort of analogous with the effects that physical activity has on muscle cells." In short, then, starve yourself for a short time and, rather than crumbling in the face of self-inflicted torture, your body will respond by enhancing its defences.

Traditionally, of course, fasting has religious roots. Here the point of fasting - for the month-long period of Ramadan in Islam, for example, or the 24-hour fast for Yom Kippur in Judaism - is to concentrate on faith rather than the less prosaic details of everyday life, such as eating and drinking. Many who have fasted, however, claim they experience health benefits and improved vitality. Which may partly explain why pilgrimages involving fasting are attracting followers other than the devoutly religious - people drawn in by the weight-loss and detox benefits. One of Ireland's most popular retreats, to Lough Derg island in Co Donegal during August, attracts more than 20,000 people a year, including some non-Catholic celebrities. It is not a glamorous experience: pilgrims begin fasting at midnight and travel by boat and with no luggage to the island where, on arrival, they must remove their shoes. They then have only one daily meal of black coffee and dry toast until they leave three days later.

Mary McCullough, 35, from Hampstead in north London, describes herself as "non-religious". Nevertheless she makes an annual pilgrimage to Lough Derg for the fast, and also to Westport, Co Mayo, where she joins 30,000 people for an exhausting hike totalling 10 miles to the top of Croagh Patrick, a 2,500ft holy mountain, on a diet of ... nothing. "A cousin, who is a practising Catholic, told me about these pilgrimages and the first time I arranged to go with a friend from work who wanted to lose some weight," she says. "Although there were old ladies with their rosary beads, most of the people we met there were like us - just there for a break with a difference.

"We climbed Croagh Patrick barefoot in July and fasted for 24 hours. It was tough, but nothing like the Lough Derg experience where you aren't allowed to sleep until the second night of the three-day fast and then you sleep on stone beds. It is hell for the first 36 hours, but then you start to feel strangely energetic. Without fail, I return feeling healthier than I have done in ages and I usually lose about 8lb."

While excessive fasting is not the kind of approach leading nutritionists recommend - and many take just as firm a stand against trendy "detox" diets - not all are strictly opposed to the odd short fast. Sarah Stanner, a scientist at the British Nutrition Foundation, says that fasting should not be used as an excuse to gorge at other times, but if a 24-hour fast makes you feel better, then go ahead. "Fasting for longer than 24 hours is not advisable, but avoiding food for a day won't really present any problems for most people," she says. "You need to make sure you stay well hydrated by drinking plenty of water and other fluids. A lot of people get headaches either because they are dehydrated or because they are suffering the effects of caffeine withdrawal. With your blood sugar levels low after eating no food, you will also feel tired."

Despite Dr Mattson's findings, the debate about whether or not fasting is actually good for you lingers on. By far the most popular reason for fasting is not to lose weight, but to improve health and cure niggling ailments - several studies have suggested a variety of benefits from a sharp, temporary reduction in calories, ranging from improved insulin resistance (which can help to prevent adult-onset diabetes) to an improved ability to deal with stress.

Jenni Preston, 33, from Leeds, is certain that periodic fasting has helped her to combat irritable bowel syndrome brought on by her stressful job in marketing. "I had tried everything from medication to a wheat-free diet, but nothing seemed to cure it," she says. "Then a friend introduced me to a nutritional therapist who prescribed a juice fast. Basically, I eat a high alkaline diet with lots of whole grains, vegetables and nuts and cut out caffeine and all animal produce, including eggs, to ensure my body and organs are well prepared for the fast, which I do every eight weeks."

During her three-day fast, Preston consumes only freshly squeezed fruit juice or water at every meal in the belief that afterwards, her digestive system will use nutrients more efficiently. She has been staggered by the results. "After the first time I tried it, the bloating stopped. Over the year since I started, I no longer suffered from water retention and I feel much less sluggish. It is not something I particularly enjoy because I feel weak and faint when I am fasting, but the after-effects are definitely worth it."

However, leading diet experts such as Dr George Blackburn, director of nutrition at Harvard Medical School, continue to cast doubt on whether fasting is useful as a medical tool. "Purging yourself by fasting has no scientific or medical backing," he says. Ms Stanner agrees, saying: "There is no proof that it does you any particular good." Many scientists also remain to be convinced about fasting's "detox" benefits. "There is some mysticism behind the idea which stems from the fact that a lot of religions advocate fasting for cleansing the body," says Dr David Bender, a senior lecturer in biochemistry at University College London. "But there are absolutely no health benefits. The body is perfectly adept at ridding itself of unwanted toxins in food.

"We each consume a lethal amount of caffeine every day, but it doesn't kill us because, like all harmful dietary substances, it is broken down in the liver and excreted. No diet fad such as fasting is capable of influencing a basic physiological process."