When Annie Bell stopped eating for a few days in Lent, she thought she'd bitten off more than she could chew
When my husband asked where my idea to go on a fast had come from, I found I couldn't remember. Fasting does not assist mental agility, as it is usually undertaken when you're feeling in need of rejuvenation.

It had all started because of Lent, and the example of Jesus's fast in the wilderness. There may be some clerics who still fast for a short period of time but not for 40 days and 40 nights. As Dr Tom Sanders and Peter Bazalgette point out in You Don't Have To Diet, even if you still drink water, death normally results after about 50 days.

For most people, Lent is about abstaining from an indulgence they hold dear - be it Cadbury's Creme Eggs, ciggies or gin. Other religions take fasting more seriously. The Islamic observance of Ramadan requires 29 to 30 days of total abstinence from eating, drinking, smoking and sex from dawn to sunset - although I have always thought it a bit of swizz, with all that midnight feasting. As well as observing a fast on both Lord Krishna and Lord Rama's birthdays, Hindus eat only lightly once a day for a month during the rainy season. And the most evolved members of the Essene desert community still observe a 40-day absolute fast, and are noted for their longevity.

Aside from this, strict fasting, as in water only, has become the domain of naturopaths. It is not as fashionable as it has been in the past, although the Germans are pretty big on it, and one or two clinics specialise in it here. One of the better-known clinics prepared to starve you in the name of good health is Shrubland Hall Health Clinic in Suffolk, which has been going since 1966. Originally the family home of Lord De Saumarez, his wife still runs it and is the principal dietitian.

Lady De Saumarez is quick to point out that there is a fundamental difference between fasting and starving: the latter is about deprivation, the former, spiritual enlightenment. As Jean Smith says in Hungry For You, "Hunger hardens people's hearts; it also anaesthetises the emotions of those who have to endure it." De Saumarez says that anyone who has ever been starved cannot fast.

Now in her seventies, she recalls how much more popular fasting was in the Thirties. Nowadays, her clients are mostly those who feel they've overindulged in junk food. The regime at Shrubland would test anyone's will power: a three-day fast of hot water and lemon, with honey if there is a drop in blood sugar (there are four doctors on hand), followed by two days on fruit juice and a potassium broth. The longest anyone has fasted there is for three weeks, and the longest on record to her knowledge is an Indian woman who fasted at Champney's for three months on fruit juices before the war in order to treat a skin condition, and did so successfully.

During the initial days of the fast you can expect to feel lousy. This, apparently, has to do with the body cleansing itself. You have to drink a lot of water during this time. The tongue becomes furry: when this clears, it means the body has rid itself of toxins. Hunger, too, is a sign. It occurs on the second or third day and then disappears again: when it reappears, the cleansing process is complete. All the body's reactions slow down, including the brain, so it is best to be prepared to do little more than sit around. Driving a car is considered dangerous, although De Saumarez says: "I told one guest not to drive a car, but I forgot to tell him not to fly an aeroplane." Basically, though, if you want to fast you have to be prepared to sit and space out.

"Prolonged fasting can also lead to vitamin deficiencies and heart failure," warn Sanders and Bazalgette. At Grayshott Hall in Surrey, they are extremely reluctant to place guests on a fast. "If you want to fast, you have to come here and do nothing," says Paula Gilbert, the dietitian there. At best, Grayshott recommends a diet of steamed vegetables and fruits. Though Gilbert adds: "We do get some elderly patients who come here to fast because they have always fasted and, yes, I would recommend a two-day fast rather than colonic irrigation." Here, the approach is holistic and diet is used in conjunction with other treatments.

Lying somewhere between Shrubland and Grayshott is Ayurveda, the traditional Indian treatment, which believes in fasting to achieve spiritual enlightenment as well as to maintain good health. Dr Asmita Jani, an Ayurvedic physician, says: "Ayurveda recommends that each individual should observe a fast according to their natural constitutional body type, the imbalance of biological energies, strength, age, season and state of health."

I couldn't go any further without taking the plunge myself. So, I began fasting over the weekend, having indulged myself as fully as possible on the Friday night in anticipation. I awoke feeling suitably dreadful on Saturday morning and failed to rally for the rest of the weekend. Apart from the occasional patch, I didn't feel that hungry, just exhausted. By the Monday morning, when I moved on to vegetable juices, salads and vegetables, I was feeling quite unwell. By this stage, though, I was at Grayshott, where there was the distraction of 70 different treatments to choose from, from aromatherapy to Swedish massage.

Tuesday morning, I again woke up feeling grim, went for a walk on the common and came back feeling even worse. Fasting was a lousy idea and I was beginning to wonder why anyone ever did it. Suddenly, at mid-morning, my energy started to filter back and my head cleared. There had been periods over the weekend when I couldn't concentrate sufficiently to read a newspaper; now it was crystal clear.

At the moment, I'm still easing myself out of it and have no great desire to rush back into the maelstrom of coffee, alcohol and rich food. And I do have far more energy than when I set out. In fact, this morning, I would go so far as to say I felt fantastic. That may just be a combination of not having had any tea, coffee or alcohol for days, along with the algae body wrap (like being the rice inside a sushi roll). But, beneath this, I think there is something to be said for fasting, although I wouldn't choose to undertake it unless it was under the guidance of a clinic, and I don't see the point unless it is part of a larger holistic plan. A few days down at a health farm that makes fasting - either absolute or partial - a part of its regime is a fine recommendation. Not least because food tastes extraordinary afterwards

Shrubland Hall Health Clinic (01473 830404); Grayshott Hall (01428 604331); Dr Asmita Jani (01273 563340)