Early Christians fasted during Lent for spiritual gain. Today, we also undergo gruelling regimes - but for our health.
F asting, along with other observations of the church calendar, is not what it used to be. Both the motives and methods have changed drastically. In days gone by, devout Christians would pile leftovers of meat and vegetables into a pancake in preparation for 40 days of austere abstinence during Lent. This Shrove Tuesday, many of us will be piling maple syrup and chocolate sauce on our secular pancakes, and those embarking on fasting regimes will observe strict diets of fruit juice, or go to a health farm, or, in the most extreme cases, simply starve themselves. The motivation? sometimes health, more often vanity.

"Dieting is part of a new ascetism, like jogging," says Karen Armstrong, author and former nun. "We can't have the old immortality, so we live for as long as we can. The old pre-Reformation ideal was of the spiritual, fasting virgin, cut off from the rest of the world. It is a sign of our spiritual barrenness that the body has become an end in itself - beautiful, lean, exercised."

In centuries past, fasting had spiritual goals aimed at concentrating the mind on the values of food. The more pious would take self-denial a great deal further and exist on nothing but bread and water for Lent - or longer. Some went to even greater extremes - Catherine of Siena, for example, almost certainly starved herself to death. St Simon Stylites was the only man in history to be celebrated for spending 36 years up a pole (he is in the Guinness Book of Records for this). In 423 he started building himself a tower which, over the years, reached 60ft. He lived on top on a 12ft square platform, lowering a basket every day for his followers (whom he harangued relentlessly as miserable sinners) to fill with bread and water.

Self-denial and fasting, especially in the early Christian church, were seen as a means of purifying the soul and heightening perception during prayer. Mortification of the flesh, including self-flagellation and sleep deprivation, could lead to mystical visions or starvation-induced hallucinations. Today, eating disorders may create similar hallucinations, but denying oneself food has become more likely to be regarded as an illness. Eating disorders such as anorexia also punish the flesh; in this case, the aim is not spiritual wisdom but a physical ideal - that of an unnatural emaciation masquerading as beauty.

One thing is clear, though: pursuing masochistic regimes of food denial, whether for beauty or beatification, is not new. Dr Turner was particularly fascinated by wall paintings in the tombs of the Valley of the Kings in Egypt, depicting people vomiting after banquets as part of normal court life in ancient Egypt. The Romans even had a special room for it - the vomitorium where, in the society's last decadent stages, feasters would regurgitate after overeating, then return to the table and start again. The modern-day equivalent is presumably the lads out for a Saturday night on 10 pints and a curry.

Controlled fasting, today's short-cut to good health, is enjoying a renaissance. This time the goal is health, specifically detoxification, ridding the body of all those poisons put there by late 20th-century living - junk food, pollution, coffee, alcohol, chocolate. The most popular version of this is the juice fast (see below), though there is at least one naturopathic clinic where they still sometimes use water fasting. When I went there, they did quite a bit of flesh mortification, too. Take the sitz bath, for example. It involves two large, low sinks, one filled with very hot water, the other with icy cold water. You began with your bottom in the hot and your feet in the cold. This is bad enough, but nothing compared with how it feels when you swap over.

Then there is the terrifying Scottish douche. This takes place in a shower with handrails, which you hang on to for dear life to avoid being swept down the corridor by the water pressure. You get hosed down from the back with fearsome jets of water - hot, then icy, alternating several times. Finally, you are rubbed down with coarse salt. The purpose of both punishing treatments is to stimulate the circulation. No pain, no gain, as Jane Fonda would say.

The newest detoxification aid is colonic irrigation (known to its friends as colonics). Leor Cohen at the Welbeck Clinic in London carries it out in conjunction with a fast. Until recently, the whole idea was something to be sniggered at. After all, the goings-on of your lower digestive tract hardly seems a suitable subject for the dinner table. Now a steady stream of people from pop stars to MPs, whom Cohen is too discreet to name, are beating a path to his door for the Seven-Day Tissue Cleansing Programme.

The process is not painful, messy or smelly and, after some initial embarrassment, you begin to develop a fascination with your bowel movements that you have not known since potty training. You may be spotty and irritable to start with, but by the end you have bright eyes, glowing skin and about half a stone weight loss. All fasting, whether for reasons of religion or health, also involves a re-education process - now you have this cleansed body, do you really want to start throwing junk in it again?

The church's feast-and-fast attitude to Lent may reflect a pattern more ancient than the religious symbolism. Historically, this has always been the time of year when food has been at its most scarce, the leanest point in the rhythm of the seasons. Perhaps our bodies' metabolism is still in some way geared to that of the early hunter who would feast for a few days, then have a long wait before killing his next bison. Whichever way you look at it, a short respite may do no harm. So what are you giving up for Lent?