When photographer Anthony Cullen heard the clank of glass on porcelain, he didn't need to examine the contents of the toilet bowl between his legs. He instinctively knew he had just passed the marble he had swallowed as a five-year-old; the small coloured sphere - "I think it was a bluey" - had lodged in his colon for 22 years. His nonchalance was understandable. Having flushed 400 pints of coffee and vinegar solution around his large intestine through 10 enemas, and taken 100 herbal laxatives, he had become hardened to extraordinary sights. He had already excreted yards of long stringy mucus "with a strange yellow glaze", several hard black pellets and numerous pieces of undigested rump steak. Like an iceberg breaking away from a glacier, the marble was simply the latest object to drop off the furred up wall of his colon.

Within 30 minutes it had become a burning topic of conversation among guests at The Spa resort on the Thai island of Koh Samui. Most listened, nodded earnestly and smiled, a flicker of mutual support, before describing their own bowel movements in unnervingly graphic detail. It was just another day at the tropical health farm where conversations that would be deemed unpleasant, if not obscene, in any place outside a gastro-intestinal ward, are mere idle chit-chat among the sun-soaked clientele.

They may have travelled across the world to The Spa's thatched beach huts, encircling its renowned restaurant whose Pod Ka Pow Nam Many Hoy - prawns and chilli, stir-fried in oyster sauce - is a house speciality, but not a morsel of food, nor a single calorie, will pass their lips. Instead they order around 70-odd gallons of coffee and vinegar, lemon or garlic solution - lightly warmed, please waiter - to be squirted up their anus. You are unlikely to find this particular dish on Masterchef.

The roots of their truly alternative activity holiday lie in our modern lifestyle. Some doctors, such as Richard Anderson, inventor of the Clean-Me-Out Programme, claim our high stress existences and over-processed diets - chips, pizzas, burgers - have left us with clogged-up digestive systems. And that, according to advocates of intestinal cleansing, makes us disease time bombs, at increased risk from cancer, heart trouble, infertility, diabetes, premature ageing and, pass the smelling salts this instant, wrinkles.

Their solution is to fast: to put nothing in one end, while simultaneously purifying yourself by propelling significant amounts of liquid up the other. "It's like changing the oil in your car," says Guy Hopkins, the 60-year-old owner of The Spa, whose eyes glint with evangelical zeal when he talks about colonic irrigation. "If you don't do it every so often [your body] isn't going to run that well. We constantly put the wrong fuel in our bodies and, sure, they keep on going, but cleanse yourself and you'll be amazed how much better you'll feel."

A tempting sales pitch, yet when my editor suggested a first-person report, I had grave reservations. As someone whose only concessions to healthy eating had involved switching from butter to olive oil and occasionally cutting the fat off my steak, the fast sounded frankly insane. Then I began hearing about the "lifestyle benefits" of the cleanse, of the 90-degree heat and tropical beaches. Words such as "de-stressing" and "life-changing" were tossed around.

I weakened, dithered and finally relented. The photographer, Anthony, it was agreed, must also fast.

Our preparation began well before we spotted our first palm tree. The Spa recommended we prepared with a fortnight of abstinence from meat, processed foods (adios my daily staples, pasta and bread), milk, cheese, booze, coffee or soft drinks. Instead, our gastric juices were stimulated by salads, fruit, slightly cooked vegetables, herb teas and water.

It wasn't easy. Both Anthony and myself are what might charitably be termed "stocky", enjoying cooking and, more importantly, eating. Within days, food, or lack of it, had become an obsession. We had long phone discussions about interesting ways to grill aubergine; Anthony bragged about his spicy ratatouille. Life was changing.

As the first toxins were expelled and severe caffeine withdrawal set in, I experienced headaches, aching muscles, a lack of energy, and an increasingly short temper. I also faced a new menace: the liver flush drink. Designed to sluice out your system, it's a vile mix of olive oil, raw garlic, and cayenne pepper blended with orange juice. I've no idea if it worked, but my urine turned clear and I always got standing space on the tube.

We stuck rigidly to the diet until disaster struck: an upgrade on the flight to Bangkok. Our willpower collapsed and over the next "lost" 12 hours we demolished peanuts, smoked salmon and oyster mushrooms, roast goose, cheese, port, champagne, Baileys and chocolates.

We had four more days before the fast, but while I got back on track, the photographer went totally off the detox rails. He consumed beer, Pringles, coffee and, as we waited for the Koh Samui connection at the airport, slipped in two Burger King chicken sandwiches, a huge pile of fried onion rings, a large Coke, followed by a chicken dinner on the plane. He was clearly heading for a remarkable first enema.

By the eve of the cleanse, I'd already lost over 2kg, weighing in at 86kg. Anthony was a little heavier, at 91kg. After demolishing an emotional last supper, we met our fellow fasters. They appeared a cosmopolitan crowd, confounding fears of being stranded among the sandals and lentil brigade.

There was Derek James, an engineer from Leeds, and Margaret Barrett, a sales rep from Cambridge, both in their mid-20s and aiming to clean up their acts after "caning it" while working in clubs in Tokyo. Nicky McCulloch, a 27-year-old Australian teacher, hoped to sort out a range of allergies, including wheat and alcohol. She was travelling with Mez Hay, a worm farmer with a shock of blond hair and strident ocker accent. Passionate about Italian food, along with steak, chops and sausages from her parents' farm, Mez admitted she was keeping her friend company and hadn't put in a single second's preparation. "I didn't know about it," she snapped. "Who the hell are you, the bloody fast police?"

Others also had tangible goals, including tackling stomach complaints, severe constipation and mystery lumps. Most were keen to stress - a cynic might say too keen - that losing weight was not the goal. "It's a bit extreme to travel half way round the world just for a diet," argued Mez. "You'd be a bit superficial. Mind you, I wouldn't mind shedding a few pounds."

That didn't promise to be a problem. After checking our pH levels - too low and the fast isn't advisable - we immediately learned that while we wouldn't be eating, a great deal would still pass our lips. The relaxed, stress-free week on the beach would involve a Stalinist adherence to a pill-popping timetable. Each day started with a charming 7am detox cocktail of psyllium husk and bentonite clay. It had the texture of liquid cotton wool, but would be crucial for pushing toxins and garbage through my system.

Ninety minutes later, we had to swallow eight tablets. They looked like rabbit droppings, tasted like rabbit droppings but were, in fact, a mix of chompers (herbal laxatives and cleansers to attack the accumulated gunge in our colons) and herbal nutrients to help compensate for those missed during starvation. We had to repeat these two doses every three hours, every day, with a final handful of pills at 8.30 each night. There was just one more lesson, the small matter of the self-administered enema. Our teacher was the sickeningly lean, tanned resident alternative health expert, Chris Gaya, who appeared to have stepped straight out of a Californian aerobic video. He made the colonic irrigation equipment - bucket, piece of wood, plastic tube, bulldog clip and nozzle - sound like straightforward DIY, although it's unlikely to feature on Blue Peter in the near future.

All we had to do, he informed us, was to lie on the wooden board between a stool (stop giggling at the back) and the toilet basin. There's a hole at one end of the board over the loo; above it a nozzle connects to a tube, which in turn leads to a five-gallon bucket of liquid hanging from the ceiling. We would liberally coat the nozzle, which was the width of a Biro ink tube, with KY jelly, lie back, think of profiteroles with chocolate sauce, and slide on.

Controlling the flow of liquid with a bulldog clip, we were to let it flow until we felt full, before massaging it round the colon (roughly following three sides of a square around the lower belly) and releasing. Fluid would, apparently, be flowing in and out of our backside at the same time. "We'll be on the board for around 40 minutes," cooed Chris. "So let's make ourselves as relaxed as possible. Put on some soft music, light a candle, create a romantic atmosphere."

We clearly took different approaches to seduction. But mastering the enema, once I'd got over muscle-clenching nervousness, really wasn't difficult. I somehow ended up with my right foot half way up the wall, but five gallons went in and out without major trauma. By that night I'd shed another kilo, and although light-headed after 24 hours without food, felt strangely satisfied with the mix of supplements and detox drinks.

Next morning, my first enema of the day down the pan, I sat in the restaurant staring longingly at the menu, and found inspiration in the shape of two women nibbling their post-fast fruit. They exuded some of the rudest health I'd ever seen.

Carol Beauclerk, a "global nomad" with a mop of curly black hair, was a vegetarian, practised yoga, meditated and warmed up for her fast with a 17-day hike in Nepal. At 54, she had the energy and enthusiasm of someone half her age. "This place is really jumping," she enthused. "I'm now hoping to do a week-long fast each year."

Two tables away, scribbling in a diary, was Claire Lyons, a 32-year-old British journalist who had recently completed 21 days without eating. Having not gone near a set of scales, she had no idea how much weight she'd lost, but told me, "I feel great. Once I got past day 10, over the hump, it was surprisingly easy." Claire oozed serenity, but three weeks without food is unlikely to leave anyone hyperactive.

By mid-afternoon, their shining example was all but forgotten. I was feeling awful. Tired, lethargic, simply lousy. Having not eaten for 36 hours my body was apparently going into detox mode. Margaret, who had felt nauseous since waking, had actually thrown up, and was questioning her motivation. Nicky, meanwhile, had produced "something about nine inches long, it was very dark, very scary".

Things were no better for Mez. Already ravenous, she was spending an inordinate amount of time sniffing around plates of steaming Thai curry in the restaurant. She had also failed to grasp the basics of colonic irrigation. Instead of letting the liquid flow out, she had taken a massive amount in - until she was about to burst - before struggling to sit on the toilet and release it. "I had a huge stomach," she gasped. "I was thinking, this must be wrong. If anyone can take the whole bucket in one go, they're sensational." I made a mental note to watch out for spectacular explosions from chalet six.

It wasn't all bad news, however. I discovered we were allowed the luxury of a daily bowl of vegetable broth. It made me pathetically happy, savouring every drop as if it were a Gordon Ramsay creation. Filling perhaps, but it did little to halt the weight loss, and by the end of day two, a further two kilos had vanished.

By next morning, tiredness had been added to my hunger. I seemed to have been up half the night on the loo, the result of drinking a copious amount of fluid. My bodily functions had also taken a turn for the truly bizarre. I experienced flu-like symptoms as I started to expel 36 years' worth of toxins with headaches and aching muscles; my nose ran constantly, my eyes were sore and weepy, my ears waxy. I felt like something out of The Omen. I had also plucked up the nerve to put a colander down the toilet. Close examination showed I had passed several feet of long brown string that shimmered as if subtly illuminated by a photographer's light.

And I wasn't alone. Margaret had picked through her colander with chopsticks to reveal yellow fatty chunks, Mez had filled hers to the brim with brown stringy "chicken skin" mucus ("We're talking litres"), as had Derek, whose output included a strip about eight inches long, while Anthony described his as "patchy, like rabbit droppings". Similar surreal conversations with virtual strangers became the norm, achieving levels of intimacy beyond the range of couples who have been together for years. Perhaps avoiding frank discussion of bowel movements is one secret of a long-lasting relationship.

That night, as I escaped the dense tropical warmth, and flicked through books on diet and nutrition in The Spa's library, I discovered a remarkable document: The Healthview Newsletter. Inside, octogenarian bowel specialist, V E Irons, attempted the Herculean task of selling colonic irrigation on its erotic potential. I would lose my frigidity, he promised, my sex life would go stratospheric.

"How could anyone fully enjoy sex when he has up to 15 years of encrusted fecal matter and mucus in his colon?" asked Irons. "HE CAN'T - and HE WON'T. If you want to remain sexually potent for your entire life, start cleaning your colon today. I'm 87, and I still enjoy sex. And if I can at my age, I know you can at your age... so get on with it!" It was of little consolation to Mez, whose hunger had now assumed epic proportions. She was considering eating her apricot moisturiser, she told me.

That night produced the most vivid dreams of my life, a typical symptom of detox, with blockages disappearing from the mind as well as the body: I'd attacked Vietcong gun positions in a hot air balloon, I'd played golf with exploding balls, I'd been savaged by a grizzly bear. Other guests' dreams were more grounded in reality: Anthony and Mez had raided their parents' fridges, with the worm farmer devouring steak, potatoes and cheese sauce.

And some simply begged for the psychiatrist's couch. Nicky, who in reality sees her divorced father only sporadically, dreamed he had turned into her boyfriend. Freud would have enjoyed that. Indeed, in private conversations with guests, well away from my notebook, many fasters admitted to having recently split up, or having travelled to Koh Samui to get a long-distance perspective on relationships. I had unwittingly stumbled on Relate-On-Sea.

There was further physical fall-out, too. Day four was supposedly the worst of the week, with toxins expelled through the skin and lungs, as well as the kidney and colon. I didn't disappoint. My nose, ears and eyes deteriorated, my sinuses throbbed, I was yet more sluggish. It felt like a beer, wine and whisky hangover. Increasingly strange things appeared in our colanders. Derek was shocked to find rubbery nuggets, Mez had found black oval shapes "up to five inches long", my offering had an almost luminous green tint.

As if to celebrate crossing the halfway point of the week, many of us switched enema solutions. Abandoning coffee and vinegar, I flamboyantly opted for garlic, claimed to get rid of parasites. It seemed as natural as ordering gin and tonic instead of margarita, but when I casually told my girlfriend in a telephone call to London, there was a long silence. "Are you aware how tenuous your grip is on reality?" she asked. "Are you with a cult?"

I clearly needed to get out more. Many people hadn't left The Spa for days, it was developing its own micro-culture. But when I summoned up the energy to sip mineral water in a bar in nearby Lamai town, I felt instant paranoia. The lights, the noise, the crowds, the smell of food. It was a world in which I didn't belong.

I returned to the womb to find new guests. John Twigg, a burly 37-year-old Kiwi, had prepared by drinking more wine. "It's made of grapes," he argued. "Grapes are vegetables, so what's the problem?" He was joined by the Lycra-clad Mimi and Dave Hatherley from Fairbanks, Alaska, who had an unnerving habit of finishing each other's sentences. Forty-two-year-old Mimi ran, biked and did step classes five times a week; Dave, 43, ran, skied, hiked, climbed and mountain biked. They were both "into vitamins and nutrition" and while fasting were also exercising hard because "the results will be better". After talking to them, I felt strangely giddy.

My mood and physical condition, however, were about to go through a dramatic change. By lunch - sorry, by the second dose of herbal laxatives - on day five, my nose, eyes and ears had cleared, and I had more energy. Remarkably, without nibbling a single shred of food for 120 hours, the irrigation still washed out huge amounts of gunk. I passed six-inch strips of gristle and what appeared to be large chunks of fillet steak. I don't know how I ever afforded them, let alone swallowed them.

At least I could contribute to the increasingly competitive enema discussions. Someone had always passed something harder, brighter, more bizarre. Margaret's chopsticks had unearthed some gristle, about a foot long, and hard, black pellets. She was so impressed she took a photograph. A few chalets away, Mez had passed "rubbery brown, fat worms" with a strange purple glaze, which she insisted on showing to me in her bathroom. But the clear winner was Anthony's 22-year-old marble. Perhaps the most bizarre thing, which I didn't appreciate until days later, is that it all seemed perfectly normal at the time.

When I next bumped into Alaska Dave, he was jogging rapidly between the restaurant and his chalet. As panpipe music played in the background and he told me about today's three-mile hike, I noticed he wore a strange electrical device. It was a zapper that emitted an electrical current to kill parasites, and carried the printed warning: "For research only. Not approved for use on humans." Even for The Spa, that clearly wasn't normal.

The improvement continued into day six. A nearly detoxified brain and bloodstream meant I awoke clear-headed, and full of energy. The enemas now produced less, but it was darker and harder as the fast broke away the older, more ingrained plaque.

It was the same story the next day. Our bodies seemed to reflect a mood of demob happiness. I had rarely felt so healthy, so energised, in my adult life. That didn't, however, mean the end of the bizarre revelations. John passed "something from an alien movie" into his colander - and then videoed it for his office colleagues. He was joined by an outsized oil worker, Pipeline Pete, embarking on his 10th fast. "The first time I came," he boasted, "they needed to dig three cesspits."

And there were more. Early that evening, I found Mez huddled over a well-thumbed tome in the library. "Jesus, have you read some of these?" she groaned, handing me a book of ex-guests' awed testaments. "I'd have bet £1,000 my bowels were clean," wrote Chris Markvert, 67, "seldom have I been so surprised." "Great pooing," said Roy from San Francisco, "the best month of my young life." And RTM contributed seven pages of increasingly manic scrawl, which included interesting facts about the Vikings.

It also contained graphic photographs of people's enemas, footnotes in The Spa's history to go alongside stories of legendary guests, such as the alcoholic whose detox included hiding whisky bottles and wandering naked into neighbouring resorts; and "Kathmandu Joan", who fasted for 140 days over two and a half years, passing over 70 green and black "buttons" and clearing up an abdominal disorder.

We couldn't compete with that, but by the morning of day eight, the fast was being credited with impressive results. It had, people claimed, got rid of allergies; removed worrying lumps that had necessitated appointments with gynaecologists; eased severe period pains and sinus problems; helped people lose kilograms while improving their skin and strengthening their nails. I'd lost well over 6kg, had an indecent amount of energy and, as people kept observing, had developed unnaturally bright eyes. I wasn't aware they were cloudy before, but felt I had earned some flattery after 14 enemas and no food for roughly 170 hours, 35 minutes and four seconds. The cost of the seven-day programme, by the way, is £184, and accommodation in a chalet for the week adds another £60 or so.

The first post-fast meal of papaya made my toes curl with pleasure, but, as George Bernard Shaw observed, "Any fool can fast, but it takes a wise man to break a fast properly." Raw fruit and vegetables should be the order of the next three days, but within hours Anthony had consumed two Snickers bars and a fish supper. It appeared to have no ill effects. They came 24 hours later. After demolishing piles of local prawns, we unwisely sipped a shot of Mekong whisky. Toxins tasted good, very good indeed. So good in fact, that by midnight, we had drunk a bottle each. The next morning, on the beach, my glasses were smashed, toxins pulsing around my bloodstream, the hangover indescribable.

But the week was not wasted. As a nutritional Philistine, I was inspired to read more, to learn some basic lessons. It's hardly double-blind scientific research, but I defy anyone to examine a post-irrigation colander with its chunks of apparently undigested family roast and not make some small changes to their diet. I love meat; the smell, the taste, the texture, but now it only makes a rare appearance on my plate.

Frankly, even that's too much for the gurus of cleansing, who believe a truly health diet revolves around fruit, vegetables, nuts and pulses - the more that's raw or steamed the better. Along with fish, they've become the staples of my diet. If I occasionally lapse - and nothing will make me give up Christmas turkey or goose - a flashback to The Spa reins me in.

While I'll take caffeine, alcohol and chocolate to the grave, I've also cut back on most dairy and wheat products. It might make me the dining companion from hell, but I do, at least, have the stories. People are constantly appalled yet fascinated by the idea of cleansing, and for some masochistic reason, demand the grim details between starter and main course. As they wait for their medium rare fillet or pork Dijonnaise, they crane forward to hear more about the decaying contents of people's colons.

As for Anthony, he never considered giving up meat. Or cream sauces. Certainly not Snickers. Life, as he sees it, is too short. And who am I to argue? But remember, this is the man who has lost his marble.

· The Spa, www.spasamui.com, tel: 00 66 77 230 855.