It's a feast: an alfresco banquet of chicken curry, Thai vegetables and tropical fruit, served on board a sun-licked boat above a reef. But not one of the passengers touches a morsel.

We gaze at the spread adoringly until Tim Warmath, a London-based recruitment expert, finally cracks and grabs a slice of watermelon. “It’s OK,” he explains. “I’m only going to suck. It’s just juice. It’s allowed.”

The food voyeurism and abstinence are easily explained. We’re on the Thai island of Koh Samui for a detoxification fast — seven days for most, 17 for Canadian engineer Dean Janzen — and not a sliver of mango, certainly not chicken curry, can pass our lips.

We can, however, swallow 35 bentonite clay drinks and 105 herbal laxatives to help the cleanse, along with 105 vitamin and mineral supplements. We’ll also consume 560 pints (318 litres) of coffee, squirted up our behinds to flush out toxins.

If it sounds like hell, particularly in a country with a reputation for spicy food, then I’m a sucker for punishment. It’s a return visit: my second bout of starvation that Gordon Ramsay would describe in F-words — and they wouldn’t include fast.

But it wasn’t hair-shirt masochism driving me back to the coconut palms and humid heat. My first detox had dramatic results. I shed lard, boosted my energy and ironed out several wrinkles. Surprisingly, it was also an entertaining holiday with an eclectic bunch of travellers: Aussie Mez, who was so hungry that she considered eating her apricot moisturiser; “Pipeline Pete”, a man mountain from Texas on his tenth fast; and Anthony from Ipswich, who passed a marble he had swallowed as a child.

Now, exactly a decade later, I wanted to see how it compared — because the Spa itself has received major cosmetic surgery. Instead of one beach resort with a hippy vibe and several tatty A-frames, there are now two centres on the island: the vastly improved, if slightly claustrophobic, original, and the smarter Spa Village, its chalets spread through the jungle on a nearby mountain.

A third spa, including a retirement village, will open in 2009 near Chiang Mai, taking the value of the complexes to £5 million. The company’s DIY cleansing regime is also being franchised out to a new resort on Koh Chang. A few more and the Spa could become the Starbucks of colonic irrigation — although a skinny latte enema is as yet unavailable.

What hasn’t changed is the pre-trip preparation: two weeks without booze, pasta, caffeine, meat and dairy products, unleashing headaches, stiff muscles and an alarming erotic dream about Vanessa Feltz. At least I stick to the diet. Last time, after a surprise upgrade, I demolished roast goose, cheese, port, champagne, Baileys and chocolate en-route to the detox.

I opt to stay in the newer, more luxurious Village — a tropical Château d’Yquem to Spa Beach’s Blue Nun. My teak bungalow with serene views of the Gulf of Thailand has a king-size bed, air-con, a huge deck and outdoor massage sala, along with a TV showing movies including Fast Food Nation, and Inside Britain’s Fattest Man. It will never be Chiva Som — some bathrooms clearly need a makeover — but with chalets starting at £14 a night, nor are the prices.

Within a day it’s clear that while my fellow fasters may be older and more professional than before, they’re no less cosmopolitan or entertaining. Where else could a UN aid worker, an Australian fashion photographer, a Hong Kong banker, a retired Brit and a chic Mexican party organiser discuss the contents of their intestines?

“Passed any bright yellow buttons?” asks American property developer David Smart over what would have been breakfast.

Colonics became something of an obsession. Anthony’s marble has achieved cult status, and rumours circulate of a subsequent visitor who passed a toy soldier — with bayonet attached. Andie Stafford, a slim Londoner, is hoping to get rid of a Scrabble piece she swallowed years ago.

I, however, just want to relax and emerge unscathed. For the first few days I crave coffee and contemplate drinking rather than squirting the cleansing solution. As toxins are expelled, I spend days four and five with a shocking sore throat. Mike Taylor, a 17-stone (108kg) former rugby player, has the same symptoms, along with a splitting headache. Other have mood swings and muscle aches.

I find more trauma in the jungle enveloping my chalet. It squeaks, rattles and hums, as well as supplying unwanted room service. It starts with a millipede, followed by an oversized gecko, a praying mantis and finally a mosquito that bites my left buttock mid-flush. It’s a tricky one to scratch.

Hunger is less of a problem. After 36 hours digestive enemas hibernate and stomachs shrink. It’s taste rather than actual food that people miss. When the Spa shows Supersize Me on the in-house movies, guests start hallucinating about McDonald’s. I find Shelley Evans, a 32-year-old Londoner, stroking the restaurant menu and sniffing ketchup bottles. “I was fine until the evil bitch in the next chalet put on Supersize Me. Now I’m obsessed with cheeseburgers.”

But there are distractions. I snorkel off nearby Koh Tan and spend hours in Koh Samui’s vast Tesco. It’s incredible, with Buddhist monks blessing the in-store bowling lanes and a ladyboy behind one of the tills — something I haven’t seen in Clapham.

There are also myriad activities at the Spa, from yoga, meditation and massage to therapies guaranteed to make your GP roll his eyes: numerology life reading and the ancient Chinese exercise chi gung, although the instructor is away “recharging his energy in India”.

By the time I break the fast — with raw fruit and vegetables rather than the lunatic bottle of Mekong whisky I downed last time — I feel amazing. I have brighter eyes, tighter jowls and have lost about a stone, half as much as David, who says: “It feels like taking off a 30lb backpack.” Shelley and Andie have lost less — they had less to lose — but in the right places. “It’s gone from the bloated stomach and thighs,” says Andie, “where women want to lose it.”

So did anyone rival Anthony’s marble? Mike, who morphed from alternative health cynic to evangelist, passed pieces of plastic bag — “I sometimes chew them” — but the real shocker was Dean’s 18in tapeworm. “I didn’t know what the hell it was. Now I’m wondering if there are any more up there. And, more important, did they mate?”

Need to know

Ian Belcher travelled with Opodo (0871 2770090, www.opodo.co.uk), which offers flights to Koh Samui via Bangkok from £763 return. Spa Village (00 6677 424 666, www.spasamui.com) on Koh Samui has double chalets from £14 a night. The one-week Clean-Me-Out Fast programme costs about £150. A Semi Fast three-day learning programme is also offered for £100.