Stale bread, regular colonic irrigation and liver packs... Inspired by tales of dramatic weight loss, junk-food addict Johann Hari thought he could cut it at the Mayr health spa. Little did he know

The editor was inexplicably reluctant to send me on the Jihad Diet, but my granny was (as always) on to something: to lose weight, I needed something drastic. I am a junk-food addict, scoffing KFC buckets and Wimpey Double-Burgers as casually as a butterfly flaps its wings. I may as well have a saline drip running lard into my veins. Once, I found an old, cold Chicken McNugget in my bed and, reader, I ate it.

Sure, I tried taking small, incremental steps: every now and then, I would book a personal trainer, but each time, I left in despair after a few sessions. My last trainer handed me a small device that runs an electric current through your body to find out your Body Mass Index. It turns out I'm 35 per cent pure lard. If I were a sandwich, nobody would eat me. Except me.

But then, this August, a sliver of salvation appeared on the horizon. A friend returned from a fortnight in a magical clinic in Austria, two stone lighter and eulogising about the theories of one Dr Franz Mayr. The late doctor practised something called "intestinal cleansing therapy".

It's simple: our bodies are clogged with toxins that damage our health, Mayr said, and they need to be cleansed and given time to heal. Once this happens, our bodies will no longer crave toxins and my pining for chicken popcorn will fade. His latter-day disciples include Sarah Ferguson, who allegedly paid £2,000 to be subjected to his structures for a week. I am a militant proselytiser for science and evidence - but when it came to my own diet, these principles disappeared and I booked myself a fortnight in the Mayr Clinic.

As my taxi driver circled around Lake Worth trying to find the clinic, my gut was still digesting the KFC Mega Meal I had gorged on in Gatwick. Eventually we found a lumbering wooden edifice looking directly over the lake's blue cool calm, and I disembarked with my bags. At the clinic, there was a gaggle of beaming Austrians waiting to explain the "extraordinary cure" I was about to experience. My guide then began to talk about the life and work of Dr Mayr. He had that look of submissive reverence that North Koreans are required to show towards Kim Jong Il.

"Up until his death at the age of 90 - when he was incredibly mentally alert and active - Dr Mayr outlined visionary theories about the intestines," my guide said. "How happy he would have been about the spread of his theories! How happy he would have been to see you here!"

"Ah, Dr Mayr," I sighed. His portrait smiled at me from the walls. Then I was taken to the "restaurant", and suddenly he did not seem so kindly. A bowl - no, a dribble - of soup was placed before me, along with a stale bread roll that had the texture of concrete. The woman next to me whispered, "I hear this is our last meal for the whole first week!" I gave her such a severe glare that she physically recoiled.

"You will be given an eating plan by your doctor tomorrow morning," the restaurant hostess said by way of introduction, "but the bread you see before you is a key part of the Mayr diet." The 40 other people in the restaurant were silent. I was beginning to panic. "Excuse me," I said. "Why is the bread stale?" "This is a good question," the hostess replied. "It is stale because we want to teach you to chew."

Chew? Didn't I learn that skill sometime before my first birthday? "No. Nobody in the Western world knows how to chew. Dr Mayr showed this. Most people today swallow their food after giving it one or two chews, and it enters the intestines very hard. This puts stress on the gut. Here, you will learn to chew each mouthful of food 40 times." Forty? "Yes. Do not swallow anything until it is a thin liquid pulp. And you must not speak to each other or read when you are eating. This is distracting and wrong. You will sit in silence. And chew."

Afterwards, I retreated to my bedroom, my jaw sore and my stomach in disarray . The room was wooden and bare, and I quickly passed out in a gloom. I was awoken by the fierce Austrian sun and taken down for another scrap of stale bread before being led to see Herr Doktor.

Tall, in his late thirties, and earnest, he looked at my tongue and my eyes with a torch and a concerned expression. He took me to a mirror. "Please put out your tongue and tell me what colour it is," he said. "Pinkth," I replied. "No. Please look again." I peered: it was true, there was a distinctly non-pink fuzz. "Mr Hari, your tongue is grey with elements of yellow." He paused, shook his head, and added: "This is not good."

"And now look at your eyes. They too are yellow." I looked carefully: this I couldn't accept. "Yes, they are. Look. There is a yellow tinge in the corners. This is a sign of a stressed and unhappy liver." He scribbled something down on his pad and told me I must lie down "so I can get the feeling of your intestines." I lay back on his table in my pants, and he prodded and felt my stomach for approximately a millennium. He kept muttering to himself. "Ah, the lower intestine - it is obscured," he said at one point. And then - triumphantly - "There is much gas here."

"I think, Mr Hari, we will put you on the T Diet," he said. I assumed he had 26 plans, running from A to Z, and he had plucked a special one for me. "What does this diet involve?" I inquired. "For breakfast, you will have tea. And for lunch, you will have tea. And for dinner, you will have tea - with a hint of honey," "Ah," I said. "And when will I eat?" He paused. "You will eat tea - as you like. But there is a strict limit on the honey."

I laughed out loud. He ignored me. "And I see you take antidepressants, Mr Hari. You will stop taking them while you are here." "I have been taking them for seven years - they are a serious medication," I spluttered. "You are at the bottom of a mountain," he warned, "and you will struggle to climb to the highest heights..." "I am not stopping my anti-depressant without consulting my GP," I insisted. He shook his head and replied: "Very well. But you must learn this is not a depressing place."

All my alarm bells were ringing. What sort of doctor was this? I staggered out to begin the Mayr Clinic's programme of activities. They discouraged vigorous exercise - my philosophy exactly - so I was led to my room to have a rest (after being awake for three hours) and to have a warm hay liver pack laid on my stomach.

I wandered down to the lake and met an Austrian woman, who had been on the tea diet for two weeks. "How do you feel," I asked. "Terrible," she replied. "And there is an Irish gentleman on the tea diet over there who has begun to hallucinate."

I hurried away to telephone The Independent and check they had an obituary ready for me. Next, my timetable stated I had an appointment in the clinic's "saline cabin". I was shut into a chamber while simulated sea-air began to slowly surround me. I waited for 15 minutes, breathing this, and... nothing happened. I felt exactly the same.

Three days passed in a blur like this, talking to disoriented, people, trying useless "treatments", and feeling my stomach digest itself. Soon, I had a Hiroshima-force headache. When I asked for an Aspirin, I was offered a tube. "Attach this to the tap in your bedroom and give yourself an enema," the nurse said. I began to wonder what would happen to me if I successfully detoxified. What if I was in fact a living, breathing toxin, and once the chemicals had gone there was nothing left?

On day four, I awoke at three in the morning, drooling after a dream where I had drowned inside a gigantic strawberry milkshake. In a frenzy, I gathered up the fluff underneath my bed, and seriously considered eating it. I scampered down to the kitchen determined to raid it, but it had been fatty-proofed: even with all my strength, I could not break into the stock of stale rolls.

Enough. I demanded an appointment with the doctor and told him I could not take this any more. He stroked his facial hair and said, "I think you are lacking in courage, Mr Hari." "No, I am lacking in food," I replied. "Very well. We will give you a meal." A meal! I nearly kissed him. I went to the restaurant - and was given something that would barely constitute a snack in the outside world - a tiny chunk of pizza. I wept and realised I will never, never be thin.

In despair, I checked myself out. Yes, I was half a stone lighter - but how sustainable was a diet where I eat nothing? How credible was the science of "inner body cleansing" anyway? But perhaps a small part of their theories turned out to be true.

At the airport, I looked at the succulent array of sandwiches and burgers - and something miraculous happened - I craved a fruit salad. I slowly, carefully chewed an array of berries and melons and kiwi fruit and gurgled with pleasure. The cravings for lard had leeched out of my system.

Even now, three months later, I have not regressed to my pre-Mayr level of obesity. But my granny was right all along: the only way I will ever be truly thin is with the help of Abu Musab al-Zarqawi.

The Mayr Clinic, Dellach, Marie Worth on Lake Worth, Carinthia, Austria (00 43 4273 2511)