His weight and cholesterol shot up, his liver was on its way to becoming pâté. He fought headaches and depression. Even sex held no allure. Morgan Spurlock, who gave himself to junk food for a month in the name of art, talks to Donald Clarke

A year and a half ago Morgan Spurlock, whose extravagant moustache looks as Victorian as his name sounds, found himself down on his luck. In the aftermath of September 11th, his New York-based film company was ailing and he had run up a quarter of a million dollars in credit card debt.

"I was paying the rent with credit cards. I was paying for food on credit cards. And then I was paying the credit card bills by credit card," he explains. Evicted from his apartment, Spurlock was reduced to sleeping in a hammock in his production office.

Now, not quite back in the black, but close enough, he finds himself sitting in the Dorchester Hotel in London ordering exotic room service while the world's journalists sit at his feet. He achieved this turnaround not by building the proverbial better mousetrap or discovering a cure for the common cold, but by eating so many hamburgers that his liver began to form itself into pâté. What a strange world we live in.

The idea for the project that became Super Size Me struck Spurlock on Thanksgiving Day as he was watching a news report about two young girls from the Bronx who were suing McDonald's for contributing to their chronic obesity. Like most sensible people, Spurlock's first reaction was to scoff. "We are going to sue a food company about choices that we ourselves have made?" he puffed to himself.

But then he began to listen closely to the defences put forward by McDonald's public relations wonks.

"They came on and said you can't link our food to these girls' weight gain. It's healthy. Well, I thought if its so healthy then you should be able to eat it all the time. Shouldn't I be able to live the all-American lifestyle: plenty of food and no exercise?"

His plan was, for a calendar month, to let nothing pass his lips - not even water - that he hadn't bought from McDonald's. To increase the pressure on his unfortunate metabolism, he would decline to take any serious exercise and every time a McDonald's employee asked him if he wished to "super-size" - that is, to increase his portion from large to gargantuan - he would accept their offer.

It all seemed like a bit of a lark at first. He would record the results for a documentary. If he put on a little weight or his skin became a tad blotchy, he could, extrapolating the results over a lifetime, make some interesting deductions about the effects of eating too much fast food. By the end of the month Spurlock's cholesterol had increased by 65 points, he had gained a 10th of his previous body weight, and he was developing serious liver abnormalities. He also suffered from headaches, depression and - as his girlfriend has ungallantly explained - lowered sex drive.

Super Size Me has become something of a sensation. Winner of the best director's prize in the documentary section at the Sundance Film Festival, the $65,000 film went on to take $10 million at the US box office. But what exactly is the point of it? McDonald's has not unreasonably pointed out that nobody is likely to live exclusively of its food throughout his or her life.

"Nobody eats it for breakfast lunch and dinner?" Spurlock says. "Well no, but I do know people who order McDonald's for breakfast and then have Taco Bell for lunch and then Dominoes pizza for dinner. It is actually not that unrealistic. One out of four people are what McDonald's calls super-heavy users: people who eat the food more than five times a week. That is out of 46 million people. Now, if you are doing that then you are not eating tofu sandwiches the rest of the time. I am not saying all this is entirely McDonald's fault. I think the film is about us making bad choices in life."

Spurlock seems like an honourable chap, but I did find some parts of the film a little difficult to, ahem, swallow. On the second day of his experiment, we see him tucking into a super-size meal in his car. He just about manages to get it down, but shortly afterwards vomits out the window. Considering that there is nothing actually toxic in McDonald's food, I couldn't quite understand why his body rejected it so violently.

"I'm a big believer that your body tells you things," he says. "It will tell you what it wants and what it doesn't want. I had already gone through one whole day of eating nothing else and here I am getting my first super-size meal: quarter-pounder with cheese, massive Coke, huge fries. And the body says: 'What are you doing to me, bro'? Let's go get some broccoli.' A super-size Coke is almost a litre and a half. A super-size fries is a quarter of a kilo."

But it is possible (so I've heard) to drink eight pints of Guinness and then eat a kebab without throwing up. This is just food. "Yeah, eight pints of Guinness might be fine in itself. But it was the full day before that did it."

And what about his medical results? It is surprising that a poor diet could cause life-threatening damage so soon. Perhaps if he continued the experiment his body might have started to adjust to the massive ingestions of stodge.

"Towards the end I started to feel a bit better," he admits. "I pushed through and towards the end of the film you see that my liver function is settling down and my blood pressure is getting better. I asked them: is this going back to normal? And they said no, it's not doing that. But my body was starting to get used to this type of torture."

Unsurprisingly, McDonald's didn't co-operate with the project. Spurlock wryly admits that, after glancing at his undistinguished CV, the company probably didn't think he was worth worrying about. When he came up with the idea he was best known for a lowbrow reality show on MTV named I Bet You Will. He had directed a few pop videos and written a play, but there was little to suggest that this obscure documentarist might produce one the following year's big sleeper hits. "If I had been Michael Moore or Errol Morris they might have responded to my calls," he laughs.

To what extent McDonald's recent corporate policy has been affected by Super Size Me's success is in question.

"Oh yeah," dismisses Spurlock, "they say it is all just a coincidence that just two weeks ago they eliminated super-size meals and that they announced that Ronald McDonald is coming out with an exercise video. The launch of the Go-Active Adult Happy Meal is a coincidence. These are all just amazing coincidences. It seems they expedited a lot of their decision-making. It is better for them to be proactive in their decisions than reactive to the film."

And, since Spurlock and I talked, the coincidences have continued. Readers may have seen the recent Irish television commercials in which a bungling, hapless (how subtle) investigative reporter pesters supernaturally cheery McDonald's staff in crisply ironed uniforms about the contents of the company's food. One advertisement in particular bears a quite uncanny resemblance to a key sequence in Super Size Me. The reporter is searching for nutritional information and is embarrassed to discover that, contrary to his preconceptions, it is clearly on display. In the film, Spurlock has less success tracking down similar documents.

When I innocently phoned up McDonald's to inquire why it felt it necessary to respond to the film in this way, I was told that the campaign had absolutely nothing to do with Super Size Me.

"We would have planned this in 2003, just as we are planning our campaign for 2005 now," says Siobhan Murray, communications manager of McDonald's, Ireland. "We would have had no idea that Super Size Me was coming out when we planned our present campaign. . . We are not reacting and we will not react to the film Super Size Me."

So let's just get this straight. There is no connection between this particular commercial and the very, very similar scene in Spurlock's film. "Absolutely no connection whatsoever." (Murray's comments relate only to Ireland. In Britain, as reported last week in The Ticket, McDonald's has responded with a series of coolly worded, vaguely conciliatory newspaper advertisements.)

One of the difficulties with Super Size Me is that it is hard to discern exactly how Spurlock wants McDonald's to respond. He agrees when I point out that introducing salads and little bags of fruit (as the company has done) is really rather pointless. Who in their right mind goes to McDonald's for lettuce or apples? We go there to enjoy guilty pleasures.

"Yeah, we go there for the burgers and the shakes and the fries," he says. "But they really need to provide their consumers with more information. What happens is that fast food is an impulse buy. You need to give people all the information. If you see a Big Mac and you know it's 590 calories, you might say: no I don't want to super-size that."

Well, maybe. But surely the main problem is the sheer unavoidability of McDonald's restaurants. The only way you are going to get people to eat beneath the arches less often is to decrease the number of outlets. And in a market economy that isn't going to happen.

"Yes. You can't work backwards," he agrees. "I picked them because they are iconic; they are the industry leader. Everybody follows the leader. If they began labelling food properly, then everybody else would do it. You will have raised the bar."

Speaking of how ubiquitous McDonald's restaurants are, I wonder if Spurlock, who travelled the length and breadth of the country making the film, ever had any serious difficulty locating an outlet.

"We were flying to Wisconsin and we had to go through Detroit," he laughs. "I couldn't get breakfast at La Guardia and we got to Detroit and there wasn't a McDonald's in the airport. How is this possible? This is the one airport in America which doesn't have a McDonald's. So when we finally got there I was absolutely starving."

He doesn't look too bad for the experience. Throughout the film his girlfriend - who, with you-couldn't-make-it-up deliciousness, is a vegan chef - becomes increasingly concerned about what Morgan is doing to himself. But the 33-year-old seems to be a proper weight now and appears to be full of energy.

"I looked terrible then," he says. "I feel much better now, that's for sure. It took me 14 months to lose all the weight. The last few pounds were the yo-yo pounds. I'd lose two and then gain three and so on. After you lose weight all those fat cells in your body don't vanish; they just get smaller. You have to make a lifestyle choice every day to stay thin and healthy."

So, will he still eat fast food? "Oh I love a good cheeseburger. I recommend In-n-Out Burger in California." But he won't go to McDonald's? "I think they have my picture on the back of the door of every restaurant with darts in it," he laughs.