Gain weight, lose your love life with McDiet?

Normally sane actors have been known to gain or lose huge amounts of weight for their art. Think of Renée Zellweger in Bridget Jones's Diary. Directors, of course, never have to undergo such torture. Or so it used to be, until Morgan Spurlock had a bright idea for a film project.

The first clue to his particular misery comes in the title of his documentary, which has become the darling of this year's Sundance Film Festival. It is called Super Size Me: A Film of Epic Portions and it is a sometimes comic but serious look at America's addiction to fast food.

Spurlock, a 6ft 2in New Yorker of usually cast-iron constitution, made himself the guinea pig in this dogged investigation into the effects of fast food on the body. He ate only at McDonald's for a month - three meals, every day - and took a camera crew along to record it. If a server offered to super-size his order, he was obliged to accept - and to ingest everything, gherkins and all.

Neither Spurlock, 33, nor the three doctors who agreed to monitor his health during the experiment was prepared for the degree of ruin it would wreak on his body. Within days, he was vomiting up his burgers and battling with headaches and depression. And his sex drive vanished.

When Spurlock had finished, his liver, overwhelmed by saturated fats, had virtually turned to pâté. "The liver test was the most shocking thing," said Dr Daryl Isaacs, who joined the team to watch over him. "It became very, very abnormal."

Spurlock put on 25lbs over the period and his cholesterol level leapt from a respectable 165 to 230.

He told the New York Post: "I got desperately ill. My face was splotchy and I had this huge gut, which I've never had in my life ... It was amazing - and really frightening." And his girlfriend, a vegan chef? "She was completely disgusted by me," he said.

Making the film over several months last year, Spurlock travelled through 20 states, interviewing everyone from fast-food junkies to the US Surgeon General and a lobbyist for the industry. McDonald's, for whom the film can only be a public relations catastrophe, ignored his repeated entreaties for comment.

Spurlock had the idea for the film on Thanksgiving Day 2002, slumped on his mother's couch after eating far too much. He saw a news item about two teenage girls in New York suing McDonald's for making them obese. The company responded by saying their food was nutritious and good for people. Is that so, he wondered? To find out, he committed himself to his 30 days of Big Mac bingeing.

The film does not yet have a distributor and, given the advertising clout of McDonald's, that may prove problematic. But the critics at Sundance seem to have been captivated. Certainly, the film is blessed by good timing. Obesity has in recent months captured headlines as America's new health scourge. The humour of the approach - and Spurlock's own suffering - obviously helps.

At the festival in Park City, Utah, he has had teams handing out "Unhappy Meal" bags on the streets with a few "Fat Fun Facts". For instance, one in four Americans visits a fast-food restaurant every day. And did you know that McDonald's feeds more people around the world every day than the population of Spain? The makers have self-rated the film "F" - for "fat audiences".

McDonald's has finally been forced to comment. "Consumers can achieve balance in their daily dining decisions by choosing from our array of quality offerings and range of portion sizes to meet their taste and nutrition goals," it said in a statement last week.

Spurlock claims that the goal was not to attack McDonald's as such. Among the issues he highlights is the willingness of schools to feed students nothing but burgers and pizza. "If there's one thing we could accomplish with the film, it is that we make people think about what they put in their mouth," he said. "So the next time you do go into a fast-food restaurant and they say, 'Would you like to upsize that?' you think about it and say, 'Maybe I won't. Maybe I'll stick with the medium this time.'"