His mission: To eat three meals a day for 30 days at McDonald's and document the impact on his health.

Scores of cheeseburgers, hundreds of fries and dozens of chocolate shakes later, the formerly strapping 6-foot-2 New Yorker - who started out at a healthy 185 pounds - had packed on 25 pounds.

But his supersized shape was the least of his problems.

Within a few days of beginning his drive-through diet, Spurlock, 33, was vomiting out the window of his car, and doctors who examined him were shocked at how rapidly Spurlock's entire body deteriorated.

"It was really crazy - my body basically fell apart over the course of 30 days," Spurlock told The Post.

His liver became toxic, his cholesterol shot up from a low 165 to 230, his libido flagged and he suffered headaches and depression.

Spurlock charted his journey from fit to flab in a tongue-in-cheek documentary, which he has taken to the Sundance Film Festival with the hopes of getting a distribution deal.

"Super Size Me" explores the obesity epidemic that plagues America today - a sort of "Bowling for Columbine" for fast food.

As well as documenting his own burger-fueled bulk-up, Spurlock travels to 20 cities across America, interviewing people on the street, health experts and a lobbyist for the fast-food industry.

Despite making dozens of phone calls, Spurlock fails to get anyone from McDonald's to agree to an on-camera interview.

A spokeswoman for McDonald's told The Post yesterday that no representatives from the corporation had seen "Super Size Me."

"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," McDonald's said in a statement.

Over the course of the film, Spurlock is regularly examined by a gastroenterologist, a cardiologist and SoHo-based general practitioner Dr. Daryl Isaacs.

"He was an extremely healthy person who got very sick eating this McDonald's diet," Dr. Isaacs told The Post.

"None of us imagined he could deteriorate this badly - he looked terrible. The liver test was the most shocking thing - it became very, very abnormal."

Spurlock has since returned to normal health. "The treatment was to just stop doing what he was doing," Dr. Isaacs says.

Spurlock, who says he ate at McDonald's only sporadically before his total immersion in the Mickey D's menu, says he even began craving fat and sugar fixes between meals.

"I got desperately ill," he says. "My face was splotchy and I had this huge gut, which I've never had in my life.

"My knees started to hurt from the extra weight coming on so quickly. It was amazing - and really frightening."

Spurlock's girlfriend, Alex Jamieson, was horrified - she's a vegan chef.

"She was completely disgusted by me, not happy at all," he says. "But she realized what my goals were in trying to educate people."

Spurlock, a film producer who grew up in West Virginia and studied ballet for eight years, was spurred to make his first feature film while watching TV on Thanksgiving Day, 2002.

"I was feeling like a typical American on Thanksgiving - very bloated and happy on the couch - and at some point on the news they were talking about two women who were suing McDonald's.

"People from the food industry were saying, 'You can't link kids being fat to our food - our food is nutritious.'

"I said, 'How nutritious is it really? Let's find out."

Not surprisingly, Spurlock has steered clear of the Golden Arches since filming wrapped.

"I have not had McDonald's for seven months, but yesterday, during an interview, I had a bite of a Big Mac," he says.

"I chewed it up, swallowed it and I said, 'You know what, I'm pretty much done after that bite.' "