Low-budget documentary Food Inc has become the shock DVD hit of 2010, even outselling Michael Jackson's This Is It.

The film, which exposes the cruel treatment of animals at the hands of big business, is one of the most disturbing releases you will see all year.

Eric Schlosser, the author of best-selling junk food exposé Fast Food Nation, is one of the experts featured in the film.

He says: "Most people think their food still comes from a little farm with a red barn and white picket fence. If they really saw where it comes from, they'd be shocked."

The documentary, out here on Friday, reveals how chickens in the US are raised and slaughtered in just 49 days. That is half the time it took to raise a chicken 50 years ago - yet they are twice as big as the birds in 1950 when killed.

Eric, 49, adds: "People like white meat so chickens have been re-engineered to have bigger breasts. The chickens never see sunlight.

"It is an assembly line with mass production. A lot of the internal organs and bones can't keep up with the rate of growth, so many of the chickens can only take a few steps and then they fall down." The film, which has been nominated in the Best Documentary category in next month's Oscars - includes footage showing what happens inside the biggest cattle yards in the US.

Cows are seen wading through their own manure. Many are fattened on corn instead of grass, and farmers force-feed them antibiotics.

Eric warns: "This could be a preview of coming 'attractions' for the UK. We filmed a gigantic pig-processing plant in North Carolina - but huge plants are already opening in Eastern Europe and shipping to the UK."

Monsanto - one of the food giants criticised - says the film "demonises American farmers and the agriculture system responsible for feeding more than 300million people and presents an unrealistic view".

But just four firms produce 80 per cent of the beef in the US, and the country has only 13 slaughterhouses. The Smithfield processing plant in North Carolina - the world's largest - kills more than 32,000 pigs a day.

Stella McCartney was among the stars who attended the film's London premiere last night. The vegetarian clothes designer said: "This is information everyone should have. It is terrifying and empowering."

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