Bob Freibaum surveyed his loaded tray for a second then opened the styrofoam box containing his hamburger. With a salt-frosted rustle he dumped a sleeve of French fries into the lid, squirted a pile of tomato ketchup on them and punched a straw through the plastic lid of his super-sized chocolate milkshake. Bob was ready for lunch.

 With earnest concentration he ploughed his way through his burger, fries and shake before throwing a deep-fried apple pie down his throat for good measure. He leaned back, a broad grin on his face.

 "Damn, I love a Big Mac!" he said. The Big Mac is not so loving to him. Bob acknowledges there is a connection between his love affair with McDonald's most famous product and his near-inability to fit his 18-stone frame behind the table. "They reckon these things are bad for you," he said, "but they sure taste good, and that's OK by me." Freibaum's satisfaction is as plain to see as his 46-inch waist but to a growing army of lawyers and nutritionists he is little more than a victim of a ruthless fast food industry they claim is responsible for America's growing struggle with obesity and its $ 117bn annual cost to the national economy.

 The charge is that these companies are peddling high-fat, high-sugar food and drink without telling their customers it is bad for them. Critics scoff that it is ridiculous to hold restaurants accountable for the long-term impact of the food they sell, but the same argument was once made about suing tobacco companies for the damage inflicted by cigarette smoking. History proved that argument wrong.

 It is no coincidence that leading the charge against companies such as McDonald's, Kentucky Fried Chicken and Burger King is Professor John Banzhaf from George Washington University in Washington DC, one of the foremost figures in the legal battles that ended with Big Tobacco paying hundreds of millions in reparations to states for the cost of smokers' healthcare.

 Key to victory in the tobacco wars was the contention that cigarette manufacturers failed to tell smokers precisely how addictive their products were. Banzhaf claims fast food companies are guilty of a similar crime. "I think when a fast food company deliberately doesn't tell you important information that both legally and morally they bear some responsibility," he said. "It is deception by omission."

 Bolstering Banzhaf's position is a recent case in which McDonald's paid 12m dollars damages after being sued by American Hindus who claimed the food chain should have advertised that it added beef extract to its French fries. In 1990 McDonald's had proudly claimed it was switching from using beef tallow to fry its chips to vegetable oil as part of its commitment to healthy eating. It did not announce it was adding the beef extract to replace the lost taste of beef that had attracted so many customers.

 Whether that case is a legitimate precedent or not, Banzhaf and the anti-fast food campaigners are clearly faced with a more diverse target than that presented by tobacco. If a smoker dies of lung cancer, the cause is clear. If an obese person dies of a heart attack, the cause is harder to pinpoint. Was it the hamburgers, the fries, the chocolate, the doughnuts, the endless soda pop or all of them?

 Kelly Brownell, a psychology professor at Yale University says the tactic should be different this time.

 "It's important for us to look at this from a public health point of view," he said, "where we are not concerned with how overweight an individual is but how overweight the population is."

 Brownell believes that means attacking the culture of the fast food industry that sees targeting young children in a bid to hook them into fast food, just as one tobacco company did by making a cartoon character out of one of its cigarette brands.

 "We take Joe Camel off the advertising billboard because it is marketing bad products to our children," he said, "but Ronald McDonald is considered cute. How different are they in their impact, in what they are trying to get kids to do?"

 Finding a weapon to use in this war is proving harder than finding indignant rhetoric. Kelly Brownell once famously advocated a "Twinky Tax", named after a gooey American confection filled with sweet cream. It got nowhere. He still believes some form of tax on fatty food should be imposed, the revenue being spent on health education programmes.

 Civil libertarians ridicule the idea of taxing food. "We can only imagine - different tax brackets for different cholesterol levels," said Brian Cohen of America's Future Foundation. "Itemised tax deductions for healthy food? Extra exemptions for dependents who make the soccer team?"

 Congress appears to agree it is a bad idea and has consistently refused to take up the cause. John Banzhaf prefers to use the legal system against fast food, much as he did in the Tobacco Wars. "There is a movement afoot to do something about the obesity problem and see it in terms of costs," Banzhaf said. "It is appropriate that the cost shouldn't be borne by everybody but confined to those who use the products and/or produce them."

 He says this means fighting individual battles over misleading advertising or misrepresentation of fast food's fat content. Gradually, he believes, the industry would be sufficiently wounded for states to get involved, just as they did against the tobacco companies, to recover the cost of caring for sick citizens. He reckons that would get the companies' attention.

 The campaign finds sympathy with the man who has done more than any to document the impact of the fast food business on America, Eric Schlosser, whose book "Fast Food Nation: The Dark Side of the All-American Meal" caused an uproar on its publication last year. Schlosser agrees that there is a health risk associated with fast food but goes one step further, accusing the companies of deliberately marketing to poorer Americans, attracted by the flashy advertising campaigns and the cheap, plentiful food on offer.

 "They are serving extremely high-fat food to people who are at the greatest risk of the health consequences from obesity," he said. "And by heavily marketing unhealthy foods to low-income children they are encouraging health problems among the segment of the population that can least afford them."

 If there is a link between fast food and obesity, evidence for Schlosser's contention is available throughout the land. It is not easy to spot very fat people in prosperous metropolitan areas of America but in poorer rural regions they are hard to avoid.

 Schlosser believes the damage wrought by fast food companies on America is even deeper than adding inches to the national waistline. His book documents the industrialisation of the meat industry that geared itself to meet the demand of the fast food business and the growing prevalence of deadly bacteria in chicken and beef that has resulted from it. He also reports on the appalling employment conditions in both the meat and the fast food industries that draw on immigrants, often in the US illegally, or non-unionised young people for their staff. His most stomach-churning evidence is of appalling hygiene conditions in fast-food restaurants that project a gleaming exterior to their customers but harbour filth and disease behind the counter.

 What all sides agree on is that the rise in the fast food culture has gone hand in hand with growing sloth in America epitomised by an ever-greater reluctance to walk anywhere, and a booming entertainment and media industry that keeps Americans' eyes glued to computers, TV and video games and their backsides glued to the couch.

 All they disagree on is whether it is the fault of the companies peddling the lifestyle or individual Americans for buying into it.

 "The ultimate 'villain' in America's epidemic of blubber has more to do with lifestyle choices than nefarious businesses peddling fattening foods," said conservative commentator J D Tucille. "Obesity comes as a trade-off for the pleasure that people take in eating and leading sedentary lifestyles. Some of us may not agree with that trade-off, but we are free to make other choices."