John Banzhaf likes to pose this challenge to students who enroll in his graduate class on legal activism at George Washington University, in Washington, DC. Think of something that really irritates you or smacks of obvious civil injustice, he tells them. Then think of a way of using the law to right the wrong and seek redress.

In other words, as Professor Banzhaf himself puts it with the freewheeling candour we have come to expect from both heroes and villains in the American legal system, let's sue the bastards.

It's a unique approach to legal education that has had some astonishing results down the years. Banzhaf's students successfully forced the stuffy Washington Cosmos Club to admit women for the first time, and got dry-cleaners to stop charging women more than men for laundering their shirts. Back in the 1970s, they sued Spiro Agnew, the Vice-President who left office in disgrace shortly before his boss, Richard Nixon, forcing him to return the bribes he had received.

Most famously, Banzhaf pioneered the notion of suing tobacco companies for the deleterious health consequences of smoking. He started doing it in the mid-1960s, when everyone thought he was nuts, and he was still doing it in 1998 when the US states successfully pried hundreds of millions of dollars out of the Big Five tobacco companies as compensation for their smoking-related health-care costs. If tobacco advertising is now banned on television, and smoking no longer tolerated on planes or in shops and restaurants in many parts of the United States, it is largely due to Banzhaf's 35 years of campaigning and savvy application of public-interest law.

And now, he has a new target: the junk-food industry. America, as we all know, is the fattest nation on the planet and getting fatter all the time. According to a report by the US Surgeon-General, released a few months ago, 61 per cent of Americans are now significantly overweight, compared with 55 per cent in the early 1990s, and 46 per cent in the late 1970s. Obesity generates $117bn in annual medical bills and triggers 300,000 premature deaths each year.

Is this a health problem on a par with the effects of tobacco-smoking? Banzhaf thinks so, and the government's figures are there to bear him out. Can the fast-food companies and the agribusiness giants, the packagers and marketers, be held responsible for the problem? Banzhaf argues that they are certainly the ones stuffing the nation's consumers full of fat, sugar and chemical additives. With a little statistical analysis, he believes, it should be possible to assign specific shares of the blame to specific companies.

And so, he is embarking on a new adventure in legal activism. Already, his graduate class has inspired one lawsuit, against McDonald's, and at least three others are in the works around the country. And that is just the beginning. As a recent magazine headline memorably put it, he wants to see whether Americans can sue their own fat asses off.

Banzhaf, it must be said, is far from your stereotypical litigation lawyer, forever looking out for an opportunity to screw a corporation or public institution and make a fast buck. Not only does he not make a penny from the suits that he inspires, he would, in fact, much rather not bring them in the first place. He would love it if the government would overhaul the food industry to make Americans healthier, just as he would have preferred the government to take action on smoking unprompted. But America is a country where recourse to the courts is frequently the only way to effect social change, since Congress and the federal government are all too often beholden to powerful industry lobbies, and public activism is rarely effective on its own because of the country's sheer size and deep-rooted conservatism. As the mantra goes, "If you can't regulate, litigate", and that is exactly what Professor Banzhaf has in mind.

"If government is willing to regulate, force disclosure of fat and calorie content, get fast food out of schools, put more health foods in vending machines, install bike racks and showers at public buildings to encourage more exercise, and so on, great," he said in an interview. "But if government does with obesity what it did with tobacco, which is largely nothing, then we may be forced to go to our third branch, the legal system."

The big question is how to go about it. It's one thing to say that diet has a lot to do with the growing obesity problem in America, quite another to prove in court that client A's heart attack was caused specifically by McDonald's hamburgers, or by excessive bingeing on Cherry Coke. Nobody sticks to one brand of food like they stick to one brand of cigarettes, so individual suits are out of the question and class action suits would have to depend on highly complicated statistical analyses of food intake and medical cause and effect. Also, unlike smoking, there is nothing intrinsically unhealthy about eating. From the standpoint of food chemistry, at any rate, the worst that can be said about junk food is that it contains large amounts of sugar and fat, both of which are actually important parts of a balanced diet as long as they are consumed in moderation. Can individual food companies really be held responsible for the immoderate appetites of their customers? Clearly, if there is a legal case to be made, it is going to have to be fairly ingenious.

Banzhaf's approach is a gradualist one, to start with the relatively easy stuff and see how far he can take it. The first line of attack is to go after food companies that misrepresent their products by understating the fat content, say, or omitting to mention certain ingredients. That is the basis of all the suits currently going through the courts. The second, slightly harder one is to accuse companies of making misleading health claims for their products – proclaiming pork to be "the other white meat", for example, when its fat and cholesterol content are in fact closer to beef than to chicken.

The third approach would be to pick up on sins of omission, or failure to warn consumers of certain health risks. Is it wrong of a fast-food chain to fail to point out that its triple-bacon double cheeseburger supersize meal contains more fat than any sane human being should consume in a week? Arguably so. Is it grounds for a lawsuit? Maybe, if the plaintiffs can work with laws on "clear and conspicuous disclosure of material facts".

And finally, the real zinger, if it can be made to work: an onslaught on the junk-food industry as a whole, in which McDonald's et al would be made to pay their share of responsibility for the adult-onset diabetes, sclerotic arteries, heart attacks and strokes that fast food helps to cause. Legal analysts are highly sceptical as to whether such an approach could ever work, and even Professor Banzhaf describes it as "a reach". But there are some promising avenues to explore, including the possibility of describing fast food as something akin to an addiction deliberately fostered by manufacturers through their marketing, especially to children.

"We know that people can become biologically predisposed to getting overfed, that once they grow extra fat cells, their bodies become accustomed to having that fat," he said. "Those fat cells never die, and even if you lose weight they lie dormant and constantly try to get you to eat more. It's not an addiction exactly, but it doesn't leave people with a completely free choice in what they eat, either."

Banzhaf has other strategies up his sleeve, first developed in the tobacco campaigns, for exerting pressure on government. One is to push for higher health- insurance premiums for the overweight, a measure that would act as an incentive for people to shed some pounds, and would also shift more of the health-care costs towards the people who incur them. Another is to push for higher taxes. After all, if one of junk food's principle attractions is that it is cheap, taxation is a simple way for governments to ensure that it does not stay that way.

Banzhaf does not pretend that any of these strategies would be a golden bullet, legally speaking, or even that successful lawsuits, on their own, would solve the problem of obesity. What he does believe is that intelligently mounted lawsuits can help change the climate of public opinion and pressurise junk-food companies and government regulators into changing some of their ways. "None of the things I'm suggesting are panaceas," he said. "But even the threat of lawsuits might be enough to make some helpful changes." On the tobacco issue, it was the change in public perceptions that turned the cultural tide, and that was due to a mixture of government action, anti-smoking health messages, public exposure of dishonest practices by the tobacco companies and, yes, the lawsuits.

His is undoubtedly an idea whose time has come. Drive along just about any stretch of highway in the United States, and the evidence of a nation addicted to junk is all too abundant in the endless string of signs for McDonald's, for Burger King, for In and Out Burger, Arby's and Kentucky Fried Chicken, for Taco Bell and 99-cent Tacos. There is rarely any healthier alternative. According to Eric Schlosser, author of last year's bestselling book Fast Food Nation, which has itself helped to stimulate debate on the subject, Americans now spend more on fast food than they do on movies, books, magazines, newspapers videos and recorded music combined. They spend more on mass-produced burgers than on higher education, or computers, or cars. More than 90 per cent of American children eat at McDonald's at least once a month, and the average American eats three hamburgers and four orders of fries every week.

There is certainly no problem in eating well in the cosmopolitan big cities, where the health kick has long since brought in its wake organic vegetables, farmers' markets, sun-dried tomatoes from Italy and home-made bread. But once you head inland from the coasts, away from the big population centres and the college towns, you find not only that the fancy olive oils and foreign speciality foods have vanished; and so, too, have most of the fruits and vegetables and, with them, the very notion of unprocessed fresh food. It's a straightforward question of availability, giving the lie to food industry claims that consumers can exercise free choice in deciding what to put in their mouths: in the heartland, the chains and big supermarkets have, by and large, taken over, and the few remaining family-owned businesses tend to survive through imitation rather than by providing any significant alternative. Thinness and healthy eating are increasingly becoming the preserve of the wealthy and the educated living in privileged urban cocoons.

Fast-food chains and soda vendors have penetrated college campuses and even state-run schools, where they have successfully offered sponsorship to cash-strapped school districts in exchange for the right to install their vending machines outside the classrooms. They have even invaded hospitals. While the cafeteria at the UCLA Medical Center in Los Angeles, one of the premier research hospitals in the country, offers sushi made on the premises and a full salad bar, at the main hospital in Toledo, Ohio, in the heart of the Midwestern rust belt, much of the catering is provided by fast food chains.

With such excesses and disparities, not to mention the manifest effect on American bellies, chins and thighs, have come the beginnings of a backlash. The junk-food merchants themselves have felt it, and have changed their strategies accordingly, with Coca-Cola and Pepsi diversifying into fruit juice, and McDonald's going on a corporate buying spree to move upscale into so-called family restaurants and fresh-sandwich chains. "Mad cow disease" hasn't hit US cattle yet, but the scare has prompted some reassessment of a food economy excessively reliant on the poorly regulated mass-production of minced beef. There are also signs that a new generation of mass-market restaurant chains might grow up with a greater emphasis on quality – the Starbucks craze is one manifestation, and so, too, are smaller initiatives like the Wolfgang Puck cafés (founded by Hollywood's most prominent celebrity chef) springing up across the West. California, with its reputation for health-consciousness, has been an obvious battleground for the first stirrings of an anti-junk food movement – just last week, the city of Oakland, across the bay from San Francisco, voted to ban fast food and soda from all of its publicly run schools.

John Banzhaf's own involvement in the issue began with a student of his, a vegan who had avoided fast-food fries for years because he knew that they were dipped in beef tallow. The student was appalled by a McDonald's advertisement claiming that its fries were cooked in 100 per cent pure vegetable oil – a statement that was literally true but omitted to mention that the fries were pre-cooked in beef fat. Soon, classwork on the issue evolved into a full-blown class action suit brought by Hindus in Seattle and a number of other cities, who said that the failure to disclose the beef content was an offence to their religion and constituted an "intentional tort". McDonald's has acknowledged the oversight and, according to Banzhaf, is on the verge of settling the case for around $12.6m.

The hidden beef issue has prompted another, more recent lawsuit against Pizza Hut, the allegation being that the chain's Veggie Delight pizza has beef products in it. Hidden fat content, meanwhile, is behind two other actions, one against an ice-cream manufacturer in Florida, and another against a line of corn and rice puffs called Pirate's Booty that, according to a test conducted by Good Housekeeping magazine, contains 340 per cent more fat than is stated on the bag it comes in.

Naturally, there are some pretty powerful interests anxious to stop any anti-obesity campaign in its tracks. Already, food-industry lobbyists and laissez-faire economic thinkers have lambasted Professor Banzhaf as some kind of food Nazi, seeking to dictate what people should put into their mouths. (The epithets "grease Gestapo" and "calorie cop" have been hurled in his direction.) In a particularly bruising appearance on one of the more vulgar discussion programmes on the Fox news channel, the presenters sought to trash Banzhaf as a hypocrite because he is somewhat overweight himself. A burgeoning "fat power" movement, meanwhile, argues that any attempt to get people to lose weight is tantamount to discrimination, that it is perfectly possible for a woman of average height to have a "natural" body weight of 20 stone or more, and that airlines, car manufacturers and restaurants should be obliged to provide larger seats.

Talking to Banzhaf, one senses that it is going to take a lot more than a few gratuitous insults and fat-is-good activists to undermine his determined sense of purpose. Unlike many consumer advocates – especially those involved in the anti-smoking movement – he is no pious moralist trying to tell people what is good for them. He is a legal thinker first and foremost, and his primary motivation is his belief that the law can be used as an activist tool for the public good.

"As a lawyer, I have two choices," he said. "I can litigate on behalf of whoever brings the buck into my office. Or I can look around and ask what kinds of problems I can attack through legal action. I find the latter much more interesting." It's the kind of thinking that caused his alarmed detractors to describe him down the years as a flame-thrower, a troublemaker, even a "legal terrorist".

Banzhaf has no illusions about the fact that what he is doing is profoundly political, and, indeed, he is plotting out his war on obesity in political terms. "What we are seeing is a large number of groups that might not previously have had much in common, coming together – vegans, Muslims, Hindus, conservative Jews, scientists, physicians, animal-rights groups, children's rights groups, sports organisations, and so on," he said. "Once they start joining forces, lawyers are going to smell the money, and legal action will gain its own momentum."

It could take years, or even, like the tobacco campaign, several decades. But Banzhaf is perfectly lucid about what he can and can't achieve: "Are suits possible? Yuh. Are some already successful? Yuh. Can we predict which ones will go and which ones will not? No we can't," he says. "But we'll soon find out."  
 
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