Ricky Gervais recently said: "If there's a woman in leggings, eating chips with a fag in her mouth, sterilise her." The comment was made in the context of over-population and also included a reference to "stupid fat faces". It caused a mild ripple in the media, but it quickly died down, largely because, ironic or not, Gervais's observation was a reflection of what most of the country thinks. In Britain, we're finally enlightened about the importance of respecting race and sexuality: but fat? In one sentence, Gervais nailed it: fat equals thick; fat equals lack of control and overeating; fat equals poverty and moral bankruptcy, a potent combination of social and body hatred. Fat equals a life sustained on fizzy drinks, mass-produced food and no boundaries. The obese, with body mass indices (BMI) of more than 30 and with their lumbering walks and rolls of flesh, Type 2 diabetes and heart problems, are a sub-race less deserving of the rights of the rest of us. As we are told, these adults are to blame for everything from rising healthcare to creating a new generation of fat children (one in four in the UK).

The US spends $147bn (£96bn) a year fighting obesity – more than it spends on cancer and twice as much as it spent in 1998. Michele Obama, in her campaign against childhood obesity, has put her 8-year-old and 11-year-old daughters on a diet (their BMI was "getting off balance" her paediatrician told her). Two thirds of Americans are overweight or obese (defined as a BMI of 25-30 or 30-plus respectively). In the UK, The Foresight Report: Tackling Obesities: Future Choices project (2007) predicted that in the absence of action, 60% of men, 50% of women and 25% of children would be obese by 2050, putting the bill for the NHS at £45bn (costs are estimated to rise to £6.3bn a year by 2015).

We have entered an era in which phrases such as "moral obligation" and "economic imperative" are being used to get fat people thin. The Food Standards Agency endorses British restaurants (such as Pret A Manger) displaying calorific content on menus, just as New York mayor Michael Bloomberg has forced cafes, restaurants and fast-food outlets in the city to do the same. Billboards around New York are showing glasses of thick, yellow fat, marbled with blood vessels, accompanied by the words "Are You Pouring on the Pounds?" Congress is considering proposals for employers to use financial rewards or penalties to promote healthy behaviour by employees – such as losing weight. In the UK, David Cameron has called for an end to the "moral neutrality" of not passing judgment. The message could not be clearer: we can no longer let fat people stuff their porky faces with fast food and feed their children burgers in front of the TV, because the rest of us are picking up the bill.

Even hand-wringing liberals (myself included) allow ourselves to see overweight people as degenerates, and luckily for us, the catastrophic social and economic predictions lend our views a respectable medical justification. We cheer Jamie Oliver for patiently trying to tell fat mothers about the vices of over-processed food, and he's now doing the same in the US. "Overweight" people (usually the poor), we think, just need to be taught the right skills. They need to learn how to cook properly with fresh ingredients, how to exercise, how to gently guide their children towards fruit and vegetables and how to show restraint in their own eating habits. Because, we think, in all those millions of fat people out there, there are nice, thin people like us trying to get out.

The focus on fat has created a backlash of "fat pride". Just as obesity and the diet industry have their roots in the American culture of excess, so too does the movement set up to challenge our obsession with them. The National Association to Advance Fat Acceptance was founded in 1969, followed three years later by the Fat Underground, a radical therapy group that began so that its female members could resolve fat oppression and become thin. (Discovering the futility of diets, the group quickly abandoned any notion of the "thin" ideal.)

After 40 years, then, the "Weight Diversity" movement has come back to the fore. Made up of activists throughout America – with strong links in the UK – its retaliation against fat-bashing has found its way to Capitol Hill. It is based on three things. Science: the strength of evidence of the abysmal long-term failure of dieting (it makes you fatter in the end) combined with new research showing weight per se is not indicative of health; social: the unacceptable persecution of a new "underclass"; legal: the violation of an individual's human rights.

With the combination of all three, the Weight Diversity movement is sending out its own message: "Back off!", it is saying. "We are fat, we are happy and we are healthy!"

Can we dare believe them?

Before my trip to San Francisco to meet members of the Weight Diversity movement, I had been reading about one of its members, Marilyn Wann, author of Fat! So? Because You Don't Have to Apologize for Your Size! Wann is one of the fat activists instrumental in getting anti-weight discrimination laws passed in San Francisco (one of only six cities where they exist) after she saw a billboard outside a gym showing an alien with the slogan: "When they come, they'll eat the fat ones first."

From Fat! So? I know more about her than I do some of my best friends. I know she is 5ft 4in and weighs 20 stone; that such a weight is no barrier to a great sex life ("you can so get on top"); that she has fat parents; that she has always been fat, has never – unusually for fat people, I will learn – been a yo-yo dieter; that she was bullied as a child; that she loves broccoli and cottage cheese but hates cheesecake, milk chocolate and caramel; that she has no health insurance because of her size but suffers no major health problems; that she was educated at Stanford; is funny and smart and likes to wear hot pink; and has in her wardrobe a custom-made crop-top bikini with hot pink piping and a thong bottom – yes, thong – for all those poolside occasions that leave most of us cowering under a sarong.

If I understand one important thing, it is that I am never to use the words "overweight", "obese" or "oversize" in her company. These words are labels given by the medical profession as a "diagnosis". She has reclaimed the word "fat" for herself: "Do you want to feel good about yourself? Silence your tormenters? Look better in miniskirts? Use the F-Word," she writes in Fat! So?

She meets me off the plane dressed, as predicted, in hot pink. It is an odd feeling being with somebody who is proudly fat – it is so against cultural conditioning to ignore it – let alone calling them by the word. On the first few occasions I stammer it out, my face flushing with embarrassment. This feeling intensifies when we meet again later in a tiny Thai restaurant. Marilyn brings along her boyfriend of two years, Orian, who is quite possibly the tallest and fattest man I have ever seen. They seem very at home in such a miniature setting, and very at home with each other (First Rule of Fat Club: fat people are not all living in shame.)

I had been worrying about the meal with Marilyn because, naturally, everybody wants to know how much a fat person eats and she knows I'll be watching. Orian does keep a beady eye on the dishes but Marilyn seems only casually interested in the food. At one point she eulogises over a mussel, but that's as far as it goes.

I ask how much she eats. She sighs: "People never ask thin folks to justify what they eat" – but is graceful enough to comply. "I eat twice a day and have a few snacks. I maybe have takeout a couple of times a month, and if I'm having a bad day I might have a bag of cheese puffs, but not by any means every day. I'm not really into sweets. I like a bit of cheese but I'm not going to sit around eating pounds of the stuff. There are great restaurants here and I love going to them, but… it's a myth that fat people overeat. I eat vegetables every day. For me, food is not the problem in my life. My fat ass is not the problem in my life."

"Why do you think you are fat?" I ask her. "Do you have to defend why you are thin?" she snaps back. But it is a necessary question, I respond, because people – particularly doctors – think you over-consume, and that is the basis of how the medical profession treats weight control: calories in, calories out.

Marilyn sighs again. "Calories in, calories out is only part of the explanation of why a person weighs what they weigh. If you look at the data on identical twins, the genetic component is very large – as it is for me. My mother is fat, my grandmother was fat, we ate totally normally as a family. There's also how much disease you've had in your life, what drugs you've taken, mental health stuff. I get people shouting at me on the radio, 'You can't beak the laws of physics!' and I say to them, 'I bet you know thin people who are completely sedentary and eat vast quantities of food and never exercised. Well, I say, that person by your equation should be fat!'"

Marilyn is not at her strongest on science – her responses do not go beyond the anecdotal. When I challenge her, for example, on why the medical profession is so worried about the fat problem if fat people can be so healthy, she retaliates in activist speak: "By denying me healthcare, they are effectively saying to me that they want all fat people dead!" When I ask why doesn't she just tell the doctors what she has just told me – that her diet is not excessive – and she says: "Oh they'd say I was lying! To them or to myself."

Are you, I ask? "Why would I?"

She looks me in the eye, but I detect she is slightly uneasy.

Anyway, Rule Number Two of Fat Club is clear and an accepted fact by all: fat people are not always fat because they eat a lot. Consider also genetics, depression, isolation, poverty, a messed-up metabolism through a history of yo-yo dieting, combined – to a lesser or greater extent – with a degree of personal choice.

The brother of novelist Lionel Shriver died in December weighing 24 stone. After a terrible accident, he became immobile and depressed. He began to overeat, she tells me, as a way of "solace and self-torture". He developed diabetes and his spine compressed under the weight so that he lost 4ins in height. He needed an oxygen tank. Eventually, he had a sudden respiratory crisis and died of a cardiac arrest in a New York hospital.

"It was utterly heartbreaking," Shriver tells me. "One of the most painful things for me was to watch how society treated him. But this weight diversity movement is flying in the face of medical fact. I am not concerned about it because the absurdity of it is self-evident. If people want to say 'I like my size!' – fine, each to their own, but morbid obesity leads to unhappiness – it is not what my brother wanted to be and in terms of health costs, it puts a burden on everyone."

I put that case to Marilyn: "He didn't want to 'love his body'" I tell her. "He hated his body."

"First of all, I'm sorry to hear about this," she says. "There are a lot of things that can go on in a person's life that are pathological in themselves. I would not lay the blame here on weight but on the things that were happening to him."

OK, I say, would you support a fat person coming to you who said they didn't want to be fat? "I don't know why I have to support anybody in anything," she says. "I don't want to present myself as having to tell people how to live in order to guarantee them any kind of health outcome."

She then says: "Being a fat person means that you are not fully a person. You are a second-class citizen at best, untouchable, subhuman. You are less likely to get jobs, dates, cool clothes, respect, to have your own sense of importance and then you get all these doctors saying to you, 'You are going to get sick!' I say give up the false hope where weight defines a person and instead adopt a real sense of hope.

"Using the word obese makes it sound like there's some kind of scientific difference between fat and thin people."

But people would say there is, I tell her. She replies: "Historically, science has served an unofficial function of justifying popular prejudice. I don't think the majority count of doctors are right. I am healthy, healthier than some thin people. I advocate good nutrition and I move my body – I've been doing circuits five times a week – but these things should only be health enhancing, not to produce weight loss."

Wann is now 43, and after her teens and 20s, which she spent being miserable about her size, she insists her life is better fat. I feel guilty for thinking that perhaps she uses the activism to justify her weight, to lend it a cause, and that if she were truly at peace with herself, a degree of weight would come off anyway. She is, after all, extremely fat. But then maybe I'm stuck in the thin mindset.

Wann sees herself as a libertarian. "It's our right to sit on a couch and eat a bucket of bacon all day. I don't think I have to prove I am healthy," she says.

"I would claim human rights – if you agree I am a human being, I have certain rights. I am not getting in your way by being fat, even if we accept that health and weight are immutable, they [my thin critics] are still wrong, they are infringing my rights."

Do fat people have a right to be fat for whatever reason? Should we even be asking the question?

Marsha Coupe, a fat activist friend of Wann in the UK, told me how she was beaten up on train in Kent for being fat: "A woman, who was drunk, started shouting, 'You fat bitch' and hitting me. I couldn't defend myself because she was on top of me. By the time she had finished with me, I looked as if I had been hit by a train. Obesity is paraded as the cause of all evil, and it's bear baiting."

Sondra Solovay is the principal anti-weight discrimination litigator in the US. She is considerably fatter even than Wann ("I wouldn't want to be 50lb heavier than I am," Wann admits, "only because it would make my life more physically complicated"). Solovay, who looks as if she is wearing special orthopaedic boots, will not talk about her weight history, except to say that she had fat parents and has been a vegetarian since she was eleven. She has a brilliant mind, degrees from America's finest law schools and has faced discrimination inside the courtroom and out. Her manner is gentle and considered. "Weight is a deep prejudice that goes beyond the green of the dollar," she says. "We have a long history of oppressing groups in the US. We never learn – we just keep applying those labels in different variations to the next group that comes along. All I want is for us to learn from our history and not to continue to oppress fat people to the point where we need affirmative action to bring them up to where they should be."

But can fat be treated the same as race? As Shriver told me: 'While discrimination against heavy people should be illegal, to equate fat with race, gender and sexual orientation is to cast obesity as an unassailable state over which we have no control. Quite apart from anything else, isn't it nice to be able to move around quickly?"

To appreciate the multi-faceted approach of the Weight Diversity movement, I go to Berkeley, California, to watch Phat Girls Fly, a dance group for fat women which specialises in burlesque (this is San Francisco, after all). The point of my watching is clear. Third Rule of Fat Club: Not all fat people are sedentary. The women possess a million times more body confidence than I do and all of them are a good deal heavier. Rule Number Four: thin people – particularly women and girls – live under the oppression of a fattist society, too. Witness the influence of the fashion and diet industries in making teenage girls anorexic and size 10 women still think they are fat. Would I want to be the same size as the Phat Girls? No. Would I like their body confidence? Yes please. And herein lies a wonderful illustration of how, as a slim woman, I am restrained by weight oppression.

The best chance we have casting aside this way of thinking lies in producing scientific data. Luckily for the Weight Diversity movement, Dr Linda Bacon, the lead investigator for a clinical research study that evaluates a Health At Every Size programme, has added her much-needed voice to the fight.

Linda Bacon is both thin and a serious player in nutrition science in the US. She has at her fingertips mountains of epidemiological evidence, which show that, subject to exceptions for the most extreme cases – no doubt Lionel Shriver's late brother being one – it remains not at all clear that being overweight is an independent health risk of any kind, let alone one that kills. While a sedentary lifestyle or a lousy diet – both contributing factors to weight gain – do pose health risks, there remained no evidence that being fat, in itself, is bad for you. In other words, while lifestyle is a good predictor of health, weight isn't. Bacon also knew of a review of research in 1996 conducted by the National Centre for Health Statistics and Cornell University that showed that in the majority of studies, groups of people labelled "overweight" – note, not the obese – were found to have equal or lower mortality rates than supposedly normal weight people. The message seemed to be clear. Fat people might be less healthy if they didn't move from the sofa, but if they were fat and active – with good blood sugar, cholesterol and blood pressure levels – they had absolutely nothing to worry about.

Health At Every Size is a movement that has existed since the late 80s as an alternative to the destructive patterning of dieting (it has a small branch in the UK, co-founded by NHS nutritionist Lucy Aphramor). It was only in 2002, however, that Bacon tested the theory scientifically: "I am a scientist," she tells me. "I have three graduate degrees all related to the field of weight science and it's amazing to me that in none of the fields are our stereotypes about weight at all based in science. There are people who are at unhealthy weights but addressing it as a weight issue doesn't make the problem go away."

Bacon embraced this herself in her 20s after years of borderline bulimia. Once she let go of the idea of weight loss and become interested only in her health – exercise and good food sourced at local farmers markets – she lost 30lb. By that stage, ironically, she did not care.

Helped by two researchers from the US Department of Agriculture, and by Judith S Stern, a respected diet researcher, and funded by the National Institutes of Health, she conducted a clinical research study on 78 women with BMIs above 30 but no more than 45. Half went on a conventional diet programme, with calorie control and weigh-ins, and half went on the HAES program that encouraged group exercise and acceptance. At the end of the six months, there was another six-month period where both groups met monthly. At the end of the year, the dieters had regained all their weight with no health benefits. The HAES women, however, had significantly lower cholesterol and blood pressure, had quadrupled the amount of energy they spent being active and had been freed from the tyranny of diets. But the HAES women did not lose weight either: "Our bodies are invested in maintaining fat for 'lean' times," says Bacon. "Of course, they might lose weight over the long term as the work they did in HAES becomes more ingrained in their lives. We just don't know. What's most important is that these women got healthier and felt better about themselves."

Lucy Aphramor agrees: "There is a vast amount of research to show that weight-controlled behaviour doesn't work, and also evidence to suggest it leads to a negative effect on health and more weight gain. If we are to reduce population weight gain, we have to prevent people from going on diets. We have to change social attitudes to 'fatness' and 'thinness'. It should be about health."

In other words, it could be that our war on fat is actually helping cause the very disease it is supposed to cure.

On a bright, cheery Sunday morning, Marilyn Wann is out on the streets of San Francisco with her Yey! scales, specially modified weighing scales that substitute numbers with readings of "You're sexy", "Fabulous", "Beautiful".

"Come on the Yey! Scales," she shouts from a cloud of hot pink. "Come get a free compliment!" Passers-by eye the scales as if they were a nest of vipers, but gradually, nervously, men and women approach and climb on board.

From this random, non-scientific sample, I learn this: they all dread getting on scales; one girl told me her mother said she was too fat; one guy had lost two-and-a-half stone through the cabbage diet – and put most of it back on. Another woman had finally abandoned diets for exercise and had never felt better – or looked slimmer. There was nobody for whom weight was not an issue. (Admittedly, those people might have kept on walking.)

Two hours later, waiting in the queue at the airport, I get chatting to a retired doctor from Glasgow. He shakes his head at the concept of the Weight Diversity movement: "Calories in, calories out," he says. I start to explain the theories I have just learned, but he says: "Nobody came out of Belsen fat!" I'm silenced.

A week later, a man I am sat next to at dinner almost explodes with fat rage. Other people join in. I hear the same thing again: "Look! There were no fat people in concentration camps!"

So is this what it's really come to, I think. In the debate to get fat people thin, we recall the starvation methods of Hitler's concentration camps?

While nobody is suggesting rounding fat people up, if people start discussing the issue through an extreme of no choice, might we not be better off in a world that advocates free choice, however extreme? And with views so polarised, do we ever stand a chance of becoming a nation in good health, eating good food?