A woman walks to her local corner shop. She walks slowly, like a duck or a dodo, her legs apart. Her eyes are lowered, as if paying avid attention to the pavement beneath, and when she passes another human being, she is careful not to raise them. In a pale green shirt the size of a small curtain, and a long black skirt that is almost coffin-like in its rectangular bulkiness, she is not only clammy and uncomfortable; she is in fear of ridicule. Every time a white van passes, England flags flapping in the summer breeze, her heart hammers in her chest. She is, you see, a size 30. White-van man tends not to favour girls who are a size 30. If she is lucky, he will merely shout: 'Oi!' and laugh should she bother to turn her head. If she is unlucky, he will have a mate with him, and together they will make farmyard noises: an 'oink!' or, far worse, a 'moo!' Poor thing. She cannot wait to get home. If she could run, she would. But she is clean out of bread and milk. What else is she supposed to do?

In the shop, things are no easier. It is a small shop, and overcrowded. Her body fills the aisle, and making a sharp turn is impossible, unless she wants to bring a shelf full of baked beans clattering to the ground. Then everyone will stare. A few people may even click their tongues disapprovingly against the roofs of their mouths. Not that people aren't looking now. As she approaches the counter, she is painfully aware of their eyes following her hand, waiting to see if, at the last minute, she adds a Bounty or a Mars or an Aero to her basket. When she fails to do this, she can almost taste their disappointment. No matter. She has the door within her sights now, and she cannot wait to squeeze herself through it and out on to the street beyond, where she will fan her face with the evening newspaper before making her way home once more.

Front door safely closed behind her, she considers her day. She had planned on going shopping but, now, she cannot face it: the clothes that will not fit; the faux-kindly smiles of the shop assistants as she struggles with their tiny wares. Nor does she relish the thought of a bus, where her buttocks will spill over the edge of the seat like meringue over a pie crust. Exercise is out of the question. She is a good swimmer, but swimming involves the revelation - to other pool users - of her body, and all those Shallow Hal-style jokes about how much water she will displace. Then the telephone rings. It is her close friend, the editor of a fashionable magazine. Let us call him Michael. They talk for a while, and Michael suggests that they go out for dinner. It so happens that he has a table booked at J Sheekey. She would love to go to J Sheekey. But the thought of manoeuvring herself into a banquette while a dozen social X-rays gaze meanly on is just too unbearable. Michael will be embarrassed. Tonight, then, she will stay home, and eat lamb chops in front of yet another makeover show. Tomorrow will be a better day.

As it happens, tomorrow is a better day. Our heroine wakes up, and she is no longer fat. This is because this woman is me and, underneath the fat suit that I hired from the BBC, I am still my usual size 10/12. When I first had the idea of wearing a fat suit as part of my research for this piece, I worried that it was something of a journalistic cliché. In fact, it was highly instructive. Wearing it was so much worse than I had thought it would be - especially the enormous breasts, which comprised a giant bra filled with lentils, and made my back incredibly sore. I only managed to force myself to go out in it once - on the day described, with some artistic licence, above - and my idea of visiting a smart west-London restaurant with a friend who is perfectly open about his body fascism soon started to seem like a very bad one. Even though I knew my flab was only fake, no one else did; it was humiliating to be, at best, invisible, and, at worst, the object of disgust. If wearing a lot of cotton wool is this bad, what must it be like to bear the equivalent in flesh?

Truly terrible, is the answer. You may think that you know this already, but in order to come even close to grasping a fat person's misery, you should probably take that imagined desperation, and triple it. Then consider your attitude to this unhappy person. Do you pity them? Or do you despise them? If the latter, do you feel able to say so out loud, in public? I bet that you do. These are critical times in the great obesity debate. In the West, all we talk about is our increasing weight, and what we can do to keep it at bay. Emotive words like 'epidemic' and 'time-bomb' are thrown about like so many hand grenades. Open a newspaper, and a story will certainly be there (most recently, it was reported from the British Dietetic Association conference that the risk of fatal disease increases by one per cent for every pound a person is overweight). The seemingly well-established connection between fat and disease has meant - so far - that it has been somehow acceptable to criticise the fat; it's for their own good, after all. But now there are rumblings. The fat and their supporters have had enough of what they regard as discrimination. They are angry, and they are going to fight. They regard their cause as just. They believe that fat is the new race.

The first person I heard make a direct comparison between fat and race was Malcolm Gladwell, author of the best-selling The Tipping Point and Blink. At an event in London, a member of the audience asked him what subjects he thought were hot. Gladwell, off the top of his head, wondered aloud whether fat wasn't the new race. The comment stuck in my mind - it sounded incendiary, and possibly mad - but I didn't act on it until later, after I saw a programme on More4 presented by the journalist Giles Coren. This was a piece of polemic entitled Tax the Fat: Coren argued that, because treatment of obesity-related illnesses now costs the health service so much money - some £1bn - there was a case for the fat, just like smokers, to be taxed. His argument wasn't especially subtle, but it was - at the time - funny and energetic. It was only afterwards that it occurred to me that it was also unkind. So, I started digging. Gladwell is certainly right that this subject is hot - it's hot as Hades. It's also complex and fraught; you meet lots of fine minds on the way, but you also encounter those whose position is so extreme, the experience is a bit like trying to talk to a creationist about Darwin.

On the telephone from his home in Boulder, Colorado, Paul Campos, a professor of law at the University of Colorado, is telling me about his own weight. A couple of years ago, Campos's weight fell from 215lb (combined with his 5ft 8in height, this made him, at least in the eyes of the government, 'obese') to 160lb (an 'ideal' body mass). This is because he took up running again. Did he feel happy about it? Of course he did. But he is unwilling to accept this was because he felt, or looked, better. It had far more to do with the praise he got from colleagues. 'I was happy,' he says. 'But I was happy precisely because of the invidious social messages that are out there: that fat is bad, and thin good. I'm a 46-year-old professional white male. Being fat for someone like me was a trivial matter.' You may not think that being fat is a trivial matter; he disagrees. This is because Campos does not accept the link between fat and disease. This link is, he argues, largely unproven. Fat has been demonised when, in fact, there is nothing at all wrong with it per se.

Two years ago, Campos published a book, The Obesity Myth, in which he argued that the public has been misinformed about obesity; when it comes to fat statistics, doctors and the media cherry-pick their data. He also argued that fat people are discriminated against in daily life by a skinny elite, which projects its more general anxieties about the world's over-consumption on to an easy target: a poor and heavy underclass. More bizarrely, he made the case that the impeachment of Bill Clinton was the result of fattism (Clinton is a notorious guzzler of pizza, and Monica Lewinsky was a yo-yo dieter, whose friendship with Linda Tripp was largely based on their shared need to lose weight - so the skinny political elite despised them). His book, which did not appear here, was much talked about - coming as it did on the heels of Greg Critser's Fat Land, which had urged people to take responsibility for their greed. Campos, who was thin by the time of publication, became the poster boy of the backlash against the long-running war on obesity.

Campos believes that there are indeed striking parallels between what he calls 'identity movements'- those who campaigned for equal rights for black and gay people - and those who are working for fat or body-shape acceptance. 'There is an increasing level of consciousness in the media and the scientific community about the extent to which weight should be the focus of public health. There is more scepticism about the claims made for the connection between weight and disease, and about the idea that fat is a chosen state. People do not choose to be fat in any meaningful way; most of those who try to change their body mass fail.'

Campos does not believe that it would be right to discriminate against fat people even if their state were chosen; but the fact that, in his view, it isn't, means that, ultimately, they should be protected under the law in the same way that, for instance, the disabled are. Is this necessary? In Campos's view, it is. Fat people are less likely to be promoted. They earn less than the thin. Worst of all, they are unable to get medical insurance, irrespective of their general health.

But people are getting fatter, aren't they? My eyes tell me this. 'We are somewhat heavier, 'he says, carefully. 'We've added 10lb in the last 25 years. You should be cautious of impressionistic responses. You are sensitised to seeing fat people because you've been saturated with a high level of propaganda.' OK, but how can someone who is fat also be healthy? If they can't move, they can't exercise. 'There are some levels of immobility that might result in a relationship between weight and health. But an appropriate response to that is not to focus on weight loss, but on fitness.' He points out that a larger active person is more likely to be healthy than a thin but sedentary person. What about the increase in type-2 diabetes? 'Diabetes is about high-sugar diets. A high-sugar diet may also contribute to weight gain, but weight is not an issue so far as the diabetes goes.' Finally, Campos reminds me that the International Obesity Task Force, source of many of the stories we read in the newspapers, is funded by drug companies, who stand to make a lot of money if they develop effective weight loss pharmaceuticals.

'There is no question that this is a civil-rights struggle,' says Campos. 'It's about basic human dignity.' He believes that the disgust I feel when I see someone fat eating a hamburger is a socially acceptable way for me - a new puritan - to express my anxiety about overconsumption. He thinks this should not be socially acceptable; but he also thinks it's neurotic because it's so trivial. I am a hysteric. 'Why focus on the man with the cheeseburger?' he asks me. 'The millionaire with a private jet is committing the greater crime.'

So how long will it take for attitudes like mine to become socially unacceptable? How long before more anti-discrimination laws are added to the statute books? Campos draws a comparison with the phrenologists of the 19th century, who regarded bumps on the head as evidence of personality traits. 'Weight has been pathologised in the same way. Phrenology was a respectable movement even though it was based on a phoney notion. It, too, had an ideological bent. The bumps on the heads of those of European extraction were supposed to prove that they had superior skills.' He thinks that the obesity industry could disappear as fast as phrenology, in the right circumstances. What would the right circumstances be? 'There will be a tipping point,' he says. 'A famous person will have weight-loss surgery and die. Or ... girls of 13 are now having weight-loss surgery. One of them will die. What happens then?'

Although I fail to see how Clinton's pillowy stomach had anything much to do with what happened to him as president, a lot of what Campos says seems reasonable. Others are more strident. Probably the most famous fat-rights activist in America is Marilyn Wann. As a result solely of her weight, Wann was turned down for health insurance and, ever since, she has dedicated herself to the cause of fat rights. She runs a magazine called Fat!So?; she is a director of the National Association to Advance Fat Acceptance (NAAFA); and she runs the Bod Squad Cheerleaders, a fat activist group. When it comes to comparisons between fat and race, Wann is extremely specific. I start off by saying that I am wary of such a comparison because weight can be chosen; race can't. 'Mutability is not a justification for prejudice,' she says, fiercely. 'People choose their religion.' But, in any case, she thinks the jury is still out on choice and weight. The geneticists could yet - forgive the pun - weigh in. 'It may be that it is like height in terms of inheritance. And we're not freaked out by height, or big feet, are we?

'Different groups have different stories. But if we accept that fat people are thought of as stupid, smelly, ugly, out of control, lazy, you can see the comparison [these are some of the stereotypes that were also attached to black people in the 1950s]. Fat people are discriminated against in employment, housing, in access to theatres and restaurants, and they are subject to cruelty - the crushing burden of fat hatred.' Like those who reclaimed the word 'black' or 'queer', Wann likes to use the word fat. 'Obesity' is offensive and meaningless; so is 'overweight'. Doctors who fear for the health of a generation are 'scare-mongering'; their research is 'junk science'.

As for people like me, who see only clogged arteries when they see a fat person, we're beyond the pale. 'You can't hate people for their own good,' she says. In an email, she also compares the struggle to that of gay men pre-Stonewall: 'We don't have places of pride to gather together on a daily basis ... there's no fat-pride bar. When fat people experience oppression, we experience it alone and our first reaction is not to fight back, our reaction is to give money to fat-hate industries (Weight Watchers, stomach amputation, etc). Instead of demanding self-respect, we seek approval from our oppressors.'

Unsurprisingly, Wann is in favour of legislation that will make it illegal to discriminate against someone on grounds of shape. This is the only way society's 'aesthetic' will change, and only once that has happened will fat people claim 'full humanity'. The idea is not outlandish: in some states such legislation already exists. In Michigan, for instance, people are protected from discrimination on grounds of height and weight, as well as colour, gender and age. In 2000, a San Francisco gym put up posters featuring an alien, that said: 'When they come, they'll eat the fat ones first.' Wann organised a group of fat people who did aerobics outside the gym, and waved 'eat me!' signs; San Francisco has since brought in legislation making it illegal to discriminate on grounds of weight. Other fat people have used existing disabilities legislation in the courts, on the grounds that fat people are perceived to be disabled.

Wann's body mass index is 46. According to the experts, this means she should drop dead pretty soon. But she points out that her mother, who is built the same way, is 82. Wann is well, and she is happy. She doesn't care about people accepting her fat; she is only interested in rights. 'I would never want not to be fat,' she says. 'I just want the things that are denied to me.' How far away is change?

'I don't care how long it takes. But I think this is an exciting, historic moment. Society is moving in the direction of easing alienation - and it's such a profound alienation for a person not to be at home in their own body.' Her only worry is the reluctance of people - fat and thin - to stand up and be counted. In this, too, her campaign has something in common with the early days of the black civil-rights movement. 'Some people feel that it's too dangerous. In the middle of a witch hunt, it's dangerous to ask: what's wrong with being a witch?'

So far as the statistics go, Campos and Wann may be right: perhaps the obesity crisis has been exaggerated. But that's not to say that, in the West, we don't have a serious problem with weight. I think that the balance of the science suggests that this is so. What about their other ideas? That fat people don't choose to be big? That fat people can be fit, and happy? That those of us who are shocked by a person's weight are prejudiced rather than concerned? This is where it gets more difficult. 'Troops are being organised on both sides of the fence,' says Greg Critser, whose book about how the US became the fattest nation on earth caused outrage in fat-rights circles (they threatened to sit on him). 'You can slice up the epidemiology lots of ways. Obesity may be the third or the seventh leading cause of death in the US. It's still a leading cause.' He hates the way the fat-rights lobby tries to turn moral judgments into fascism. 'Of course only an asshole makes jokes about a fat person. If you see someone who is 30lb overweight, you might think: I don't want to look like that. But if you see someone who's 150lb too big, another kind of discrimination kicks in. It is troubling, and silence doesn't help.' He is not convinced that fat people - of which he used to be one - are happy. An anti-discrimination law will protect them from abuse, but it won't help them lose weight, and they need, and long, to lose weight.

It's hard to shift a sneaky feeling that some fat-rights campaigners are merely dealing with their misery in the only way available to them - indulging in a kind of transference by making their weight our problem rather than theirs. They, of course, would say that this feeling is simply evidence of my own prejudice. In Britain, where the argument is less advanced than it is in the US, you can find people who also subscribe to this view - that prejudice is so internalised, debate is basically pointless - but they are rare. One example might be Charlotte Cooper. The leader of an alternative girl gang called the Chubsters, Cooper thinks that the 'rhetoric of the global obesity epidemic stinks'. She believes that fat is 'not necessarily unhealthy - and even if it is, it doesn't matter'. She worries that scientists will try to eliminate fatness through genetic engineering. I wanted to meet Cooper, author of the book Fat and Proud, but she does not talk to 'mainstream journalists' on account of their extreme bigotry.

One who has had experience of Cooper is Shelley Bovey. 'She tore strips off me,' she says. In 1989, Bovey wrote Being Fat is Not a Sin, which called for an end to discrimination against fat people, and she followed it with another book in similar vein, The Forbidden Body. Then, in 2002, she published What Have You Got To Lose?, a book for large women who want to be thinner; by this time, she had lost six-and-a-half stone. At this point, the fat-rights world turned on her. 'I didn't change my position,' she says. 'Discrimination is always wrong. But I did lose weight, and terrible things were written. I got hate mail. I lost all of my friends in the size-acceptance movement. They wouldn't even return my calls. People were very angry.'

As she points out, this attitude makes a mockery of the term 'size acceptance'; if the aim is for all people to be treated equally, whatever their shape, why was there less room for a 12½-stone Shelley Bovey than a 19-stone Shelley Bovey?

Bovey is still a supporter of fat rights: she is in favour of legislation, and thinks the medical establishment is hugely discriminatory against fat people. But she is reluctant to go all the way down the fat-is-the-new-race road because she believes that, in an evolutionary sense, human beings are not meant to be big. 'When you see someone who is very fat, your response is primal. I feel it myself.' She says my hunch about transference is a fair one. 'I can't believe anyone can be happy being very large. They can have happy things in their life, but they can't be happy. I wasn't. I was miserable. It was awful.' At her most fat, she felt short of breath; a steep hill was not an enticing prospect. 'I was in denial. I was not fit and healthy, and I am very afraid of going back.'

There are others who agree. Last year, the journalist William Leith published his memoir, The Hungry Years, in which he confessed to a food addiction, railed against his own hateful white body and then, finally, revealed how he lost weight. In some ways, Leith's book is the final word on this subject. It isn't scientific, but it does, unlike almost every other piece of Fat Lit I have ever read, express the joy of hunger. I mean earned hunger, the kind where you could eat a ranch because you have, say, been walking all day in the fresh air. Feeling that hunger, and satisfying it, is one of the best things in the world - and it's something that, for one reason or another, we have lost. I speak to Leith as I am writing this piece, and he tells me that he, too, thinks some of the fat-rights people are in the business of compensating themselves for their lot. 'Large and in charge,' he says. 'You don't have "Size 10 and in charge", do you? That gives the game away immediately.' Like Bovey, he equates being fat with being unhappy.

So is fat the new race? I don't believe that it is, though it could become so in the future. But that's not to say that thinking about it in these terms isn't a useful corrective. If we're allowed to want fat people to lose weight, then they're allowed to want thin people to be kind - or, better still, blind. Best not to forget, then, where we started - with a woman walking down a street, feeling as though she might as well be stark naked.

'I know ... that when a thin person looks at a fat person, the thin person considers the fat person less virtuous than he,' writes Judith Moore in her memoir, Fat Girl. 'The fat person lacks willpower, pride, this wretched attitude, "self-esteem", and does not care about friends and family because if he or she did care about friends and family, he or she would not wander the earth looking like a repulsive sow, rhinoceros, hippo, elephant, general wide-mawed flesh-flopping flabby monster.'

Imagine feeling like that. Think before you click your tongue against the roof of your mouth.