On my way to meet Susie Orbach, it occurs to me that it would be very hard to find a single week when her work isn't relevant to the headlines. It's a depressing sort of tribute, but I certainly haven't managed to find such a week. On the journey to her Hampstead home, I read a report in the day's paper of an Australian beauty pageant featuring a contestant so thin that she would not look out of place in hospital. That same morning I had happened to drop into a chemist and found a queue of women lining up to buy a supply of Alli - the new "miracle" slimming pill currently thrilling the beauty press, despite a long list of side-effects that include faecal incontinence.

"Oh my God," Orbach exclaims as soon as I mention Alli. "So, I'm in Boots yesterday, and there was a queue of women at the pharmacist, each with a pack of it. Not one of the women in front of me conceivably had a BMI of 28 [the minimum body mass index the new drug is supposed to 'treat']. And every one of the women said to the pharmacist, 'How much does it cost?' '£50.' And these weren't women with money. But they were saying to each other, 'I think it's worth it.' And the pharmacist was saying, 'If you'd lose 2lb normally, then you'll lose three by taking Alli.' So I'm like a one-woman campaign; I couldn't restrain myself. I said to the pharmacist, 'If these women needed this drug, they would be getting it on prescription, and not be needing to pay £50 a month for it.' And I said to the women in the queue, 'I guess we all walk around feeling fat in the head, but couldn't you dare to feel OK about your body?'"

The psychotherapist gives an incredulous, defeated smile. "I didn't stand there saying, 'Actually, the question isn't losing weight, the question is will you learn how to eat, and feel comfortable with your body?' I didn't do any of the things I might as a propagandist, because I just didn't expect to find this great queue."

Orbach didn't expect to see most of the mess we've got into with our bodies when she wrote Fat is a Feminist Issue more than three decades ago. The seminal bestseller was a groundbreaking work, the thesis of which was so simple that no one who read it could dispute its logic. At 31, Orbach had been bingeing and dieting for much of her adult life when she wrote that diets make us fat by distorting our relationship with food, and urged us to restore trust in the natural rhythm of our self-regulating appetite. Yet 31 years later, Orbach acknowledges that we are more disconnected from our bodies than ever before.

Her latest book, Bodies, maps the progress of our alienation, from a time when we took our bodies for granted to one where they are an endlessly perfectible work in progress. "When I was growing up," she explains, "one or two girls were beautiful, but it was not an aspiration, right? We didn't expect to be that sportsman or that beauty queen. That was OK, that was what movie stars were for. That wasn't something that was essential for all of us." Yet today, movie-star looks are not just an aspiration but an imperative, and ordinary people think nothing of starving or surgically enhancing their bodies in a tireless campaign to make them look as though they belong to somebody else altogether.

Just as Donald Winnicott identified the "false self", whereby a neglected baby will blame itself for its carer's lack of interest, and create an artificial version of itself in the hope of winning love, so Orbach argues that we are creating false bodies. Assailed by media imagery that celebrates only one type of body and one type of beauty, we assume any discrepancy between our own appearance and this digitally airbrushed "ideal" must be our fault, and that it's not merely necessary but morally virtuous to do whatever it takes to correct our deficiency. The simultaneous rise of anorexia and obesity is not a paradox, but rather two sides of the same psychological coin - both manifestations of our panic about hunger, in which normal appetite becomes pathologised as the enemy. Crucially, whereas once we might have experienced the pressure to look different as an onerous tyranny, today we tell ourselves that it's empowering.

"We transform the sense of being criticised," Orbach writes, "by becoming the moving and enthusiastic actor in our own self-improvement programme. We will eagerly repair what is wrong ... We see ourselves as agents, not victims. It is the individual woman who feels herself to be at fault for not matching up to the current imagery ... She applies herself to the job of perfecting that image for herself and so makes it her own, not assaultive or alien."

Orbach's writing is closer in tone to cultural studies than to the jaunty self-help register of most contemporary books about eating, but in person there is nothing abstractly academic about her. Framed by a mass of curls, she is small, even birdlike, but her sprightly energy conveys a vivid sense of aliveness. Her accent has a faint American inflection, which can sound almost antipodean at times, particularly when her sentences end in a question mark - "right?" - and she is surprisingly relaxed, even imprecise, with her words, often letting sentences tail away unfinished. But she is very clear about where we are going wrong.

I've been half looking forward to the meeting, and half dreading it because, although it must be 20 years since I first read what Orbach calls "Fifi", her work feels uncomfortably relevant to my own current state. She is the sort of woman you find yourself confiding in, and I admit to her that halfway through my first pregnancy, my overriding preoccupation is with weight gain. To my dismay, what I'm really thinking about most of the time is how I'm ever going to lose it. And right there, according to Orbach, is the source of our troubled relationship with food. Mothers transmit their own anxieties to their babies; it all begins in the family.

"The only way to solve the problem is to provide very different help to new mums," she says briskly. "Because every mother wants to do right by their kid. It would mean training health visitors and midwives; you'd raise a mother's awareness of her own body. This is an opportunity for both of you to find the rhythm in terms of relation to appetite. I don't think it would be difficult to design. And it would be very cheap. And new mums would really benefit from it. But instead, they are being told to do sit-ups straightaway, and why not even consider having a C-section, so you don't have to get that last month's weight gain? All of that nonsense. It's completely counter to what a baby's mental health requires - and what the mother needs as well, actually."

What can parents say, I ask, to a 16-year-old girl who is convinced that a regime of dieting and beautification is not self-punishing but empowering? "Well, it's awfully late at 16. But I'd be saying to the mums: 'Watch your own behaviour - how often do you criticise your own body in front of your daughter?' Stop making the body the cause of the problem, or the solution to the problem. The problem isn't how she looks."

But surely a teenage girl would say that how she looks is precisely the problem? "But what she'd be picking out aren't imperfections, they're just what makes her her, right?" What if she says she's overweight? "Well, they all feel overweight. Even when they're tiny, tiny, tiny. But where are they getting that idea? That's why I think the mums are doing something."

If Orbach were just another voice in the cacophony of finger-pointing that surrounds most discussion about weight, I would be feeling unpleasantly guilty by now. But her analysis of what she calls "disordered eating" extends beyond mothers and the family, to encompass everything from the diet industry - "which relies upon a 95% recidivism rate" - to the media, which produces glossy magazines in which "not a single image is not digitally retouched, up to hundreds of times" - and globalisation, in which a culture of "aspirational bodies is the mark of entry".

It is a comprehensive critique, yet one that resists apportioning blame for overeating to individuals, which you would have thought would make her hugely popular, a sort of Claire Rayner of the kitchen. Yet strangely, at 62 she is not the national treasure you might have expected. Before this interview, I'd met Orbach several times, and always found her warm and intensely compassionate. Yet time and again, interviewers report that she is cold, uptight and unforthcoming.

Does she find the characterisation upsetting? "Yes," she says simply. "I do."

One reason for the public relations problem is probably her reluctance to volunteer much about her private life. Born in 1946 to Jewish parents - her father was a Labour MP, her mother an American teacher - she grew up in north London before moving to New York, where she was married briefly and became involved in the women's movement, before returning to London and training as a psychotherapist. Last year she split from her partner of nearly four decades, a fellow psychotherapist with whom she has a son, 24, and daughter, 20.

"It's just not appropriate, is it?" she says, when I ask why she has always chosen to say little about her personal life. "I think what's most interesting about me is the work that I do." In the 90s she found herself besieged by journalists when they discovered she was treating Princess Diana for her eating disorder, and Orbach's refusal to discuss their relationship probably inflamed the media's frustration at knowing so little about Britain's most famous shrink.

"The other thought," she offers, "is to do with the role of public intellectuals, and for some reason women don't come off so well in that. So perhaps there is something there about feeling intimidated, and thus narrowing the conversation that occurs, and focusing on personality rather than the ideas one might have."

I suspect it may also be because the combination of a psychotherapist and a feminist campaigner confuses people. Orbach is not just a therapist and writer, but an activist; she is a consultant to the Dove ad campaign, and runs a website, any-body.org, "giving women a voice to challenge the limited physical representation of females in contemporary society". In a media discourse that is endlessly blaming women for being too thin or too fat, for looking hatefully perfect or shamefully flawed, her contribution is too nuanced to quite fit. For example, when I ask if she feels angry with feminists who opt for cosmetic surgery, she says: "Well, that's where I think being a psychoanalyst puts me in a different category. I'm really not there as a moralist. I've always felt very sympathetic from the first days of writing about women that, whatever the woman, whether she is trying to be a woman in the conventional sense or breaking the boundaries, those struggles are quite difficult."

So she doesn't think it's helpful to criticise them? "Well, I don't think it's helpful for me. I think I'm of more use saying, 'Right, let's try and work out why we feel this way. What does it mean that women post-pregnancy are being offered reconstructive surgery, as if pregnancy were a disease? Let's think about that. That's where I come in. Rather than say, you shouldn't do this or you should do that. That's irrelevant."

As a campaigner, Orbach wants to see our visual landscape filled with images of people who look real. "And gorgeous!" she interrupts. "I mean, I'm not an anti-stylist, for Christ's sake. I think it is one of the capacities of human beings, to create style. It's the mono-imagery, the one look, that is so disturbing."

Some feminists take the view that it doesn't matter if models look like Twiggy or Beth Ditto; it's the act of objectification that is so destructive. But Orbach shakes her head. "I don't buy that because I think it's unrealistic. Maybe this is too pragmatic but we live in this world, we have got the democratisation of beauty, we have got the notion that we can, should, enter into the representation of ourselves in certain kinds of ways. We're going to do that. So the question is, can we do it and have joy about this, rather than only regarding ourselves critically."

She is very happy to criticise the diet industry, though, and once tried to sue WeightWatchers. "I really hoped we could do it, and there was some interest, but you can't do it without a massive amount of money. I think there's still an important discussion point, though. Why haven't they been prosecuted under the Trade Descriptions Act? Why haven't they?" Similarly, when I suggest that the government should take action against the media for irresponsible reporting of a drug such as Alli, just as it would on false advertising, she says at once, "I think that's an interesting argument, I really do, because the question is: where are the pressure points?

"What could journalists do? It's not enough to blame the media, it's what could they bring to the party. What could they do to change the visual culture? They could write about Alli not in a tongue-in-cheek, maybe-this-time-a-drug-will-work kind of way, but by saying the stats show that 95% of slimming preparations will have a serious failure rate, and you're likely to end up larger than you started. And then you deconstruct what this new drug is about - the marketing of it, the production of it, and all that. So that would be a nice way to do it."

How can it be that, after all these years, we're still queuing up to buy what we know will probably not work? "Because of hope," she says sadly. "It's about transforming that sense of feeling powerless into feeling powerful. It transforms the image of you as the victim into thinking, 'Oh, this is a real opportunity! I could do it this time!'".