In her first book, Fat Is a Feminist Issue, published 31 years ago, Susie Orbach told us why lots of women had a dysfunctional relationship with their bodies. Some women got fat, she said, not because they were greedy, but because being fat made them feel safe. For a woman, she said, being slim might get you the wrong kind of attention - not just from men, but from other women, too. So lots of people have an unconscious drive towards being fat, even if they think, on the surface, that they want to be slim. What an insight! That's what I thought, when I read it. And also: what a twisted world we live in!

As a psychotherapist, Orbach had asked women what their fat was actually doing for them. It was giving them a certain relief, they said. It "took them out of the category of woman and put them in the androgynous state of 'big girl'". It gave them something concrete to worry about, so they wouldn't have to think about all their other troubles. "Above all," Orbach wrote, "the fat woman wants to hide."

In this new book, Orbach tells us what has happened to our bodies in the intervening three decades. These days, we live in an even more twisted world. Or, as Orbach puts it, "the problems I sought to describe have mushroomed". Now that we no longer use our bodies to make things, she says, we make our bodies instead. Our bodies are the product. Of course, the human body has always been sculpted by what it does. And now, mostly, it does not till the fields, or mine the coal, or hump things around. In the modern age - what Orbach calls "late modernity" - it sits at a keyboard, tapping away, like I'm doing right now - or on a sofa, reading a newspaper, like you're doing right now.

And what do sedentary people in modern societies have in common? What are we doing, exactly, when we sit at our keyboards? Well, for one thing, we're not getting much exercise. Weirdly, though, the less exercise we get, the more we worship the toned, impossible body of our dreams. But the body we dream of is always out of reach and getting harder to achieve with every passing year - for women, this body is slender body, with a certain type of breasts - and, lately, a certain type of bottom, too. For men, it's the six-pack, among other things.

So what's driving all of this? Why, Orbach asks, "is bodily contentment so hard to find"? In her first book, she showed us how people's bodies can be shaped by forces beyond their control. Here, she turns her attention to the controlling forces - "the merchants of body hatred". Her point is that capitalism works much better if we hate our bodies. If we're anxious and needy, we are better consumers; if we're anxious and needy when it comes to something as fundamental as our bodies, we are putty in the hands of marketeers and diet-merchants. And if we ever start to get comfortable with what we've got, along comes another body - another piece of unattainable perfection - to keep us anxious.

Who are these merchants of hatred? Where are all these images of perfection? Actually, they're everywhere. They are, I began to think as I read this, the people sitting at the keyboards, writing the ads and marketing the holidays, airbrushing the pictures and arranging the loans. They are us, me and you, communicating, right now, via a medium partly funded by advertising. And look at the ads! Look at the bodies in the ads! And think of the meaning of those bodies, and how they got there, and what those bodies will do to us. That's one of the things I was thinking, as I was reading this book. I was thinking: it's us - it's all of us! We are the merchants of body hatred.

Orbach takes us on a world tour of body anxiety - the rock videos, the magazine covers, the ads, the people who tweak the portraits of children to make them look perfect, the high heels made for babies to wear, the mothers who diet during pregnancy, the elective caesareans, the pressure to lose "baby weight" in new mothers, the spread of cosmetic surgery, the spread of cosmetic surgery shows... and it's all repeating the same mantra about the need, particularly for women, to be slim and sexy, but with the right breasts and now the right bottom. "Visual muzak" she calls it.

She's right, just like she was right 31 years ago. Our bodies are being shaped by forces beyond our control and these forces are malign. She's right about cosmetic surgery - it's becoming more and more normalised and this just raises the bar, so that what looks beautiful today will look less beautiful tomorrow. She's right about the cult of celebrities."By creating internationally recognisable iconic figures, it appears to be inclusive and democratic," she says. But it's not. It "sucks out variety". It makes us all want to look like the same few people.

And she's right about the diet industry. Did you know that, in America, the diet industry is worth $100bn a year? And the annual education budget is $127bn? And diets mostly don't work. That's because, when you go on a diet, your body gets better at sucking calories out of the food that you do eat. So when you start to eat normally again, you balloon. And that's the whole point. That's where the $100bn comes from. Diets create a need for more diets. Or, as Orbach puts it: "Diet companies rely on a 95 per cent recidivism rate, a figure that should be etched into every dieter's consciousness."

But there was another voice in my head as I was reading this book about bodies. I was thinking six-packs, of clear, wrinkle-free skin, of better teeth and hair... and I secretly wanted these things. I was thinking of Brad Pitt. I was thinking of Brad Pitt's stomach. I was wondering how I could get Brad Pitt's stomach. What would life be like then? I kept thinking: I know, Susie, body tyranny is terrible. It's morally wrong and spiritually destructive. It was bad in 1978 and it's been getting worse ever since. But just imagine, all the same, a world of perfect toned slimness, in which...

I had to keep saying: wake up! She's right! And this is a terrific, timely book. Body tyranny has been hurting us for decades. At bottom, I think, it's about making us want things. Reading this book made me think: our system makes us want things until we're so damaged that we can't go on, and it's showing on our skinny, obese, scarred, tattooed, pierced and hated bodies. And now it looks like the system is breaking down. Which might be good news for bodies.

• William Leith's most recent book is Bits of Me Are Falling Apart: Dark Thoughts from the Middle Years (Bloomsbury).

Susie Orbach
A life

Born
6 November 1946, London.

Educated
University of New York, Stony Brook and UCL.

Personal life
Two children with partner and fellow psychoanalyst Joseph Schwartz, from whom she recently split. Career
1976 Founded the Women's Therapy Centre in London.
1978 Published Fat Is a Feminist Issue
1981 Founded the Women's Therapy Centre Institute in New York.
1982 Fat Is a Feminist Issue II; and Understanding Women: A Feminist Psychoanalytic Account with Luise Eichenbaum.
1986 Bittersweet: Love, Competition and Envy in Women's Relationships with Eichenbaum.
1993 What's Really Going on Here
1999 The Impossibility of Sex
2001 Susie Orbach on Eating

Famously treated Diana, Princess of Wales, for bulimia. Currently a visiting professor at the LSE.

She says:
"Fat is a way of saying 'no' to powerlessness and denial."

They say:
"Virtually all feminist debate on body image and beauty imagery owes its existence to Susie Orbach's enduring formulation" - Naomi Wolf Will Daunt