Susie Orbach’s first book, Fat Is a Feminist Issue, a pioneering exploration of women’s relationship with eating and body image, became an instant classic when it was published in 1978. Orbach is one of the world’s best-known psychotherapists, lecturing internationally, advising organisations ranging from the NHS to the World Bank, and helping patients who have included Diana, Princess of Wales. The daughter of an American teacher and a Labour MP, Orbach grew up in London, where she still lives and works.

What do you remember about writing Fat Is a Feminist Issue?
It came out of youth – I was 29 and I wouldn’t know how to write that way any more. I would have written it as a pamphlet but Joe, my then husband, said don’t be ridiculous, you need to write it as a book. It took just six months. I remember Penguin turned it down at the time so it went to Paddington Press and they tried to change the title on me.

Change the title to what?
Fat is a Feminine Issue. They didn’t want the word feminist.

Could you have envisioned it being so relevant 45 years on?
Did I envision the horror that is now not just girls’ but boys’ relationship to creating a body? No. I get emails from readers every week, and I feel privileged and moved that I’ve managed to enter their lives and help them, but it’s helping with a problem that’s worse than ever. We’ve got a very, very troubled population around bodies, and it gets expressed in everything from anorexia and obesity through to people whose bodies seem OK but who are desperately uncomfortable and critical of them.

How does that make you feel?
Abject, furious, quite murderous, actually, towards the corporate profits that are made on the back of making people feel shit about their bodies… It’s a heartbreak, I guess that’s the word.

What practical steps could be taken to bring about change?
I’d like to see the diet industry prosecuted for false advertising – they have a 97% recidivism rate. And I’d like all non-food foods – stuff like aspartame that originates in a lab – out immediately.

Why is there still such pressure for physical uniformity even as society embraces diversity?
The problem of uniformity has got completely out of control. Labiaplasty is at epidemic levels – as though there’s a right way to have labia!

You’ve written about cosmetics and the impact of Photoshopping but we’re now in an age of hyperrealistic AI face filters.
I guess what’s interesting about these fictive bodies is that when I see 20- or 30-year-olds they don’t look like that, so they’re managing two bodies: the body for broadcast and then the body in trainers and jogging pants without all the contouring or whatever they call it. It’s deception as a performance, we can all be a star.

Is the mainstreaming of porn also having an impact on our relationships with our own bodies?
I think of it as this false erotic – it has something to do with sex but I don’t think it has anything to do with eroticism. It’s disturbing an internal relationship to one’s body, making people ask how am I supposed to look when I’m in bed or up against a wall? How am I supposed to “do” sex?

Anorexia is a deadly mental illness. Have we become any better at treating it?
No, not at all. I still think there is the denial of human rights when you’re being treated – there’s the deprivation of telephones or company unless you eat. It just seems to me it’s always been a perfect case for a civil rights lawyer.

What advice would you give to anyone raising a daughter?
If we’re talking about bodies, I’d say help her to enjoy a body that moves, and I’d talk about bodies as adventure. When she says her tummy’s too big, just say that what’s exciting is that bodies are always changing – we can’t know how your body will develop in adolescence. I would also say don’t foist ambition on your daughters without telling them it’s a bloody struggle.

We’re much more mindful of mental health these days. How do you feel about trigger warnings?
You can’t do therapy with trigger warnings because therapy is challenging. I’m not even sure how I feel about trigger warnings for books or movies, because for me they were startling things that allowed me to experience something outside of myself, as well as something inside myself that I didn’t want to necessarily know. And yet we have to know things. I think the safety that people are attempting to find through trigger warnings is an attempt to manage a world that’s falling apart, not just politically through the drift towards authoritarianism but ecologically.

You helped stop Facebook from using the fat emoticon as a feeling. Do you still see yourself as a campaigner?
I do and I don’t, because I think who the hell’s going to listen to an old lady, really? But also, I don’t know how to campaign when people live online. It seems to me that ever since having 2 million people on the streets of London protesting against the invasion of Iraq, and that being totally irrelevant, it’s very hard.

What gives you hope?
In my working daily life as a psychoanalyst I get to see people in tremendous amounts of agony and anguish but also in pursuit of being able to develop, and that’s pretty wonderful to be part of.

I hear you felt a little awkward posing for our photographer.
It’s not my job to pose, but it’s the stillness of a photograph that’s a problem for me. Faces are so lively, that’s why I find the distortions of cosmetic procedures so upsetting, because people’s faces don’t move the way they’re meant to.

Do you pay especially close attention to faces and bodies in your therapeutic practice?
I don’t stare at people but I’m terribly aware of their impact on me. Just as someone might have a really deep understanding of music or develop a knowledge of wine, I think I understand the impact of bodies on my body. The thing that was staggering for me after having been out of the consulting room at the start of the pandemic was that people had a smell. Their physicality is in their pores, and I could sense it in a whole new way.

Talk to me about ageing.
Ageing is difficult at the physical level because one is, after all, living in a visual culture. Also, life is still very interesting to me, I’m still eager for it, but I realise it’s just not possible to go to the theatre every night, for instance. There are things I like about ageing – things that are about complexity. When I was younger, I would never have found Thought for the Day interesting because I wouldn’t have been interested in the philosophical thinking.

You’ve nicknamed Fat Is a Feminist Issue “Fifi”, which makes it sound like a small fluffy dog.
It turns it upside down, doesn’t it? I think the book is both a polemic and a solution book, but it’s playful as well.

Do you think it’ll still be in print in another 45 years?
Will we still have print?

• Fat Is a Feminist Issue by Susie Orbach is published by Penguin (£10.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply