Fat is more than a feminist issue – as this extraordinary memoir by novelist and essayist Roxane Gay reveals. Gay’s last book, Bad Feminist, became a New York Times bestseller and revealed her to be a writer unfazed by inconvenient truths and a champion of women – especially gay and black women. Hunger tells a story that must have been as hard to write as it is disturbing to read. She does not duck from telling us, early on, that at 6ft 3in tall, she weighed, at her heaviest, 577 pounds: “That is a staggering number, one I hardly believe, but at one point, that was the truth of my body.” She does – and does not – know, she says, how things got so out of hand. To some extent, she is on the side of Susie Orbach [author of Fat Is a Feminist Issue]. She remarks with devastating simplicity: “This is what most girls are taught – that we should be slender and small. We should not take up space.” But her book is a bid to take up space in another sense, to tell a story that wants to shrink into invisibility yet needs to be told. A personal story, with implications for us all.

Terrible to think of a 12-year-old child willing herself to go on as though nothing had happened

“Something terrible happened,” she writes. “That something terrible broke me.” Aged 12, she was gang-raped by “a boy I thought I loved, and a group of his friends”. They were in an abandoned hunting cabin in the woods in Omaha, Nebraska, where no one but the boys could hear her screams. She drags her account on to the page – faltering, incomplete, unsensational. “They were boys who were not yet men but knew, already, how to do the damage of men.” One reads about the unthinkable abuse she suffered – the boy holding her wrists and spitting in her face after raping her is a particularly upsetting detail – and feels as shaken as if one were directly witnessing what she describes. Yet this is no attention-seeking misery memoir. The book is an attempt to see fat in its complexity, its contrariness – as potentially more than a physical problem to be overcome. And although Gay regrets she is unable to go as far as the campaigners who rejoice in their size, she does want us to rethink what fatness can mean.

For Gay, overeating was, for a while, her solution. She makes it persuasively plain that fatness began as a response to rape. The fatter her body became, the safer she felt. Fatness was home in a game of chase: “a place where no one can get you”. Throughout the book, two selves exist in tandem: Gay as writer and as a woman living her life. As a writer, she can rise above her body and the humiliations of the flesh. Reading the book is to witness the gap between the conscious mind and the unconscious body – in combat for years.

Writing can be escapist and can be an opiate (it has been both for Gay, although neither here). But most important, in the context of this book, writing is weightlessness. Gay’s prose is unencumbered. A New York Times and Guardian US columnist, her punchy authority is in contrast to what she describes. There is a tension between her low self-esteem and the self-worth needed to write this courageous, honest book. Her mutinous body is the continuing subtext – going its own way, persisting in its compulsions, fleshing out the story. “Hating myself became as natural as breathing. Those boys treated me like nothing so I became nothing.”

Gay did not tell her parents what had happened until she had grown up. Raised as a Catholic, the daughter of Haitian immigrant parents – her father a civil engineer – she feared their reaction. She felt guilty for once fancying the boy who raped her. Terrible to think of a 12-year-old child willing herself to go on as though nothing had happened. One longs to be able to go back in time, to intervene, to find help for her. Not that she wants our rescuing pity – on the contrary, one of the complications of her life is the resentment she feels about other people’s reactions.

Gay describes strangers lifting fattening items out of her shopping trolley. Air cabin crew wonder aloud if their safety belt extensions will encircle her girth. People at the gym offer well-intentioned words of encouragement. But she is clear: “I am stronger than I am broken.” She rejects the affirmation of strangers. One of the triumphs of the book is that she not only makes one consider the way fatness is judged, she implies a larger question about the impertinence of judging others at all.

Her father’s job meant the family moved around – to Colorado, Virginia, Illinois, New Jersey. Gay was sent to Phillips Exeter Academy, New Hampshire and later went to Yale. Food became her “friend” because it was “constant and I didn’t need to be anything but myself when I ate”. There was much eating in private – an “orgy of food” – secret cure for a secret hurt. In two and a half months, she gained more than 30 pounds. She felt “a rush of solace when I ate”. The internet would later supply comparable solace: “I didn’t have to be the fat, friendless loser… I could pretend to be thin and sexy and confident.”

Without telling anyone, she left Yale after her second year and travelled to Phoenix with a strange, gentle guy she had met online. This was her “lost” year. She earned her living as a phone-sex worker. Her parents were sick with worry and eventually hired a private detective, she believes, to find her. She describes her sexual vacillations. She came out to her parents – a big deal – then went back in, thinking herself straight after all.

Nowadays, she sees hunger, in a metaphorical sense, as a driving force: “The older I get, the more I understand that life is generally the pursuit of desires.” It is good to learn she has a happy relationship with someone she refers to, with sweet ambiguity, as “my person”. But there are no easy resolutions here. She admits to Googling the boy/man who raped her [assumed name, Christopher], now an executive at a major company. She rings him and, when he answers, is unable to speak. They listen to each other’s silence. She writes: “I need to understand, at all times, the distance between him and me.” One cannot help but wonder whether this book might reach him, close that distance, make contrition possible. Unlikely, I suppose. Some weight is impossible to lose.

• Hunger: A Memoir of (My) Body by Roxane Gay is published by Little, Brown (£13.99). To order a copy for £11.89 go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99