Roxane Gay has several personae, but she first garnered Internet fame as a diarist. On Tumblr, during the platform’s embryonic years, Gay recorded her daily likes and dislikes in posts such as “Things I Am Currently Charmed By” (items included “Discussions of ‘spirit animals,’ particularly when the spirit animal is an inanimate object”) and “Me and My Man,” about Bill Clinton. Gay also used the platform to discuss the culture’s punishing relationship with aspects of her own identity: fatness, bisexuality, and blackness. She wrote about the murder of Jordan Davis and, powerfully, about her rape at the age of twelve. As Gay commuted her intimate, rigorous pontifications from Tumblr to now-defunct Web sites such as xoJane and the Toast, and later to the Times, where she is a columnist, she fashioned herself as a public intellectual of the people—mostly women, black and brown, queer—and of readers who allied with (and wanted to be seen to ally with) those people.

In 2012, Gay posted “Feminism (Plural),” a short manifesto that became the introduction of her blockbuster 2014 essay collection, “Bad Feminist,” in which she argued that “Professional Feminists” were out of touch with those who most needed the movement. (The things that she claimed made her a “bad feminist” included enjoying rap music despite its sexist lyrics, faking orgasms, and not knowing how to fix her car. “Like most people, I’m full of contradictions,” she wrote.) Since that book, Gay’s readership has only grown as she has positioned herself at the vanguard of soft, feminist punditry and much else: she is the author of two fiction best-sellers and the first black woman to write for Marvel Comics.

It is curious to be reminded, in Gay’s new memoir, “Hunger,” that she was first drawn to online forums by the promise of anonymity. The memoir deals with her rape, her overeating, and her struggles with her public and private identities. Before the dawn of avatars, she lived on IRC, “an old-school chat program with thousands of channels populated by thousands of lonely people who were mostly interested in talking dirty to one another.” The memory contrasts with the tone of the book, in which Gay is constantly defining and defending herself against others’ expectations. Increasingly, she has become not just a writer but a spokesperson. Gay, who rejects the ideal of “(th)inner woman” while also wishing that she could herself be smaller, has drawn the ire of fat-acceptance advocates, who presumably wish that Gay were a less equivocal role model. In “Hunger,” she writes candidly of her position, returning to the theme of contradictions: “I have been accused of being full of self-loathing and being fat-phobic. There is truth to the former accusation and I reject the latter. I do, however, live in a world where the open hatred of fat people is vigorously tolerated and encouraged. I am a product of my environment.”

Some of the liveliest prose in “Hunger” can be found in her previously published takedowns of the “weight-loss industrial complex,” in which she points out the depraved strategies of “The Biggest Loser” and of life-style deputies like Oprah and Kirstie Alley. (“The public weight-struggle spectacle is a popular fallback for once-famous women who yearn to recapture their former glory.”) There are also passages about her love of Ina Garten, known as the Barefoot Contessa, and of beating men at poker.

Elsewhere, Gay catalogues her daily stresses as a fat woman, recalling the blunt, confessional voice she has assumed on her blogs since her early Tumblr days. She recounts how shoppers reach into her cart and replace her food with their idea of healthier options. Flight attendants seem to take pleasure in loudly discussing whether her seat-belt extender is up to code. Armchairs are, she writes, “generally unbearable,” leaving her with lingering bruises. The medical community encourages obesity panic, yet doctor’s offices lack scales that can accommodate her weight. People are either bungling or cruel. (Last week, while promoting the book, Gay took to Twitter after an Australian podcast host made public her publisher’s request that Gay have a sturdy chair to sit on for her in-person interview.)

The opening section of “Hunger” is written as an accretion of false beginnings: “The story of my body is not a story of triumph,” she writes, and then later, “I don’t know how to talk about rape and sexual violence when it comes to my own story. It is easier to say, ‘Something terrible happened.’ ” Gay writes carefully about the attack, which occurred, we learn, when a boy she calls Christopher, whom she adored, brought her to a cabin in the woods where his friends were waiting. At school the following day, her attackers broadcast their own version of what happened. She was jeered at, called a slut. She stopped believing in God. (Her parents, wealthy and well-meaning Haitian immigrants, didn’t learn what happened until years later.)

A year or so later, Gay was sent to Phillips Exeter Academy, in New Hampshire, where she gained more than thirty pounds in two and a half months. “I needed to feel like a fortress, impermeable,” she writes. She spent her parents’ money at a campus greasy spoon. One summer, at the suggestion of her parents, she went on a liquid diet, lost weight, and returned to school svelte. Then she gained the weight back, and more. At her heaviest, she weighed five hundred and seventy-seven pounds. Her academic excellence and shyness obscured, to most people, the obvious fact that she was depressed.

She completed two years at Yale University before disappearing for ten months—what she calls her “lost year,” when she went to San Francisco to connect with an older man whom she met online. They moved together to Scottsdale, Arizona, where she got a job as a phone-sex operator. He taught her how to use a gun, and, Gay tells us, “I did the kinds of things that the good girl I had long pretended to be would never dream of doing.” She doesn’t tell us what these things are; the result is a chapter that reads like a trailer for a future book.

In fact, Gay rarely writes scenes—preferring to focus on material that directly correlates to the story of her fatness. Of interacting with lovers online, Gay writes, “At first, I did it because it felt safer and I could be sexual without having to actually be sexual.” In her twenties and thirties, while pursuing a Ph.D. in fits, Gay entered a series of abusive relationships: “I was a lightning rod for indifference, disdain, and outright aggression, and I tolerated all this because I knew I didn’t deserve any better, not after how I had been ruined and not after how I continued to ruin my body.” At the same time, she expresses disappointment that she feels the need to write the book we are reading. “The list of bullshit I deal with, by virtue of my body, is long and boring, and I am, frankly, bored with it.” Several times, Gay writes a version of “I’m a mess.” There is occasionally something proud, almost, in Gay’s lethargic prose, as if to focus on language would be beside the point.

But there are a few moments when Gay gives us a glimpse of the deeper account that “Hunger” might have been—one in which she pursues, rather than merely dispatches with, the contradictions that have so painfully defined her life. In one almost stream-of-consciousness narrative, she describes tracking down Christopher as an adult. We learn that she knows where he lives, where he works. She has called him and listened, without speaking, on the line. (“His voice hasn’t changed,” she writes.) Plunged into the act of remembering, Gay’s prose grows frenetic and textured and marvelously difficult:

I wonder if he knows I have sought out men who would do to me what he did or that they often found me because they knew I was looking. I wonder if he knows how I found them and how I pushed away every good thing. Does he know that for years I could not stop what he started? I wonder what he would think if he knew that unless I thought of him I felt nothing at all while having sex, I went through the motions, I was very convincing, and that when I did think of him the pleasure was so intense it was breathtaking.

Here, Gay is revealing herself, finally, with shocking truth and devastating poetry. She quickly moves on to other topics, but the passage stayed with me for days.