Emma Healey’s two previous novels explored themes vivid in her own life. The protagonist of her bestselling debut, Elizabeth Is Missing, for which Healey won the 2014 Costa first novel award, is Maud, who has dementia but is forced to turn detective in the hunt for her missing friend. Magnificently rendered by Glenda Jackson in the BBC adaptation, she was in part inspired by Healey’s desire to see the world through the eyes of her grandmother, who had the disease. Her 2018 follow-up, Whistle in the Dark, allowed Healey to consider what her teenage depression might have been like for her mother. Her new novel has an autobiographical element, too.

Sweat is a psychological thriller about coercive relationships, the futility of revenge and when self-control turns pathological. It’s the latter that bled into the plot from Healey’s life. An exercise and fasting regime following the birth of her daughter in 2017 became obsessive and damaging, just as it does for Cassie in the book after she meets Liam, “man of my dreams, star of my nightmares, my mentor, my shadow”. A personal trainer, Liam is physically perfect, from the curl of his eyelashes to the way his muscles “hum”, and when he meets Cassie, he wants to help her be just as perfect. Soon he controls everything from her punishing runs to her calorie intake.

There is an incident in the novel where Cassie can’t focus on a conversation with a friend, whose help she desperately needs, because she is panicking after consuming 11 calories over her limit of 500 for the day. I had thought this was satire (there is plenty of dark humour in the novel) but Healey tells me that she’d regarded it as an entirely relatable plot point until her husband read a draft and remarked upon how sad it was. “And I was like: Oh yeah, it’s sad, not normal.”

Yet not so long ago, Healey says she was making sure she never made it to 500 calories on a fast day: “It was always under, just in case. Eleven calories? No way.” A preoccupation with exercise and food began in her late teens, she tells me, but life moved on and she was able to manage it. That changed when she had her first child.

“I had been doing all the things you’re supposed to,” she says, “like hypnobirthing”, but the minute labour started everything fell apart. “It was really bad – 52 hours – and obviously very painful and scary.” The experience was nearly fatal for both mother and baby. “Afterwards, it felt like it was my fault,” she says. “I felt guilty, and pretty disgusted with myself – it was quite visceral – and it started something in me.”

She began a fasting diet. “It suits my brain, ticking things off, there’s a game with yourself: how much lower can I go? Maybe I can do two fast days in a row?” Then the couple invested in a rowing machine for her husband’s 30th birthday and Healey was hooked. She rowed and rowed until she developed shoulder pain. “But I couldn’t think about rest days. So I thought: well, I’ll just run instead. Sometimes I might do 10k rowing, and then run 10k.” Looking back, she says, it was all about regaining control of her body. “I don’t think I wanted any maternal softness.”

Today Healey, who now lives with her family in Norwich and will soon turn 40, is rosy cheeked and serene. Chatting easily over a cup of tea, she seems relaxed and happy, but this contentment is hard won. While her debut may have been a breakout hit, she points out that her path to becoming a novelist was far from straightforward. After a “sort of breakdown” as a teenager, partly brought on by exam stress around her GCSEs, she skipped A-levels for art college. And it was only after landing her “dream job” in a gallery that she realised that writing was all she really wanted to do.

She had drive, she remembers, but given what she saw as her academic “failure”, lacked confidence: “I started doing a correspondence course, because I thought I could do that, and no one would know.” Eventually she joined a writing group in London. “We used to meet at Housmans, the radical bookshop near King’s Cross.” When she read them the first 500 words of Elizabeth Is Missing, they gave her such positive feedback that, she recalls, “I just kept going”.

Making creative decisions can feel oddly weighted after a bestselling debut; how does the huge success of Elizabeth Is Missing affect her work? “I can’t ever really hope that my next book will sell as well,” she agrees. Still, she’s grateful for the profile that first book gave her, offering an enduring sense of freedom to follow what she’s interested in writing.

The roots of Sweat stretch back a long way. “For years, I’ve had this idea of someone who has been stalked or harassed, and then the person comes back and they somehow turn the tables. But I couldn’t really work out what the setting was.” Then she hit upon the “weird dynamic” between personal trainers and clients. “You’re paying them, so you’re in charge, but also they’re paid to be in charge of you. So there’s a constant shifting of power … And then immediately it was like: oh, and I can use every single obsessive thing I’ve had about fitness and eating and put them into the book.”

As the story develops, Healey explores different aspects of coercive control. “I was interested in how many women find it harder to break out of these situations because of society’s messaging around male-female relationships.” It’s normalised? “Yes. Even though there are obviously relationships where women are in control and coercive, they don’t tend to be backed up by societal norms. It felt similar to how I knew I was overexercising and not eating enough, but all the messaging is: ‘Get out and move more and make sure you’re not eating too much.’”

Healey says Sweat was a strange book to write because while it feels personal in some ways, “I also feel quite detached from it, because I wanted it to be pacy, to be a fun, dark, read.”

Novel number four is percolating already, as she moves through the new territory of parenthood, with its different relationships, rivalries and subcultures. “I mean, soft play centres are fascinating,” she laughs. “I’d kind of like a job in one just to see what it’s like to be there five days a week.”

She is thinking, in particular, about parental competition. The painful truth is that it’s not only something we deplore in others, she says, it can be something we see and cringe at in our own behaviour. “You know, why have I just boasted about everything? I’m interested in that.” But she wants to get it down on paper before saying any more. She has a plot already. “I think it’s going to be a bit weird.”

• The UK eating disorder charity Beat can be contacted on 0808-801 0677. In the US, help is available at nationaleatingdisorders.org or by calling ANAD’s eating disorders hotline at 800-375-7767. In Australia, the Butterfly Foundation is at 1800 33 4673. Other international helplines can be found at Eating Disorder Hope.

• Sweat is published by Hutchinson Heinemann on 30 January. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.