This occasion," says Lionel Shriver, "breaks every rule in the book." She does not usually eat lunch, has had to "mobilise my mind into the idea of having it". She looks around Pizarro, in Bermondsey, close to where she lives: a dark Spanish restaurant, designed as if with Mediterranean heat in mind. The menu is simple, bordering on spartan. She joins me on a semi-circular bench. "I am going to be a different person today," she jokes. "It's great." I have to glance sideways to see her: tiny, muscular, a dynamo. Fair hair pulled back and pinned up like a ballerina's. Royal blue sleeveless T-shirt. She is 55. Her profile is decided – rather like her opinions.

It was Shriver's seventh novel, We Need to Talk about Kevin, about a boy responsible for an American high school massacre, that made her name. Turned down by 30 publishers, it became a word-of-mouth sensation and a harrowing film starring Tilda Swinton. Her new novel Big Brother is going to be "huge" I venture – then stop, embarrassed. For it is about being huge, about a man whose fingers "recalled bratwurst in the skillet just before the skin splits", who climbs into a car "with the delicacy of a giant crane manoeuvring haulage from a container ship". It is about a sister who wants to rescue her brother from obesity and what makes the story delicate to discuss is that it is – in part – autobiographical.

"Do you mind if I mix and match?" Shriver is studying the menu. She chooses three starters: marinated salmon, mackerel escabeche and sautéed vegetables. And that is it, her lunch. In 2009, she wrote a column for the Guardian, describing her fear that her elder brother, Greg, might kill himself from overeating. "My brother breaks my heart," she wrote. "He drags a portable oxygen tank with him like a faithful dog." Days later, her brother died, at 55, from respiratory failure. Big Brother is dedicated to him "in the face of whose drastic, fantastic, astonishing life any fiction pales".

I want to know all about Greg, but there is something else to settle first. Pandora, the sister in the novel, defines herself in terms of food, comparing herself to white rice. What food would sum Shriver up? "Chilli," she replies. "That is what I eat more of than just about anything." She reaches into a bag, pulls out a jar of Marmite. "I travel with this." She takes the lid off: it is filled with chilli flakes. "I like sensation. I am fascinated by flavours between pleasant and unpleasant, right on the line." There follows a connoisseur's tutorial – describing her initiation into Mexican chilli powder and habañero, "so hot that – even for someone who has a heroic immunity – three pinches borders on inedible".

She gives her starters the chilli treatment – a lavish scattering – although she is complimentary about the gentle paste that accompanies the vegetables, detecting hazelnuts in the mix. Tell me about Greg, I say. Was the novel written out of a feeling you were unable to save him? "I got that question from my parents," she says. "I last saw Greg a year and a half before he died. He gained weight after that. I'd say he weighed about 400lb by the end. He was a sound engineer – self-taught. Greg did everything on the grand scale. People think I am extreme. I'm nothing compared to my brother. That tendency to extremity is very dangerous when it has to do with substances. He had a series of addictions, ending in food and pain medication." Greg also inspired devotion on a grand scale: "Iconic personalities insist themselves on your consciousness," she says.

Shriver had to confront the question of whether to return to the US – she has a house in Brooklyn (born in North Carolina she has been an expat since 1985) – to care for Greg when gastric bypass surgery was recommended: "I wasn't sure whether I was up to it. My brother was a challenging character. I mean, he was a wonderful person with a very good heart and was never malevolent but could be quite frustrating – not exactly a listener."

At this point, a complimentary bowl of fish rice arrives. Shriver pronounces it, "unbelievably intense, three-dimensional, huge…" before deciding against finishing it. She calls the waiter over – "Please tell the chef it is an achievement" – and asks if it might be possible to take it home for her husband – jazz drummer Jeff Williams.

"Almost no one," she admits later, "has a normal relationship with food." And the temptations against which she battles are bread and wine. She laughs as she hears what she has just said: "Biblical! My father would be proud." Her parents were "liberal Christians, with the emphasis on liberal". Her father was a Presbyterian minister. Lionel was the middle child between brothers. A tomboy, she ditched her Christian names – Margaret Ann – and dubbed herself Lionel. The family used to have "Hunger" dinners to focus on third world privation and raise funds. These dinners sometimes consisted exclusively of baking soda biscuits. What Lionel's parents may have missed was this: "I loved the biscuits…"

Nowadays, Shriver eats only one meal a day – often late in the evening. "I'm not a fanatic. It is not religious, not some nothing-shall-pass-my-lips-until-sundown thing." But having one meal "eliminates guilt which can contaminate the experience of food – I am fighting guilt right now." Every other evening, she goes for a 10-mile run along the Thames, thinking about her writing or which vegetables to cook for supper ("I'm big on vegetables"). Penitential? "No – but I wouldn't call it fun. It is often onerous. The scenery is good. I am always glad when it is over." Once home, she makes popcorn ("Festive – I like the explosive sound") to coincide with Newsnight. "When you are small – I am not being boastful about being skinny – you don't need much. And temperamentally, I like a sharp divide between work and play."

As she talks, she bangs her fork on the plate for emphasis. "As a young woman, I was interested in experimenting with control, it doesn't interest me any more." During her 20s and 30s, she twice went on three-week fasts: "It didn't especially imperil my health. I wouldn't do it now." She remembers the "bizarre energy. At other times, everything was grey and each minute ticked by like a year." She is glad she did it – not least because the experience has fed the new novel.

What does she make of society's contradictory attitudes towards food – focusing on gluttony one moment, denial the next? "There is a big class divide. The 'lower orders' are oriented towards quantity and cheapness. They weigh more. The middle classes are oriented towards quality – price no object. Yet both groups are equally obsessed with food." Our greatest problem, she argues, is a confusion between aesthetic and moral. Obesity is treated as a "moral issue" because it is costing the NHS money [£5bn a year]. And she has no quarrel with that. Yet, at the same time, fat is censoriously linked with failure of character and thinness with virtue.

She talks eloquently about the psychology behind the anorexic disappearing act: "Anorexics want to be admirable, to achieve. The ideal is ultimately to be so thin you are not there any more. Anorexics feel guilty for being three-dimensional." But "the equation is all wrong – fat is not evil … food should not be a moral issue."

Over coffee, I ask if writing the novel has helped settle her feelings about her brother. She replies in the slightly washed-out tone of someone who feels her emotions to be beside the point. "The remorse may be smaller," she concludes. Then she picks up a neat, silver envelope containing the fish rice and we walk out together, towards her bicycle, in the sunshine.

Big Brother (HarperCollins, £16.99) is out now. To order a copy for £13.59 with free UK p&p go to theguardian.com/bookshop