"I try to avoid reviews, but this book has been reviewed really well by almost everybody. That makes me really nervous. Seriously, it does. I'm not a consensus kind of writer. What I am writing about is really divisive. It's not coming from the normal social milieu that literature comes from. I really feel that I write about things that divide people more than they unite them."
Irvine Welsh is, by the sound of it, a hard man to please. It could be his anxiety about the near-unanimous praise for his new novel, a characteristically provocative, entertaining yet unnerving examination of obesity, fitness and power titled The Sex Lives of Siamese Twins. Or it could be the transport nightmares that beset the internationally successful writer who spends too much time in airports. "It's Heathrow's constant inefficiency. I can't believe they spent all that money on Terminal 5 and it's as bad as ever."
I catch up with Welsh, who first rose to fame with Trainspotting, his debut novel from 1993, in London at the end of a book tour for the new novel. Such is his prolificacy (nine novels, four short-story collections and 10 screenplays in two decades) that in recent weeks he has been promoting three books in three languages across Europe. "I have been in France and Switzerland doing Crime, which was two books ago. Then I was in Spain doing Skagboys [released in 2012]. I always wake up thinking, 'Is it nonces, junkies or lesbians today?'" The joke proves to be typical Welsh. His writing may have earned him a reputation as the prince of darkness, urban squalor and literary controversy, but in conversation he is chatty and funny.
These positive characteristics are in plentiful supply in Welsh's novels, but tend to be overshadowed by his talent for outré set pieces. He has a gift for misanthropy and human degradation. But in shining a light on the darkest parts of the human condition, his novels are comic, humane, even profoundly moral. "We are the only creatures on the planet who know we are going to die, and that is terrible knowledge to be bothered with. We just about make it past that knowing we have the capacity for love and the capacity to create art. They are the two big sweeteners that make our awareness of mortality bearable."
Welsh is quick to lament art that sacrifices reality for an easily-won happy ending. His problem is not so much with blinkered optimism as the peddling of irresponsible fantasies. "I always kind of squirm when I hear a novel being talked about as life-affirming. You know it means that it affirms the reader's middle-class life. The person who goes and rents a villa in Tuscany and gets lazy Italian builders to fix it up for them, and then falls in love with the postmaster's beautiful daughter."
The Sex Lives of Siamese Twins has its share of disturbing moments. It is the story of a perverse power relationship between Lucy, a psychotic, control-freak fitness trainer devoid of compassion for human frailty, and Lena, an obese artist whose traumatic life involves a compulsive cycle of comfort eating.
The drama intensifies when Lucy kidnaps Lena and subjects her to a fitness regime that has more in common with Abu Ghraib prison than a local gym. For Welsh, the conflict satirises a broader, polarised vision of Western values. "America's the home of obesity, of compulsive consumerism, but it's also the home of self-improvement and the fad diet. It is Western individualism taken to its highest extent. We want to stay healthy, good-looking, fit and active. In some ways it is based on sound logic - everyone wants to look better, everyone wants to feel fit. There's a massive narcissistic element taken to its extreme."
Welsh was inspired by his experience of living in the US, his adopted homeland: first Miami, where the novel is set, and more recently Chicago. "Miami has a very visual culture, a very beach culture, a very body-conscious culture. Practically every second retail outlet is a gym of some kind, or to do with pills, vitamins and supplements."
This exotic setting and the strong female leads propose The Sex Lives of Siamese Twins as a departure of sorts from Welsh's classic, masculine, British fiction. But the pitiless depiction of obsession as a response to trauma is typical of his finest work.
"We have created a zoo here that doesn't really suit us. It's not a really good place for us to be. So we embrace all these mad, obsessive-compulsive disorders. You can have damaging ones like drug addiction or more ambiguous ones like addiction to extreme fitness. They all distract us from the fact that life isn't offering us that much, that we aren't getting any spiritual answers to the big questions, that we are going to die. There is all this existential stuff going on."
There is also, it would seem, all this personal stuff going on too. Contrary to first impressions, The Sex Lives of Siamese Twins may just be Welsh's most personal book to date. At least, that is, according to his wife. "She's a 32-year-old American. She said to me that she has not found any two characters who are so close to me as Lucy and Lena. Even Renton in Trainspotting. I am not necessarily endorsing that, but it was interesting," he says.
Welsh admits his personality is divided between fierce ambition and an underlying vulnerability. But he is more interested in discussing other contradictions. "I see the novel largely as being about the dichotomy between sport and art. I always remember as a kid being fascinated by sports, but I also loved art - painting, drawing, writing and music. I loved my mates who played sport and my more geeky pals who were a bit more weird and socially awkward. I hated the dichotomy between the two and I hate the way the education system makes us into these dumb jocks or poncey, artsy types."
Welsh has always been an unusual writer, one influenced as much by Iggy Pop and British rave culture as James Kelman, who achieved enormous success by working outside the literary establishment: he famously recommended that readers steal Trainspotting from bookshops. This creates enormous self-confidence but also hints at insecurity. "Anybody who doesn't come from an arty background and who aspires to be an artist of any kind always has a battle within themselves to validate themselves in the world of art."
In The Sex Lives of Siamese Twins, this is expressed through Lena's lack of self-confidence as an artist and her vexed relationship with art-world critics. I ask Welsh, who has suffered his share of bad reviews, to describe his relationship with reviewers. "I don't really have one. The first couple of books got some really great reviews. It struck me when I read them that people tended to use the same string of adjectives - 'disgusting', 'vile', 'violent', 'horrendous', and then either 'brilliant' or 'terrible' at the end. Critics were reacting in a very middle-class way. There was a visceral reaction and they either really enjoyed being on that trip or they really distrusted it."
As his new novel demonstrates, Welsh is a writer at the top of his game. This is why he feels no pressure, despite press reports, to rush into a long-awaited sequel to Trainspotting the movie, based on his 2002 novel, Porno. He reveals that there have been discussions with the movie's original team including director Danny Boyle.
"We have been hanging out. We got a flat in Edinburgh for a week. We are just looking at ways it could be done, but we are not convinced yet. We are not desperate to make it - nobody needs the money. It's all down to whether we can get a decent script that would excite us and the cast."
Welsh has more than enough on his plate. He has already completed a new Edinburgh-based book for next year, and is part of two high-profile screenplay collaborations: with Harmony Korine on a "darker, crazier" follow-up to Spring Breakers, and with dance music supremo Calvin Harris on a HBO series about dance music in Las Vegas.
Before he flies half way across the globe, I ask the 55-year-old icon of youth culture how he has been coping with ageing. Has he done better than the characters of his new book?
"I am still trying to work all that out. I oscillate from kidding myself and the world that I am 20 again. That usually ends badly after a while with a terrible social embarrassment of some sort. Then I just become this staid, safe pair of hands. That usually ends badly as well because it bores me. If anyone knows [the answer] please tell me."