When I meet Irvine Welsh in an East London member's club, I almost feel guilty when I order a beer. For the Leith-born author seems to have discovered a penchant for clean living since moving to the United States five years ago. However, he admits he has fallen back into bad old habits since returning to Britain on a whirlwind publicity tour for his latest novel, The Sex Lives of Siamese Twins.
Centring around the dysfunctional rapport between officious gym instructor Lucy Brennan and obese contemporary artist Lena Sorenson, the idea for the book came to the 55-year-old while he was working out.
"I thought it was really weird because there was this trainer, who was really ripping into one of her clients and giving her a tough time," he recalls. "The girl was in tears and I thought, 'Why would you pay to have someone do that to you?' They must have some kind of relationship between them beyond that because otherwise she would have just sacked her. So I started thinking about what kind of relationship that would be and I drew a picture of the characters based on that."
Taking its name from the tawdry tale of two conjoined siblings' desperate attempt at romance that is played out on television sets in the background over the course of the novel, Welsh also drew inspiration from his favourite reality TV shows.
"I became completely obsessed with The Biggest Loser and that kind of tough love stuff with people like Jillian Michaels and Bob Harper," he says,
referring to the stars of the prime-time NBC show. "I also saw a documentary about this fat Mexican kid whose mother was feeding him to death. It was her way of showing him love but they were trying to explain to her that 'this is what your love is doing to your son', giving him things like heart disease. It was almost like it was her disease as well because she couldn't stop herself from feeding him. That kind of obsession was something I've always wanted to write about and it was basically a case of finding these characters."
Set in Miami, the glossy, artificial milieu that Lucy and Lena inhabit might seem a world away from the grim Edinburgh housing estates of Welsh's seminal 1993 debut, Trainspotting. But Lucy's constant calorie counting and Lena's love of fatty treats actually has much in common with the heroin dependency of seedy junkies like Renton and Sick Boy.
"They all have really addictive personalities," says Welsh. "We make everything into addiction and we're addicted to everything. We're very compulsive/obsessive as we basically live in an OCD society. If you're overweight, you don't think, 'I need to eat less' because we have to substitute other products for that instead. So we have all these weight-loss and diet/exercise programmes, as the idea is to just keep consuming."
According to Welsh, attitudes towards eating have radically changed in recent decades.
"Poor people used to be thin because they didn't get enough to eat but now poor people are fat because they're eating the wrong food, which is basically processed food they can't metabolise," he says. "In America, some states like Florida and Colorado have a very healthy diet but a lot of other parts of the country have fallen behind Britain in terms of their actual food.
"The food revolution we've had here [in Britain] over the past 20 years still hasn't happened at grass roots level in America. They're still growing GM crops, which we wouldn't put up with here. It's very interesting, because it's such a food-related society.
It's like when a couple of our friends come over to see my wife and me. They're not overweight - they're actually quite thin - but they talk incessantly about food. It's like how the weather is the stock conversation in Britain. With food in America, if they're not eating it, they're talking about it."
Having originally settled in Chicago, Welsh now divides his time between Illinois and Florida after falling for the allure of Miami's tropical climate and laid-back lifestyle. "I started going down there when I was DJing as I used to hang out at some of the bars during the Miami Music Conference," he says. "I had a bit of cash so I bought an apartment, so I could spend some time there. Chicago is a great town but the winters are very, very brutal, with real Siberian weather.
"It's become a well-known city through film and TV and they make a lot of programmes there because of the light. You've got a lot of artists, photographers and models, so it's a very visual culture. It's not a great book town but it's a great visual arts town."
Home to best-selling crime scribes like Carl Hiassen and Dexter creator Jeff Lindsay, Miami's glitz and glamour has fired Welsh's imagination more than Chicago's more rarified literary tradition.
"I'm more inspired to write in Miami than in Chicago," he says. "I like Chicago but it feels like a British city in a way. It's more ossified and it's an old industrial, working class town. There are some great writers who have come out of Chicago but they know more about the place than I do. They're steeped in it and I'll never be able to write about Chicago in the way they do. I feel that Miami is as valid as anywhere if you've just got off the boat or the plane, because it's being formed right before your eyes. There's nothing that's set in stone."