As twists of fate go, this one is particularly twisted. It was announced over the weekend that Allen Carr, who gave up his 100-a-day habit 23 years ago and became one of the world's best-known anti-smoking campaigners, has been diagnosed with lung cancer. "This has come as a shock but I remain very upbeat," the 73-year-old told a Sunday newspaper. Those who hope that quitting smoking will mean they avoid such diseases will probably not be feeling quite so defiant: they will want to know whether there is any point giving up at all.
The link between lung cancer and smoking was first made by Richard Doll in the British Medical Journal; on June 26 1954, he launched a 50-year study, funded by the Medical Research Council, Cancer Research UK and the British Heart Foundation, of exactly how many years smokers were lopping off their lives. On June 26 2004, he announced that it was, on average, 10. However, he also discovered that quitting at age 50 - the age Carr was when he stopped - nearly halves that. Stopping at ages 60, 50, 40 or 30 gains, respectively, about three, six, nine or 10 years of life expectancy.
"It's obviously very sad that Allen Carr has now got lung cancer," says Amanda Sandford, research manager at Action on Smoking and Health (Ash), "but if he hadn't quit, he would probably have died years ago. He has certainly extended his life."
But it's not just a question of staying alive - quality of life is also a factor.
"It's not just about death," says Professor Robert West, director of tobacco studies at Cancer Research UK, "because obviously we've all got to die sometime. It's about disability. The reason people should stop smoking is not to stop them dying, but to stop them living a horrible, disabled life."
Smoking, like any addiction, consists in large part of the triumph of hope over observable fact. For example, it is comforting to believe that when you finally quit, the body will heal itself; a popular belief is that, by a certain length of time after quitting - five years, say - your lungs will be back to normal. Unfortunately, that is not the case. "When you stop smoking, your lung cancer risk does not go down," says West. "What happens is that it stops going up. It's like a sort of escalator of death - you want to try and get off on the lowest floor possible."
As well as writing Allen Carr's Easy Way to Stop Smoking, which has been published in 45 countries, the give-it-up guru has established Easyway clinics in over 30 countries. His business partner, John Dicey, said at the weekend that Carr had "spent many years in smoke-filled rooms delivering his method to smokers; it is part of the clinic process that people actually smoke while there. He is not drawing any conclusions, but Allen feels that if that did contribute, it is a price worth paying, as we estimate he has cured around 10m smokers."
West is unconvinced. "It's more likely that he got it from his own smoking than from passive exposure." Carr was a very heavy smoker, for a long time, he points out. "Lung cancer is a multi-stage disease - it doesn't require just one event, but a succession of events eventually to become malignant and break through the body's immune system," he explains. "It's entirely possible that he sowed the seeds of this in early to middle age because if you stop at 50, you carry with you a greatly increased risk of lung cancer until the day you die."
The other major smoking-related killer is chronic obstructive pulmonary disease (COPD), a progressive, degenerative condition which eventually results in an inability to breathe; emphysema is a subset, as it were, of COPD. In emphysema, the alveolar sacs which surround the alveoli merge into each other; they lose their elasticity and breathing in becomes more and more difficult. People don't often realise how affected they are - 80% of COPD goes undiagnosed. Sufferers simply assume that they are a little breathless, which comes with age, or perhaps lack of exercise - and don't do anything about it. In fact, they are probably on a steep decline. By the time a smoker reaches 60, they can have a similar lung function to someone who has had asthma since childhood. Giving up smoking may halt that downward trend, but it can't reverse it.
"You never recover the lung function you have lost," says West.
After about 35, lung capacity and lung health decline anyway, so if they are already damaged, the decline is simply faster. "People shouldn't be fooled into thinking that they can smoke for years and then, by stopping, remove all risk of smoking-related disease," says Sandford. "There will always be some residual risk."
It is true that not all smoking-related damage is irreversible. The risk of coronary heart disease drops by 50% in the first year after quitting smoking. Skin health improves surprisingly fast.
"It is extraordinary how resilient the body is," says West. "It can put up with this insult day after day, year after year, and it forgives you - until about the age of 35. After that, the clock is ticking, quite quickly. It's much more urgent than people realise."
All warn, however, that none of this counts as a reason not to stop smoking, and to do it as soon as possible. "No matter how long you've smoked," says Sandford, "it's always worth quitting".
The gurus who failed to fix themselves
As Scott Ian, lead guitarist of the thrash metal band Anthrax, said, after stocking up on antidote during the 2001 anthrax-in-the-mail scare: "I will not die an ironic death." And who would? You want your death to provoke weeping in the streets, not wry chuckles.
One of life's more grimly enjoyable ironies is the misfortune that rebukes the principles of a life devoted to some form of self-improvement - particularly if those principles have been sold to consumers at considerable profit. Schadenfreude is not one of humanity's more appealing traits, but for those who feel preached at by faddists, some feeling of poetic justice is hard to resist whenever a lifestyle guru turns out to be fallible.
The example of Robert Coleman Atkins comes to mind. Dr Atkins, the man responsible for halitosis across six continents, had, on April 18 2002, eaten a carb-free, high-protein, Atkins-diet breakfast as usual, before his morning routine was interrupted by a sudden, non-fatal heart attack. A year later, after he slipped on an icy New York pavement, fell into a coma and died, medical records confirmed that he was suffering from heart disease.
And what was God trying to tell us with the fate of Jim Fixx, author of the Complete Book of Running, which sold more than 1m copies in the 1970s? It rapidly stopped selling when the 52-year-old New Yorker died while out jogging in 1984. Or of Euell Gibbons, who became famous in the US in the 1960s for his series of books about healthy eating (including Stalking the Healthful Herb)? Despite never knowingly ingesting a single toxin, Gibbons ascended to the Ironic Death Hall of Fame at the age of 64 by dying of a heart attack.
Another health champion floored by a bad heart was Jerome "JI" Rodale. An early evangelist for organic farming in the US, Rodale founded a successful publishing empire on a range of health food books and Prevention magazine, which advised readers on nutritious eating. Appearing on the Dick Cavett Show in 1971, he boasted to the audience about how healthy he was, before slumping in his chair and appearing to go to sleep. "Are we boring you, Mr Rodale?" bantered the host. Having died of a heart attack, Rodale declined to reply. The show was never aired.
Demonstrating closer to home that conviction in one's lifestyle plan confers no immunity is Carole Caplin's mother, Sylvia. Caplin senior, herself a health guru, has none the less suffered endless problems with her health including a chest infection, diseased colon and the removal of half her bladder. But the fact that she is still gamely in business after all that might serve to advertise, rather than diminish, her skills.