The WHO calls tobacco “the single most preventable cause of death in the world”—but cigarettes may also provide a handful of paradoxical, if pyrrhic, health benefits: Smoking will probably take years off your life, but certain things in tobacco smoke may actually do the body good. Here’s what science has to say about the smoker’s paradox.

Smoking Alleviates Ulcerative Colitis but Inflames Crohn’s Disease

Broadly speaking, smoking worsens your health. But for some people who suffer from inflammatory bowel disease (IBD), smoking seems to relieve their worst symptoms.

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IBD refers to a handful of chronic gut diseases. Crohn’s disease and ulcerative colitis, the two most-common forms of IBD, both result when a person’s immune system mistakenly attacks that person’s own gut. But while smoking cigarettes has exacerbates Crohn’s disease, it seems to alleviate ulcerative colitis.

At a recent conference on IBD in San Francisco, IBD expert Stephen Hanauer speculated that everything from our diets to tylenol to colder climates might be to blame for the recent rise in IBD diagnoses, especially in developed countries. But cigarette smoking, which is largely a 20th Century phenomenon, doesn’t gel with theories about IBD being a consequence of modern lifestyles, because it can actually ward off ulcerative colitis.

Hanauer is one of many IBD doctors who has noticed over the years that his Crohn’s patients are often smokers, while his ulcerative colitis patients are often ex-smokers. Cigarette smokers suffer from worse bouts, or “flares,” of Crohn’s disease than non-smokers, so doctors are happy to encourage Crohn’s disease patients to quit smoking. But when former smokers with ulcerative colitis relapse into their tobacco addictions, they are often rewarded with relief from their symptoms.What gives?

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At the conference, Hanauer was careful to avoid stating outright that smoking could cure ulcerative colitis, but he couldn’t deny that something in cigarette smoke was benefitting some of his patients. A handful of small studies has shown that nicotine might be a useful therapy for ulcerative colitis, though it remains unclear how it helps. Also puzzling: Nicotine therapy seems to alleviate one type of Crohn’s disease (in an unpublished study of a dozen patients), while smoking itself exacerbates Crohn’s disease. This observation would suggest that non-nicotine substances in cigarette smoke, of which there are many, may stoke and/or soothe the symptoms of Crohn’s and ulcerative colitis, respectively.

Smoking Increases the Risk of Dementia, but Protects Against Parkinson’s Disease

The same toxic compounds in cigarette smoke that damage the heart and lungs also damage blood vessels and neurons in the brain. Amongst smokers who don’t succumb early to lung cancer, emphysema, or heart attacks, smoking has been linked to an increased likelihood of developing Alzheimer’s disease, and dementia in general, later in life. What’s bad for the body, it seems, is also bad for the brain.

And yet, studies have observed that the more people smoke, the less likely they are to develop Parkinson’s disease. This inverse correlation was first observed in the 1960s, by epidemiologist Harold Kahn. When he analyzed the health insurance records of nearly 300,000 veterans, Kahn noticed that smokers who had served in the US military between 1917 and 1940 were three-times less likely than non-smokers to have later died of Parkinson’s disease, even though they were over ten times more likely to have died of lung cancer or emphysema than non-smokers. This finding has since been bolstered by numerous studies.

In cases of Parkinson’s disease, dopamine producing neurons in the midbrain degenerate and die. But nicotine appears to protect these neurons, at least in rodent models of the disease. Clinical trials using nicotine to treat Parkinson’s disease in humans are currently underway, but doctors still do not recommend using nicotine (like nicotine patches) unless you’re actually trying to quit cigarettes.

Nicotine does improve certain aspects of human cognition, and it is being explored not only to treat Parkinson’s, but other dementias, and even everyday inattention. But the relationship between Parkinson’s and smoking probably involves more than just nicotine, which, again, is but one component of cigarette smoke. While the latter is believed to increase one’s risk of developing dementia, the former may be useful in treating it.

The Fickle Relationship Between Obesity and Smoking

Cigarette ads have long touted the appetite-suppressing qualities of tobacco. Even today, much of the public believes that cigarettes can be used to lose weight. Indeed, people do eat less when they smoke, and quitting smoking can cause a temporary spike in weight gain. Four years ago, scientists finally identified how nicotine curbs the appetite via a hunger circuit in the brain, lending an explanation for decades of studies that broadly correlated smoking with lower body weight.

Bruce Henschel, CC BY-NC-ND 2.0.

But the devil is in the details of this correlation. Some people smoke multiple packs a day, while others smoke only with their friends over the weekend, and it’s only on average that smokers weigh less than non-smokers. Recent studies that examined the medical records of hundreds of thousands of people have found that heavy smoking (40+ cigarettes per day) is strongly associated with an increased risk of obesity, while light smoking (on the order of a dozen cigarettes per day) has the opposite effect. Moreover, younger smokers do not appear to benefit from the general association of smoking and lower body weight.

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Why do hardcore cigarette addicts and young people miss out on the appetite-suppressing, weight-controlling effects of smoking? It turns out that other obesity risk factors, like low levels of physical activity, and poor diet, are associated with smoking more cigarettes. Add it all up, and the weight-loss effects of smoking are quickly negated by television and fast food. It’s impossible to draw causal conclusions from these types of studies, which often rely on people’s self-reports of their own behavior, but clearly, smoking is no panacea for obesity.

False Promises: Some “Smoker’s Paradoxes” May Not Exist at All

Some smoker’s paradoxes may not be paradoxes at all. The phrase itself, “smoker’s paradox,” originally referred to the observation that smokers seem to have better short-term health outcomes than non-smokers following hospitalization for heart attacks. But this “paradox” may have a straightforward explanation: Smokers are notorious for having heart attacks earlier in life than non-smokers, when their bodies are generally more resilient.

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Science bears this idea out. Smokers are hospitalized for heart attacks eleven years earlier, on average, than non-smokers, and this age gap “generally accounted” for the formers’ more favorable outcomes. Of note, most studies claiming this smoking/heart attack paradox are over 20 years old, and some doctors have argued that the age-dependency of the smoker’s paradox has been rendered irrelevant in the face of modern therapies for heart attack victims.

Other doctors continue to debate the merits of this paradox. A recent meta-analysis of 26 studies, encompassing the medical records of over 700,000 patients, concluded that smokers do benefit from reduced mortality after a heart attack, but only for one month after being hospitalized. But the authors also note that their study couldn’t possibly account for people who never made it to the hospital in the first place, stating: “Thus, the ‘smoker’s paradox’ may be largely explained by a greater case fatality before admission to hospital in smokers.” Compared to non-smokers, smokers are much more likely to die immediately following a heart attack; less likely to die within 30 days if they are admitted to the hospital; and just as likely to die further in the future after a heart attack.

Studies on the “smoker’s paradox” highlight the risk of another paradox: Simpson’s paradox, which occurs when an apparent result (in this case, smokers recovering more quickly from heart attacks than non-smokers) is actually a better reflection of some unacknowledged third factor (like age, exclusion of pre-hospital admission deaths). Simpson’s paradox has even led scientists to mistakenly conclude that smoking could save lives.

Jane Mejdahl, CC BY-SA 2.0

A few other purported smoker’s paradoxes are worth mentioning. This study found that smokers were less likely to need knee replacement surgery than non-smokers, while other studies have shown that complications and failures of joint replacements are much higher in smokers. Another study intriguingly found that trauma patients were less likely to die in the hospital if they were smokers, but the study only looked at patients under the age of 65 and did not follow up on mortality following release from the hospital (nor could it account for trauma-related death prior to hospitalization). Perhaps there is something to these findings, but it will take some time for these paradoxes to be bolstered by further research.

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One smoker’s paradox that almost certainly does not hold water is the “observation” that smoking prevents Alzheimer’s disease. It turns out that most of the underlying studies to arrive at this conclusion were funded by the tobacco industry. In 2010, a group from UCSF compared 43 studies on smoking and Alzheimer’s disease and found one-quarter of them were funded by tobacco groups. Those industry-funded studies demonstrated a protective effect of smoking against Alzheimer’s; independently-funded studies, on the other hand, clearly showed that smoking increases the risk of Alzheimer’s disease. Why wasn’t this caught sooner? The affiliations of many of the involved doctors and scientists had been hidden for years: “To determine if study authors had connections to the tobacco industry, the UCSF team analyzed 877 previously secret tobacco industry documents,” UCSF News Center reported.

It’s not easy to study human health, let alone the potential benefits of one of our deadliest recreations. Maybe someday we’ll have nicotine-based therapies for our ailments, but the smoker’s paradox is best chewed on by doctors, statisticians, and amateur health science enthusiasts, not the average person who lights up. Doctors today are unanimous in their opposition to smoking, even as some smokers dodge Parkinson’s disease, ulcerative colitis, and Saturday night desserts while playing the ultimate game of chicken with their hearts and lungs.

Top image: Matthias Weinberger. CC BY-NC-ND 2.0.