Tobacco smoke contains literally thousands of chemicals, including the addictive substance nicotine, and toxins such as carbon monoxide and cyanide. Not surprisingly, smoking is known to increase the risk of a number of serious diseases, including cardiovascular disease, respiratory illnesses, and certain types of cancer. It is also linked to significantly worse outcomes among patients with Crohn’s disease.
Smoking is generally bad for health, and it’s clearly harmful in the context of Crohn’s disease. Cigarette smoke can cause the death of mucosal cells, which line the gastrointestinal (GI) tract. It also inhibits mucosal cell renewal, decreases blood flow in the GI mucosa, and interferes with the mucosal immune system.
Inflammatory bowel disease (IBD) comprises Crohn’s disease and ulcerative colitis. IBD is believed to depend on an interplay between host, genetic, and environmental factors. IBD is characterized by immune system dysfunction, in which the patient’s immune system inappropriately attacks the cells of the GI mucosa.
Under ordinary circumstances, this inflammatory response to a perceived threat from an external invader would abate as soon as the threat is eliminated. But for reasons that are still being unraveled, the immune system continues to attack the mucosa in patients with IBD. While symptoms may subside or seem to disappear entirely, this state of remission is usually only temporary. Of the three factors involved in the development and progression of IBD, the one that a patient has the most control over, is environmental.
An environmental factor can include anything from exposure to certain bacteria or viruses, which is largely uncontrollable, to diet, exercise, medications, and exposure to pollutants. Smoking falls under the latter category. It is an environmental factor that involves the willful introduction of pollutants into the body. It’s also avoidable.
For reasons that are not entirely clear, ulcerative colitis is one of the few illnesses in which smoking may offer a small measure of protection against flares. Scientists are exploring the possibility of using nicotine as a treatment to help sustain remission of disease activity. That’s not to say that ulcerative colitis patients should start smoking, or ignore the need to quit. Smoking remains, on balance, a harmful activity.
But Crohn’s disease is clearly aggravated by smoking. In fact, it’s considered a major risk factor for the disease. Smoking is associated with a more severe disease course in Crohn's, more frequent relapses, and worse outcomes after surgical repair procedures. Indeed, it’s been linked to a greater likelihood of disease recurrence following surgery. Furthermore, people who smoke are more likely to develop Crohn's disease in the first place. The response to medications prescribed to treat the disease is poorer among current smokers. Smokers tend to need higher doses of various medications for longer periods to achieve disease remission than non-smokers.
The goal of treatment is to induce healing in the gut mucosa. But a recent study of more than 3,000 Crohn’s disease patients concluded that smokers had more strictures (abnormal narrowing of the digestive tract), and required medications more often, including steroids, immunosuppressants, and anti-tumor necrosis factor (biologic) drugs. Strictures are associated with an increased need for surgery in Crohn's disease. Crohn's patients who were smokers were also more likely to require thiopurine drugs, and to have perianal complications (i.e. abscesses and fistulae).
Although it’s unclear how exactly smoking makes Crohn's disease worse, experts have noted several possibilities. According to Harvard-based researchers, “Smoking may […] affect the colonic mucus layer, modify cytokine production, modulate humoral and cellular immunity (innate and adaptive responses), reduce smooth muscle tone and activity, change gut permeability, and affect the microvasculature.” That’s in addition to the harm that may be done by highly reactive molecules called free radicals, which are present in cigarette smoke, and carbon monoxide, which is a highly toxic gas.
The bottom line is clear: smoking is exceptionally harmful in Crohn's disease. Smokers are more likely than non-smokers to be diagnosed with Crohn's in the first place. Smokers with Crohn's are likely to require more aggressive drug therapy than non-smokers, and to have a more severe disease course. Conversely, smokers who quit are likely to experience a decrease in Crohn's disease severity.
References Nunes T, Etchevers MJ, et al. Smoking does influence disease behaviour and impacts the need for therapy in Crohn's disease in the biologic era. Aliment Pharmacol Ther. 2013 Oct;38(7):752-60. doi: 10.1111/apt.12440. Epub 2013 Aug 26.
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