Beyond the bowel
Smoking runs in opposite directions in Crohn’s and colitis
Almost everything that affects Crohn’s disease and ulcerative colitis affects them the same way. Smoking does not — and the review that documents the pattern says plainly that it cannot explain it.
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There is one exposure in inflammatory bowel disease that does not behave like the others. Almost everything that affects Crohn’s disease and ulcerative colitis affects them in the same direction. Smoking does not. A review in Frontiers in Immunology in 2018 set out what is known about it, and the honest summary is that the association is well documented in both directions and remains unexplained.
In Crohn’s disease
Here the direction is the expected one. The review reports that early smoking significantly increases the risk of developing Crohn’s disease, with an odds ratio of 2.0 and a confidence interval of 1.65 to 2.47. It also describes worse outcomes among people who already have it: lower quality of life, higher rates of hospitalisation, greater need for surgery, and more complications.
The starkest figure concerns what happens after an operation. Among people who smoke and have had an ileal resection, the review cites 70 per cent showing visible lesions at the surgical join within the first year, against 35 per cent of those who do not smoke.
In ulcerative colitis
Here it runs the other way, which is the part that surprises people. A meta-analysis the review cites found the risk of ulcerative colitis significantly lower among people who smoke than among those who never have, with an odds ratio of 0.58 and an interval of 0.45 to 0.75. The review further reports that rates of hospitalisation and relapse, the need for more potent drugs, and colectomy are all significantly lower in patients who smoke.
And when people stop
The two conditions diverge again on cessation, and this is where the practical weight of the finding sits. In Crohn’s disease, stopping works: the review states that after two years without smoking, the disease activity and treatment requirement of former smokers match those of people who never smoked. In ulcerative colitis the association reverses — the relative risk of developing the disease rises after cessation compared with never-smokers, at an odds ratio of 1.64 with an interval of 1.36 to 1.98.
What the review reports in Crohn’s disease
- higher risk of developing it, at an odds ratio of 2.0
- more admissions, more surgery, more complications
- lesions at the surgical join in 70 per cent within a year, against 35 per cent
- after two years without smoking, activity matching never-smokers
What the review reports in ulcerative colitis
- lower risk of developing it, at an odds ratio of 0.58
- lower rates of hospitalisation, relapse and colectomy
- risk of developing it rising after stopping, at an odds ratio of 1.64
Two things follow from all of this, and neither of them is a recommendation. The first is that these are associations reported in a review of disease risk and disease course, and that review examines inflammatory bowel disease alone. It is not an assessment of what smoking does to the rest of a body, which is outside its scope and outside this post’s.
The second is that a decision about smoking, in either condition, belongs with the team who knows your diagnosis, your disease extent and everything else on your chart. What this post can honestly do is report that the two conditions genuinely differ here, and that anyone who has been told the same rule applies to both has been told half of it.
Where this came from
The odds ratios of 2.0 for developing Crohn’s disease, 0.58 for ulcerative colitis among smokers and 1.64 after cessation; the 70 per cent against 35 per cent post-resection lesion figures; the two-year cessation finding in Crohn’s disease; and the authors’ statement that the proposed mechanisms fail to explain the opposite effects.
Berkowitz L, Schultz BM, Salazar GA, Pardo-Roa C, Sebastián VP, Álvarez-Lobos MM, Bueno SM, “Impact of Cigarette Smoking on the Gastrointestinal Tract Inflammation: Opposing Effects in Crohn’s Disease and Ulcerative Colitis”, Frontiers in Immunology, 2018;9:74