A recent study by Gabriel Arefalk and colleagues at the University of Uppsala in Sweden, published in the American Heart Association flagship journal Circulation (abstract here), was purported by the authors, the AHA (here) and the media (here) to show that continuing snus use or smoking after a heart attack (myocardial infarction, MI) is twice as harmful as quitting.
We have submitted a letter to the editor of Circulation, and Carl has the full text of our letter in his CASAA blog post (here).
The bottom line is that the authors tried to spin the results as suggesting that continuing snus use is dangerous after an MI. In fact, continuing snus users actually had a lower death rate than people who used neither snus nor cigarettes.
Whatever is happening in this population, it clearly does not support the simplistic “snus is bad” mantra. There is a glaringly obvious explanation for why people who quit snus (or smoking) after an MI fare better than those who do not: Those who are healthy (except for the recent MI, of course) and hope to recover are more likely to take steps to minimize their risks. After being advised to give up snus, many also get physical therapy, exercise and maintain a healthier diet. Meanwhile, those who are less healthy may not make changes in an attempt to regain long-term health. The Arefalk analysis may not have adequately controlled for these confounding factors.
Of course, this would only partially explain the better outcomes of quitters compared to continuing users; it does nothing to explain why all of them (except those who continued to smoke) apparently fared better than non-users. There are possible explanations for this in the form of statistical artifacts or real effects. The key observation is that these unreported results do not support the authors’ main interpretation that snus use is dangerous after an MI.
With the publication of this article, peer review appears to have been woefully inadequate. The prime statistical error we discovered is the key number reported in the first paragraph of the article’s results section. Reviewers of this study failed to detect the glaring error.
Even without correcting that error or calculating the mortality rate for non-users, the (incorrect) number for the population as a whole the authors reported can still be compared to rates for people who used snus or cigarettes at the time of their MI. This is enough to raise red flags about the analysis and conclusions, since it is still higher than the rate for those who kept using snus, and far higher than the rates for those who stopped using either product.
In 2011 Arefalk was lead author on a study making dubious claims about snus use and heart failure. I described that effort as “neither legitimate nor persuasive until the authors resolve the fundamental questions about the analysis.” (here).
The current article in Circulation is a classic example of anti-tobacco propaganda. Credible epidemiologic studies do not report risks in exposed groups without reporting the comparable baseline risk among the unexposed. The authors, and the journal editors and reviewers who enabled them, omitted this critical information. Our letter to the editor gives them a chance to correct these deficiencies.