Passive smoke exposure and type-2 diabetes: is it time for action?

Abstract

In a recent meta-analysis [1] of six cohort studies, Sun et al. found evidence of a 21 % increased risk of type-2 diabetes or glucose intolerance among subjects exposed to passive smoking (95 % CI 7-38 %) compared to those who were not exposed. The authors conclude that such association may have a 'far-reaching significance' for public health. At a first glance, such conclusion might be justified. The metaanalysis appears to have good methodological quality, including an adequately powered pooled sample size (n = 154,406) and using a random effects model to address between study heterogeneity.Association, however, does not necessarily imply causation. Findings might be due to alternative explanations, although the large pooled sample size minimises the risk for random error. First, since the studies included in the meta-analysis are observational, passive smoking exposure could mask the effect of other causal factors of diabetes such as being overweight or obese, diet habits, and family history-and adjustments performed in individual studies may have still left considerable residual confounding. Second, assessment of the exposure could be biased and overestimated among those with diabetes due to lack of blinding of the assessors.