People who smoke have an increased risk of developing type 2 diabetes compared with nonsmokers, according to an article in the December 12 issue of The Journal of the American Medical Association.
Carole Willi, MD, University of Lausanne, Switzerland, and colleagues conducted a systematic review and meta-analysis of studies describing the association between active smoking and the incidence of diabetes or other glucose metabolism irregularities. A database search yielded 25 studies published between 1992 and 2006. The number of participants per study ranged from 630 to 709,827. The studies reported a total of 45,844 new cases of diabetes during a follow-up period ranging from five to 30 years.
The data indicate that smokers have a 44 percent increased risk of developing type 2 diabetes compared with nonsmokers. Further analyses suggested a dose-response relationship, with the association stronger for heavy smokers (20 or more cigarettes per day; 61 percent increased risk) than for lighter smokers (29 percent increased risk). The association also was weaker for former smokers (23 percent increased risk) than for active smokers.
The researchers pointed out that observational primary studies cannot prove causality, but that these studies met several recommended criteria for causation.
"First, there is an appropriate temporal relationship: the cigarette smoking preceded diabetes incidence in all studies," the researchers wrote. "Second, the findings are consistent with a dose-response relationship, with stronger associations for heavy smokers relative to lighter smokers and for active smokers relative to former smokers. Third, there is theoretical biological plausibility for causality, in that smoking may lead to insulin resistance or inadequate compensatory insulin secretion responses according to several but not all studies," the researchers noted.
"Conversely, there are also possible noncausal explanations for this association," the researchers continued. "Smoking is often associated with other unhealthy behaviors that favor weight gain and/or diabetes, such as lack of physical activity, poor fruit and vegetable intake and high alcohol intake.
"Considering the consistent finding of increased diabetes incidence associated with active cigarette smoking across a large number of studies, we believe that there is no need for further cohort studies to test this hypothesis," the researchers wrote. "However, there is a need for studies that include detailed measurement and adjustment for potential confounding factors such as socioeconomic status, education and exercise with a goal of establishing whether the association with smoking is causal."
