Dr. Carole Willi of the University of Lausanne led a systemic review and meta-analysis of 25 studies -- encompassing a total of 1.2 million participants -- that shows active smokers have a 44 percent increased risk of developing type 2 diabetes compared with non-smokers.

Further analysis suggests a dose-response relationship between smoking and diabetes, with an increased risk of 61 percent for those who smoke 20 or more cigarettes/day. The association also is weaker for former smokers.

"The relevant question should no longer be whether this association exists, but rather whether this established association is causal," the study authors say in a statement. "We recommend that future studies focus on plausible causal mechanisms or mediating factors such as obesity, lack of physical activity, dietary habits and stress levels."

The findings are published in the Journal of the American Medical Association.