A recent review conducted by researchers at the University of Lausanne, Switzerland, concluded that people who currently smoke face an increase risk of developing type 2 diabetes compared to non-smokers.

Type 2 diabetes is often associated with excess body weight, poor diet and sedentary lifestyle and is the increasingly common disease in many countries, including Switzerland.

Carole Willi, M.D., of the University of Lausanne, Switzerland, and colleagues reviewed 25 previous studies, published from 1992 to 2006, about the association between active smoking and the incidence of diabetes or other glucose metabolism irregularities. The studies involved more than 1.2 million people aged 16 and older.

Researchers found that 45,844 developed type 2 diabetes during the studies, which lasted for 5 to 30 years.

"We knew a few studies had already assessed this link, but we didn't expect to find so many -- we found 25 studies and all except one showed that smokers faced an increased risk of diabetes," Carole Willi said.

The analyses of the existing data indicated that active smokers have a 44 percent increased risk of developing type-2 diabetes compared with non-smokers.

Heavy smokers, meaning people smoking more than 20 cigarettes per day have 61 percent increased risk compared with lighter smokers from which only 29 percent face increased risk. The review also found that quitting smoking reduced the danger, with former smokers seeing a 23 percent higher risk that non-smokers, far lower than the risk of current smokers.

Willi added that the studies cannot prove smoking was a cause of diabetes, but she said they did meet several recommended criteria to suggest this.

"First, there is an appropriate temporal relationship: the cigarette smoking preceded diabetes incidence in all studies. 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. Conversely, there are also possible non-causal explanations for this association. 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," the researcher wrote.

The review of the studies doesn’t show whether exercise, social class or education affected the results so the researchers recommended further studies which should focus on those issues.

The review appeared in the December 12 issue of The Journal of the American Medical Association