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Smokers have a sharply higher risk of developing diabetes, according to new research.

The study, published in today's edition of The Journal of the American Medical Association, found that smokers have a 44-per-cent increased risk of developing Type 2 diabetes compared with non-smokers.

"We can already give people plenty of reasons why they shouldn't smoke and this is yet another one," William Ghali, a professor of medicine and community health sciences at the University of Calgary and co-author of the research, said in an interview.

In a commentary also published in JAMA, Eric Ding of the division of preventive medicine at Harvard Medical School, said the findings provide a "conservative underestimate" of the impact of smoking on diabetes.

Dr. Ding estimated that, based on the new research, about one in eight cases of diabetes are actually caused by smoking. Given that close to three million Canadian suffer from diabetes, smoking has a significant but largely unrecognized impact.

Dr. Ghali stressed that the research does not definitely prove that smoking causes diabetes, but said there is enough evidence that physicians should warn patients about the link.

He pointed out that other lifestyle factors such as poor diet, lack of exercise, excessive alcohol consumption and obesity, as well as socio-economic factors such as poverty and poor education "go hand-in-hand with smoking," and they all play a role in diabetes.

But Dr. Ghali also noted research shows that there are biological mechanisms at play that bolster the argument that smoking can cause diabetes, in particular that smoking can interfere with insulin production and increase insulin resistance.

(In healthy people, cells in the pancreas respond to the level of glucose - a type of sugar - in the blood and secrete insulin as necessary. In Type 2 diabetes, a person gradually loses the ability to manufacture insulin or use it efficiently, leading to a host of complications from improper blood-sugar levels.)

The study found a dose response, meaning that the more someone smoked, the greater their risk of diabetes. Current smokers also have a higher risk of developing disease than former smokers.

"Smoking and diabetes is a very, very bad combination," Dr. Ghali said. "Both smoking and diabetes damage the heart, and the most significant cause of death in diabetics is cardiovascular."

The study, conducted by a team at the University of Lausanne in Switzerland (where Dr. Ghali was on sabbatical), is a meta-analysis, a compilation and analysis of previous research.

In this case, the findings from 25 previous studies examining the link between smoking and diabetes were combined.

In total, there were 1.2 million participants, including 45,844 who had diabetes. They were followed for periods ranging from five to 30 years.

Currently, about one in every 12 deaths worldwide is due to smoking. But the number of smoking deaths is projected to increase to more than 10 million a year by 2025, up from the current rate of four million annually.

Worldwide, almost 200 million people suffer from diabetes.