TORONTO - Smokers appear to have a significantly higher risk of developing Type 2 diabetes compared to non-smokers - and that risk increases the more one lights up, an analysis of numerous international studies suggests.
In a review of 25 studies that pooled data from 1.2 million subjects from around the world, Canadian and Swiss researchers found that on average, tobacco users have a 44 per cent higher chance of developing the metabolic condition, which can lead to such complications as heart disease and kidney failure.
The link between smoking and diabetes appeared strongest among heavy smokers (20 or more cigarettes daily), who had a 61 per cent increased risk of developing the disease compared with non-smokers; lighter smokers (less than 20 a day) had a 29 per cent higher risk. The association was weaker for former smokers (23 per cent increased risk) than for active smokers.
"What we found is that smoking is indeed associated with the later development of diabetes, and it's a remarkably consistent association in the combined results of these 25 studies," said co-principal author Dr. William Ghali, a professor of medicine and community health sciences at the University of Calgary.
The analysis, which included studies published from 1992 to 2006 and followed participants in some cases for up to 30 years, appears in Wednesday's issue of the Journal of the American Medical Association.
Although these observational studies show an association between smoking and the development of Type 2 diabetes, even taken together they cannot prove tobacco use is directly responsible for the condition, Ghali said. (Only randomized controlled trials comparing smokers and non-smokers matched for age, sex and other factors could prove causality.)
"Smoking is often associated with other unhealthy behaviours that can lead to diabetes, including physical inactivity, poor diet and high alcohol consumption," he said. "It might not be the smoking, per se, but those other mediating factors that are related to smoking and separately related to diabetes."
Still, the relationship appears so strong - the risk rises the more one smokes - "it may be that smoking is associated independently . . . and (is) more likely to be a causal link," Ghali speculated.
"In all of the studies, the smoking preceded the development of diabetes."
Ghali said there is also a biological basis for pointing a finger at tobacco as a potential cause of Type 2 diabetes: some research suggests that smoking may lead to insulin resistance or inadequate insulin secretion, both hallmarks of the disease that results in the body's inability to properly metabolize glucose (sugar).
As well, smoking is associated with cancer of the pancreas, the organ that produces insulin.
"In trying to put together a case for saying this is causal, it kind of catches the attention that there is obviously some toxicity to that particular organ," Ghali said. "So it does make it compelling to say, well perhaps that's yet another argument that there are effects on insulin secretion - and insulin secretion occurs in the pancreas."
"So there's lots of reasons not to smoke."
Dr. Bernard Zinman, a diabetes expert at Mount Sinai Hospital in Toronto, said Tuesday the finding should provide a wake-up call to smokers.
"Right now, we're clearly in the midst of an epidemic globally of diabetes," said Zinman, who was not involved in the research review. "There's no question that there are many factors that are contributing to the diabetes epidemic, the most important one of which appears to be the obesity epidemic and the fact that we're far more sedentary."
"So here we see there's another risk factor - and the risk factor is a history of smoking."
Besides being a known cause of several types of cancer and a contributor to cardiovascular disease, smoking is also associated with the development of intra-abdominal fat that boosts the risk for diabetes and heart disease.
"So in the context of healthy behaviours and healthy lifestyles, this is a very, very important observation, particularly in some countries where there isn't that major effort to quit smoking," said Zinman, pointing to Asia, Japan and China as examples where smoking rates remain high.
In an editorial accompanying the paper, Dr. Frank Hu and research fellow Eric Ding of the Harvard School of Public Health say that given the rising incidence of Type 2 diabetes, it might be prudent for doctors to regularly check blood-sugar levels among current and former smokers.
As well, "recommendations for Type 2 diabetes prevention should incorporate smoking avoidance accompanied by lifestyle modification," they write. "Although a frequent concern of smoking cessation is subsequent weight gain, moderately increasing exercise can largely minimize the approximately (4.4-pound) weight gain associated with stopping smoking, indicating that the public health issues of smoking, exercise and obesity are inextricably intertwined."
"Major population prevention of Type 2 diabetes is achievable via avoidance of smoking and modification of lifestyle factors through a combination of healthy weight control, regular physical activity, moderate alcohol intake and proper diet."
