With commentary by Frank Hu, MD, PhD, MPH, professor of nutrition and epidemiology, Harvard T.H. Chan School of Public Health, Boston
The findings may come as a surprise to many people, says Frank Hu, MD, PhD, MPH, professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health, Boston, and a co-author of the research. People tend to think of excess weight as a major risk factor for getting type 2 diabetes, and it is, he says. But so is smoking, and his new analysis strengthens the link. "I think this is another reason to stop smoking," Dr. Hu says.
With his colleagues, Hu reviewed the findings of 88 previously published studies on the link between smoking and diabetes risk, analyzing information from about 6 million study participants.
"This is the most comprehensive meta-analysis of numerous published studies from around the world," he says. "I think the evidence from our meta-analysis is very compelling and that the link is real."
The study was published online September 17 in the Lancet Diabetes & Endocrinology.
In the new analysis, the researchers found:
While passive smoking may not be much of public health issue in the U.S., with smoking outlawed in many public venues, it is still an issue globally, Dr. Hu says. "The prevalence of passing smoking is very high, almost 30% [of people] around the world."
While evidence about smoking and its links to cancer, respiratory disease and heart disease is well known, the information about its link to diabetes has been slower to build, the researchers say. In 2014, for the first time, the U.S. Surgeon General included a section on smoking and the risk of diabetes, arguing that the link is cause and effect.
Hu's analysis finds a link, or association. What may explain it? Dr. Hu points to smokers' higher abdominal fat, increased resistance to insulin (which helps regular blood sugar) and toxic substances in nicotine that may damage the insulin-producing beta cells in the pancreas. All of these explanations may potentially explain the link, he says.
Most patients are largely unaware that smoking is a risk for type 2 diabetes, says Robert Gabbay, MD, PhD, chief medical officer at Joslin Diabetes Clinic, Boston, who reviewed the findings. However, he says, "I would argue that most people who smoke know that it's not good for them. This is yet another reason why it's not good."
The strength of the new research is the size of the analysis, including 6 million people, he says. For those who still smoke, the good news message from the research is that ''It doesn't matter how long you have smoked, quitting still helps. Every year of not smoking helps to reduce your risk of diabetes. That's an important message to get out."
"It's a good, observational study," says Spyros Mezitis, MD, PhD, an endocrinology consultant at Lenox Hill Hospital, New York, who also reviewed the findings. One limitation is that the smoking history and status were self-reported, which is always subject to errors, he says.
Even so, the new analysis does strengthen the link, he agrees. "We don't have a firm understanding of how smoking affects the diabetes," he says. However, ''we do understand that diabetes is an inflammatory disease, and that this inflammation can be increased with smoking. Smoking is a risk factor for inflammation, and inflammation can cause insulin resistance."
The message is clear, that quitting smoking can reduce risks for numerous health issues, experts agree. However, weight gain is common in ex-smokers and being overweight or obese, in turn, can boost diabetes risk.
Some research has found that increasing physical activity can counterbalance the weight gain linked with smoking cessation, Dr. Hu says. Physical activity can reduce cravings and withdrawal symptoms and weight gain while a smoker is trying to quit, researchers concluded after reviewing the results of 17 studies.