Passive Smoking Increases Risk for Type 2 Diabetes

Veronica Hackethal, MD

September 22, 2015

A new meta-analysis that included almost six million people has found that current smokers and people exposed to secondhand smoke have a significantly increased risk of developing type 2 diabetes, and this risk decreases in quitters over time. The study was published online September 18 in Lancet Diabetes & Endocrinology.

The results suggest that 2.4% of type 2 diabetes cases worldwide in women and 11.7% in men — 27.8 million cases total — could be traced back to active smoking, assuming a direct cause between smoking and type 2 diabetes.

"Despite global efforts to combat the tobacco epidemic, smoking remains a major public-health threat for many countries. Our findings provide strong evidence that smoking is related to increased risk of diabetes and lend further support to the WHO Framework Convention on Tobacco Control," commented first author An Pan, PhD, assistant dean of the School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, in Wuhan, China.

Dr An has also recently reported other research showing that being a smoker and having either type 1 or type 2 diabetes increases the relative risk of total mortality and cardiovascular events by about 50% and that quitting smoking can reduce these risks.

"Smoking cessation and avoidance of secondhand smoke remains a major target for diabetes prevention. Although diabetes risk remains high in recent quitters, the risk substantially reduces as the time since quitting increases," he emphasized.

Smoking kills about six million people worldwide each year and is the leading cause of avoidable disease globally.

The links between smoking and increased risk for cancer and respiratory and cardiovascular disease are well-known, but the association between smoking and diabetes has received less attention.

Accumulating research, though, has shown that smoking increases the risk for type 2 diabetes. In 2014, the US Surgeon General's report included a section about smoking and diabetes risk for the first time, pointing to a causal relationship between the two.

Secondhand Smoke Now Definitively Linked to Diabetes

The new meta-analysis included 88 prospective studies published in English and identified by searching Medline up to May 2015 and Embase up to April 2014. It included nearly six million people and almost 300,000 incident cases of type 2 diabetes. Most trials were conducted in Europe, the United States, and Japan. Five studies included only women, while 27 included just men.

Compared with never smokers, current smokers had a 37% increased risk for type 2 diabetes (relative risk [RR], 1.37; 95% CI, 1.33–1.42). This decreased to a 14% higher risk in former smokers and was 22% higher in people exposed to secondhand smoke (RR, 1.22 for diabetes).

Although studies varied largely regarding methodology and populations, these associations remained in subgroup analyses.

"For secondhand smoke, [past studies have] clearly shown that passive smoking is related to increased risk of cancer and cardiovascular disease and now diabetes as well," Dr Pan pointed out.

"Avoidance of secondhand smoke is certainly recommended. Creating a smoke-free environment is also important, at the policy level as well as at the office and family level," he added.

The study also found a dose-response relationship between smoking and diabetes risk. Compared with never smokers, light smokers had a 21% increased risk of type 2 diabetes, moderate smokers had a 34% increased risk, and heavy smokers had a 57% increased risk.

And after a slight increase initially, diabetes risk declined in quitters over time.

Compared with never smokers, people who had given up smoking in the past 5 years had a 54% increased risk of type 2 diabetes (RR, 1.54; 95% CI, 1.36–1.74), those who quit within the past 5 to 9 years had an 18% increased risk, and those who stopped smoking 10 or more years ago had an 11% increased risk.

Quitting Smoking: Benefits Far Outweigh Short-term Metabolic Changes

Dr Pan noted that some people may be put off trying to stop smoking because of issues such as weight gain.

"The short-term increased risk of smoking cessation on diabetes risk may be a barrier for some people who want to quit smoking," he pointed out, "However, the long-term benefits clearly outweigh the short-term risk.

"Clinicians need to closely monitor metabolic changes like weight gain and glycemic control after smoking cessation in patients," he advised, "Patients should also be aware of the potential short-term metabolic changes after smoking cessation. More support from family members and friends is needed as well."

Writing in an accompanying editorial, Naveed Sattar, MD, PhD, from the University of Glasgow, Scotland, and others discuss the subjects of weight gain after smoking cessation and worse diabetes control after quitting.

But they also point to recent research that suggests that smoking, while lowering body mass index (BMI), may redistribute body weight into a pattern favoring central obesity, which in turn increases diabetes risk. Although this study was observational in nature, they still recommend that doctors should advise patients that smoking increases the risk of diabetes.

"The link between smoking and diabetes should be increasingly appreciated in clinical settings," they write, "Patients who smoke should also be informed that stopping smoking and maintaining long-term abstinence will not only lessen their cardiovascular and cancer risks, but over time, might also lessen their diabetes risk.

"Of course, convincing people to never take up smoking would be even better; in this respect, public-health messages should perhaps now include diabetes on the list of smoking-related harms."

The authors report no relevant financial relationships. Dr Sattar reports no relevant financial relationships; disclosures for the coeditorialists are listed in the paper

Lancet Diabetes Endocrinol. Published online September 18, 2015. Abstract, Editorial

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