Cigarette smoking is the number one avoidable cause of disease worldwide, killing nearly 6 million people a year. Its link with cancer, cardiovascular and pulmonary disease are well established. Now, a new report published in The Lancet Diabetes & Endocrinology adds to the growing body of evidence that smoking may be linked to type 2 diabetes.

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Researchers provide further evidence that smoking may lead to type 2 diabetes.

In 2014, the US Surgeon General’s Report mentioned smoking as a possible cause of type 2 diabetes for the first time. In response, a team of researchers have carried out a meta-analysis of 88 prospective studies involving nearly 6 million participants, of whom 295,446 had type 2 diabetes, to investigate a causal link between various smoking behaviors and diabetes.

The study, carried out by Prof. An Pan, of the Huazhong University of Science and Technology in Hubei, China, and Prof. Frank Hu, of the Harvard T.H. Chan School of Public Health in Boston, MA, indicates that both active and passive smoking are associated with significantly increased risks of type 2 diabetes.

Compared with never smoking, current smoking increased the risk of type 2 diabetes by 37%, former smoking by 14% and passive smoking by 22%.

Among current active smokers, the increased risk of developing type 2 diabetes is estimated to be 21% for light smokers, 34% for moderate smokers and 57% for heavy smokers, compared with never-smokers.

If smoking does contribute to diabetes, the authors estimate that 11.7% of global diabetes cases in men and 2.4% in women – approximately 27.8 million cases in total – are attributable to active cigarette smoking.

Interestingly, there was a 54% increased risk of diabetes in new quitters – those who quit less than 5 years ago – compared with those who had never smoked. This fell to an 18% increased risk after 5 years and 11% increased risk in long-term quitters – those who quit more than 10 years ago.

Fast facts about smoking
  • 17.8% of US adults currently smoke cigarettes, down from 20.9% in 2005
  • Smoking is the leading preventable cause of death in the US
  • Cigarette smoking causes more than 480,000 deaths annually in the US.

Learn more about smoking

Although former smokers appear to be at greater risk than those who have never smoked, the increased risk in the long run is much lower in quitters than it is in those who are still smoking, making avoidance of diabetes in the long term a valid argument for quitting smoking.

Could the higher risk of type 2 diabetes after giving up smoking be attributable to the weight increase that typically occurs at that stage, of 4-5 kg on average in the first year? Type 2 diabetes tends to be associated with high BMI, but the researchers found that even with adjustments for baseline body mass index (BMI), the increased diabetes risk remained.

Moreover, one study observed that male new quitters who gained less than 3 kg weight during the first 5 years after cessation were more likely to develop diabetes on average, whereas those who gained 3 kg or more did not, supporting the view that this weight gain in itself is not to blame.

This suggests that the common belief that smoking helps keep weight down might be misleading – it may reduce BMI, but not necessarily fat or waistlines.

One limitation considered was the fact that smoking is often related to other unhealthy lifestyle factors, such as poor diet, excessive alcohol use and physical inactivity, which could confound the results. However, no substantial differences were noted when adjustments were made for these factors.

The benefits of quitting smoking, such as reducing the risk of cardiovascular disease, are well documented, in both the short and long term, including in patients with diabetes. Therefore, the researchers say:

“The long-term benefits of smoking cessation on diabetes and other health outcomes would greatly offset the short-term increased risk of diabetes.”

While Sattar cautions that the link is not “definitive,” the researchers believe there is a strong case for associating type 2 diabetes with smoking and call for further research in the field. They also emphasize the importance of adopting and enforcing smoke-free legislation to reduce the number of people exposed to secondhand smoke.

If the association between smoking and risk of type 2 diabetes is causal, public health efforts to reduce smoking could have a substantial effect on the worldwide burden of type 2 diabetes.

A recent article in Medical News Today addressed the question of why many women resume smoking after quitting during pregnancy.

Written by Yvette Brazier