A meta-analysis of 88 studies covering nearly 6 million people published in The Lancet Diabetes & Endocrinology shows that both active and passive smoking are linked to increased risk of type 2 diabetes, while the risk in those who quit smoking decreases as time elapses since their last cigarette.
Evidence has been building on the link between smoking and diabetes risk. The 2014 US Surgeon General’s Report, for the first time, included a section of smoking and diabetes risk and argued for the causal relation between them, but it did not discuss the relation of passive smoking and smoking cessation with diabetes risk. In this new research, the authors combined the results of 88 studies to establish the association between active, passive and former smoking, as well as quitting, with risk of type 2 diabetes.
The search included 88 prospective studies with nearly 6 million participants and 295 446 incident cases. Compared with never smoking, current smoking (84 studies) increased the risk of type 2 diabetes by 37%; former smoking (47 studies) by 14%; and passive smoking (7 studies) by 22%.
The associations persisted in all subgroups, and a dose-response relation was found for current smoking and diabetes risk: the increased risk of developing type 2 diabetes was 21%, 34%, and 57% for light, moderate, and heavy smokers, respectively, compared with never smokers.
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 worldwide if smoking is causally related to diabetes. The attributable fraction could be much higher in populations with a higher smoking prevalence, such as men in China.
Finally, compared to never smokers, there was a 54% increased risk of diabetes in new quitters (who quit less than 5 years ago) which fell to 18% increased risk after 5 years and 11% increased risk in long-term quitters (more than 10 years).
In addition to the need to reduce smoking prevalence, the authors say on passive smoking that:
Our findings further emphasise the importance of adoption and enforcement of smoke-free legislation to reduce the number of people exposed to second-hand smoke.
The authors conclude that:
In view of the high prevalence of smoking in many countries and the increasing burden of diabetes worldwide, reducing tobacco use should be prioritised as a key public health strategy, which could potentially contribute to prevention and control of diabetes. More efforts are needed to implement and enforce the provisions of the WHO Framework Convention on Tobacco Control to reduce cigarette consumption among current smokers. Smoke-free policies can provide protection for non-smokers and can help smokers in their attempts to quit. Finally, the mechanisms underlying the short-term increased risk of diabetes in recent quitters need to be further explored, since an improved understanding of this effect could contribute to the development of targeted pharmaceutical and lifestyle interventions that could be used to aid smoking cessation and prevent the occurrence of type 2 diabetes.
Writing in a linked Comment, five authors represented by Professor Naveed Sattar, University of Glasgow, UK, say:
(Doctors) should mention that, as well as being a risk factor for cardiovascular disease and many cancers, smoking should also be regarded as a risk factor for diabetes (albeit with a small effect relative to, for example, lung cancer)… 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.