Quitters face an almost doubled risk of developing diabetes in their first three smoke-free years.
Researchers from Johns Hopkins University, Baltimore, studied 10,892 adult smokers, none of whom had diabetes at the start of the study.
The participants were studied for nine years during which time, 1,254 developed type-2 diabetes.
In the first three years after giving up, new quitters were 91 per cent more likely to develop diabetes. This decreased over time and after 12 years quitters had no excess risk.
What is behind the raised risk?
Extra weight put on by new quitters explains around a third of the increased risk, the researchers said. A further third of the excess risk is accounted for by systemic inflammation, as assessed by increased leukocyte counts.
However, after adjusting for this weight gain and inflammation, new quitters were still at higher risk compared with participants who continued smoking. This risk may be a result of differences in the two groups that the study was not designed to detect, say the researchers.
How important is this finding?
Smoking cessation has many health benefits that outweigh this short-term effect, the researchers point out.
Patients should, however, be made aware of the risk and advised to consider countermeasures, particularly for heavy smokers, they said.
Natasha Marsland, care adviser at Diabetes UK, said that 'on no account' should people use the results of this study as an excuse not to give up smoking. 'The health benefits of giving up smoking far outweigh the risk of developing type-2 diabetes from modest, short-term weight gain,' she said.
'You can be successful at giving up smoking and keeping to a healthy weight if you combine daily physical activity with a diet rich in fruit and vegetables.'
Martin Dockrell, director of policy and research and Action on Smoking and Health, said: 'The researchers are clear that smokers should quit but - especially if you are a heavy smoker or are already overweight - you might want to gently increase your exercise when you quit. If you are a smoker who is also overweight you should talk to your doctor about how to get the best from quitting.'
- Giving up smoking leads to a short-term increase in the risk of developing type-2 diabetes.
- Weight gain following successful quit attempts explain the raised risk.
- Smokers wanting to quit should be advised about how to reduce their diabetes risk.