What do we know already?

Diabetes means you have too much sugar in your blood. Usually, the way your body uses sugar is controlled by a hormone called insulin. But if you have type 2 diabetes (the more common type) your body can't use insulin properly.

People are more at risk of getting type 2 diabetes if they have a relative with diabetes, are overweight, are older, or had diabetes during pregnancy. Smoking tobacco also increases your risk of getting diabetes, although we don't know why this is.

Although people who smoke have more chance of getting diabetes than people who've never smoked, we don't know what effect stopping smoking has on your diabetes risk. Of course, we do know that stopping smoking has a good effect on the health of your lungs, and can cut your chances of having a heart attack or stroke. But this is the first big study looking at what happens to people's risk of diabetes when they stop smoking. The researchers followed up more than 10,000 people without diabetes at the start of the study.

What does the new study say?

Surprisingly, the risk of diabetes went up when people stopped smoking, especially in the first three years. It then slowly reduced, over about 10 years, down to the level of risk of someone who never smoked. During the first three years after stopping, the risk of diabetes (compared with someone who never smoked) was about 70 percent higher. The risk for someone who continued smoking was about 30 percent higher than someone who never smoked.

The researchers think most of this increased risk is because people tended to put on weight after stopping smoking. This may be because they ate snacks or sweets when they got a craving for a cigarette. The researchers said the risk of diabetes from stopping smoking was almost cancelled out when they took account of the amount of weight people gained. So a smoker who didn't gain weight after stopping might have little or no increased risk of diabetes.

The researchers say doctors should advise patients stopping smoking about how to avoid gaining weight, and should monitor them for diabetes in the years after they've stopped smoking.

How reliable are the findings?

This was a big, well-run study and the researchers tried to account for everything that might have affected the results. But it's possible that there was something specific about the people who stopped smoking that meant they were at higher risk of diabetes, that the researchers didn't manage to account for.

Where does the study come from?

The study was done by researchers at universities in the US and in Brazil. It was published in a journal called the Annals of Internal Medicine. The study was funded by grants from the National Heart, Lung, and Blood Institute and National Institute of Diabetes and Digestive and Kidney Diseases in the US.

What does this mean for me?

Stopping smoking is still the best thing you can do to improve your health. But it's a good idea to keep an eye on your weight, and to check you don't pile on the pounds after you stop smoking. There are many ways to get help with stopping smoking. Your GP surgery will be delighted to help you quit. You can also get advice and help, including a stop-smoking kit, through the NHS free smoking helpline on 0800 022 4332, or see the website http://smokefree.nhs.uk.

From:

Yeh H, Duncan BB, Schmidt MI, et al. Smoking, smoking cessation, and risk for type 2 diabetes mellitus: a cohort study. Annals of Internal Medicine 2010;152:10–17.

To find out more about which treatments work for stopping smoking, see our information on smoking.

© BMJ Publishing Group Limited ("BMJ Group") 2010