Two of the most common lifestyle-related conditions account for almost a million new cancers worldwide each year, a new study has found.
Diabetes and excess weight were responsible for nearly 800,000 of newly diagnosed cancers, including those affecting liver, breast, bowel and womb, say researchers from Imperial College, London.
It is the first time scientists have estimated the worldwide cancer burden caused by being overweight or obese, as defined by a high body mass index (BMI), and the metabolic disease thought to affect more than four million people in the UK.
Lead researcher Dr Jonathan Pearson-Stuttard, from Imperial's School of Public Health, said: "While obesity has been associated with cancer for some time, the link between diabetes and cancer has only been established quite recently.
"Our study shows that diabetes, either on its own or combined with being overweight, is responsible for hundreds of thousands of cancer cases each year across the world."
The vast majority of diabetes cases are Type 2, which is strongly linked to poor diet, excess weight and inactivity, as well as genetics.
On its own, being overweight was responsible for almost twice as many cancers as diabetes, 544,300 versus 280,100 cases.
Cancers linked to the two conditions were also nearly twice as common in women than in men.
How diabetes can trigger cancer is still being investigated but high insulin and blood sugar levels, chronic inflammation and sex hormone disruption may all play a role, the scientists believe.
In men, liver cancer was the most common cancer linked to diabetes and high BMI, followed by bowel cancer.
For women, breast cancer and endometrial cancer had the strongest link with diabetes and being overweight. Breast cancer accounted for nearly 30% of cancers thought to be caused by the conditions.
The study, published in The Lancet Diabetes & Endocrinology journal, is based on data from 175 countries in 2012. Rates of 12 different types of cancer were combined with population figures for high BMI and diabetes.
Dr Pearson-Stuttard added: ""It is vital that co-ordinated polices are implemented to tackle the shared risk factors and complications of chronic diseases such as obesity and diabetes."
Dr Emily Burns, from the charity Diabetes UK, said: "Diabetes doesn't directly cause cancer, but this study adds to the evidence that having diabetes can increase risk of certain types of cancer.
"We also need to recognise the individual types of diabetes, and understand how they might affect the risk of cancer differently."
