The study, led by Imperial College London, found that nearly six per cent of new worldwide cancer cases in 2012 were caused by the combined effects of diabetes and being overweight (BMI of over 25 kg/m2) or obese (BMI of over 30 kg/m2). For the 12 cancers types studied, diabetes and high BMI combined were responsible for nearly 800,000 new cancer cases.
As individual risk factors, being overweight was responsible for twice as many cancers as diabetes, 544,300 cases of cases were attributable to high BMI (equivalent to 3.9 per cent of all cancers), and 280,100 were attributable to diabetes (equivalent to two per cent).
One in four diabetes-related cancer cases in 2012 (77,000 cases) were attributable to the worldwide rise in diabetes between 1980 and 2002. Just under a third of weight-related cancer cases in 2012 (174,000) were attributable to the worldwide rise in the number of overweight and obese people over the same period.
Cancers caused by diabetes and being overweight or obese were almost twice as common in women as men, accounting for 496,700 and 295,900 overall cancer cases respectively.
The authors say that if global rates of diabetes and overweight continue to rise, the share of cancers attributable to the combined factors will increase by over 30 per cent in women and by 20 per cent in men by 2035.
The research is published today in The Lancet Diabetes & Endocrinology.
To conduct the study, the researchers gathered data on cases of 12 types of cancer from 175 countries in 2012. They combined this with data on high BMI and on diabetes. They matched the data sets by age group and gender.
The two factors accounted for a quarter of liver cancers and more than a third of all endometrial cancers worldwide. In men, liver cancer was the commonest cancer caused by diabetes and high BMI, accounting for 126,700 cases, or 42.8 per cent of all cancers caused by diabetes and high BMI. Colorectal cancer was the second commonest, accounting for 63,200 new cancer cases or 21.4 per cent.
In women, breast cancer was the commonest cancer caused by diabetes and high BMI, accounting for 147,400 cases or 29.7 per cent of such cancers. Endometrial cancer was the second commonest, accounting for 121,700 new cancer cases or 24.5 per cent.
The most cancer cases were seen in high-income western countries (38.2 per cent, 303,000 of 792,600 cases), however the largest increases from 1980 to 2002 were seen in low and middle-income countries. The leading cancer due to diabetes and high BMI in high-income western countries was breast cancer, accounting for 23.8 per cent of all cancers attributable to these risks. In East and South East Asia, liver cancer accounted for 53.8 per cent of cancers attributable to diabetes and high BMI.
The new figures highlight the substantial role of diabetes in cancer worldwide, particularly in lower income countries where rates of people being overweight, diabetes and cancer cases are soaring.
Dr Jonathan Pearson-Stuttard, lead author of the study 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 authors say that although the reason for the link between diabetes and cancer is still being investigated, high insulin or glucose levels, chronic inflammation, and sex hormone disruption are potential factors.
They say the figures highlight the need for effective food policies to tackle overweight and diabetes, and for clinicians to be aware of the high cancer risk carried by people of all ages who are overweight, have diabetes, or both.
Dr Pearson-Stuttard added: "Both clinical and public health efforts should focus on identifying effective preventive, control and screening measures to structurally alter our environment, such as increasing the availability and affordability of healthy foods, and reducing the consumption of unhealthy foods. It is vital that coordinated polices are implemented to tackle the shared risk factors and complications of chronic diseases such as obesity and diabetes.
"The distinct features of cancer patients are evolving throughout the world. In the past, smoking was by far the major risk factor for cancer, but now healthcare professionals should also be aware that patients who have diabetes or are overweight also have an increased risk of cancer.
The research was funded by NIHR and the Wellcome Trust.
The Lancet Diabetes & Endocrinology