People with diabetes could be up to three times more likely to get bowel cancer, research suggests.
Insulin can be a key part of diabetes treatment
Diabetes is caused by the inability of the body to break down sugar in the normal way.
It seems that the high sugar levels that result may also increase the risk of bowel cancer.
The research, by Cancer Research UK and the Medical Research Council, is published in Cancer Epidemiology and Biomarkers and Prevention.
The scientists tested a marker of the sugar levels in blood samples taken from almost 10,000 men and women aged between 45 and 79 and then checked their medical condition six years later.
They found that people with diabetes - and those with abnormal glucose metabolism which could lead to diabetes - were more likely to develop bowel cancer. The trend was stronger in men than women.
Lead researcher Professor Kay-Tee Khaw said: "The study shows that high sugar levels, even when they are below those of diagnosed diabetes, could be linked to increased risk of bowel cancer.
"More research is needed but if the results of our study are confirmed they would be important in developing prevention strategies."
Professor Khaw believes that both diabetes and bowel cancer may share common predisposing factors. It is already accepted that a high fibre diet and regular exercise can help protect against both diseases.
Alternatively, hormonal changes associated with diabetes could promote tumour risk.
The researchers focused on a chemical called glycated haemoglobin. Not only does this give a good indication of blood glucose levels over the past few months, it also provides a clue about the metabolic processes in the body that influence levels of insulin - the hormone which breaks down sugar.
Professor Khaw said: "Understanding these metabolic changes, and the lifestyle factors responsible, may help us to prevent and treat cancer."
Around 35,600 people are diagnosed with bowel cancer each year in the UK. In men incidence rates have increased by an average of 1% each year over the last 20 years.
Around two-thirds of cases may be preventable by changes in diet, including eating more fibre, and less fat.
Jola Gore-Booth, chief executive of the charity Colon Cancer Concern, said: "If this research is confirmed, it provides another important indicator of people who might be at risk of bowel cancer."