Slim people who look outwardly healthy may be at increased risk of bowel cancer if they have raised insulin levels, research has shown.
A new study found that abnormal insulin is associated with a greater risk of bowel cancer, whether or not an individual is overweight.
The findings suggest that measuring blood levels of the hormone could help identify those most likely to develop the disease, increasing the likelihood of early diagnosis and treatment.
Obesity is already known to be a risk factor for bowel cancer.
Study scientist Dr Marc Gunter, from the International Agency for Research on Cancer (IARC) in France, said: "These results show for the first time that bowel cancer risk is associated with elevated insulin levels among lean people, as well as those with obesity.
"High insulin levels are common in obese individuals but less so in leaner people. We don't know why the lean individuals had raised insulin levels but it could be due to poor diet or sedentary behaviour.
"Measuring insulin levels could be used alongside BMI (Body Mass Index) as a new tool for assessing bowel cancer risk to better determine who is at the greatest risk and requires greater monitoring."
Bowel cancer is the fourth most common cancer in the UK and kills more than 16,000 Britons each year.
Experts estimate that nearly half of UK cases of the disease could be prevented by healthy lifestyle changes.
The study, part of the European Prospective Investigation into Cancer and Nutrition (EPIC), included 737 participants who developed bowel cancer and the same number who did not.
All were tested for levels of an insulin biomarker, C-peptide, in the blood.
The findings are reported in the journal Public Library of Science Medicine.
Dr Panagiota Mitrou, from the World Cancer Research Fund, which part-funded the study, said: "These interesting new findings allow us to identify the groups of the population who are at the greatest risk of bowel cancer.
"It's very important to have a healthy diet and exercise regularly. This won't only help reduce your risk of bowel cancer but also of a number of other cancers."