People with diabetes have a somewhat increased risk of colon cancer, an international study said -- but the reasons for the connection, and what should be done about it, remain unclear.
Researchers headed by Hiroki Yuhara, at the University of California, Berkeley, combined the results of 14 international studies and found that, overall, people with diabetes were 38 percent more likely to be diagnosed with colon cancer than those who were diabetes-free.
There was also a 20 percent increase in the risk of rectal cancer, though that appeared to be confined to men, according to the findings, published in the American Journal of Gastroenterology.
"These data suggest that diabetes mellitus is an independent risk factor for colon and rectal cancer," Yuhara and his colleagues wrote.
The findings do not prove that diabetes directly contributes to colon cancer in some people.
The results come from observational studies in which people with diabetes were found to have a higher risk of colon cancer than those without diabetes. In most of the studies, the researchers adjusted for at least some factors that might explain the link -- like older age, obesity and smoking -- and the diabetes-cancer connection remained.
"I think we can make the statement that diabetes is consistently associated with colorectal cancer," said Edward Giovannucci of the Harvard School of Public Health, who was not involved in the study.
"The cause-and-effect aspect is a bit difficult to consider since diabetes is such a complex disease," he told Reuters Health in an email.
He said it seems likely that some aspect of diabetes contributes to colon cancer, but it's not certain what.
One theory is that hormones are involved.
People with diabetes tend to have high levels of the blood-sugar-regulating hormone insulin, as well as related hormones called insulin-like growth factors. Those hormones cause cells to grow and spread, and that may include cancer cells.
If diabetes does contribute to colon cancer, it's not clear what the implications would be.
People with diabetes are not advised to get colon cancer screening any more often, or at a younger age, than people without diabetes, said Yuhara.
It's also not clear if that advice will change at any time in the future. Experts recommend that most people start colon cancer screening at the age of 50.
People with certain risk factors for colon cancer, such as a strong family history of the cancer, are told to start screening earlier. Diabetes is not currently considered one of those risk factors.
And there's evidence the link between diabetes and colon cancer risk may be weakening.
A study published last year by researchers at the American Cancer Society (ACS), and not included in the current analysis, found that among the 184,000 older U.S. residents followed for 15 years, men with type 2 diabetes were about one-quarter more likely to be diagnosed with colon cancer than diabetes-free men were.
But that increase in risk was modest, and smaller than past studies had suggested. In addition, there was no similar increase seen among women with type 2 diabetes.
The ACS researchers speculated that the findings might reflect better diabetes control among U.S. residents -- and women in particular -- in recent years. In theory, better blood control would mean lower insulin levels, which might affect colon cancer risk.
For now, Giovannucci recommended that people focus on maintaining a healthy weight and getting regular exercise.