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NEW YORK (Reuters Health) - A new research review confirms that people with diabetes have a somewhat increased risk of colon cancer -- but the reasons for the connection, and what should be done about it, remain unclear.
Combining the results of 14 international studies, researchers 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.
The findings, reported in the American Journal of Gastroenterology, 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.
However, there could still be other explanations.
"I think we can make the statement that diabetes is consistently associated with colorectal cancer," Dr. Edward Giovannucci of the Harvard School of Public Health, who was not involved the study, told Reuters Health in an email.
"The cause-and-effect aspect is a bit difficult to consider since diabetes is such a complex disease," said Giovannucci, who studies the underlying causes of colon cancer.
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.
Whatever the mechanism, 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 Dr. Hiroki Yuhara, who led the new study while based at the University of California, Berkeley.
And it's not clear if that advice will change at any time in the future, Yuhara told Reuters Health in an email.
Experts recommend that most people start colon cancer screening at the age of 50, with any of several tests -- including stool tests that look for hidden blood, or invasive tests like sigmoidoscopy or colonoscopy.
People with certain risk factors for colon cancer -- like inflammatory bowel disease (Crohn's disease or colitis) or 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 that the link between diabetes and colon cancer risk may be weakening.
A study published last year by researchers at the American Cancer Society (ACS) found that among more than 184,000 older Americans 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, the researchers said, smaller than past studies had suggested.
What's more, there was no similar increase seen among women with type 2 diabetes. (The study was not included in the current analysis, which considered studies published from the 1990s through 2009, from the U.S., Canada, Europe and Japan.)
The ACS researchers speculated that the findings might reflect better diabetes control among Americans -- and women, in particular -- in recent years. A separate government study had shown that U.S. women with diabetes improved their long-term blood sugar control between 1999 and 2004.
In theory, better blood sugar control would mean lower insulin levels, which might affect colon cancer risk.
But that's speculation. For now, Giovannucci recommended that people focus on maintaining a healthy weight and getting regular exercise: Obesity is a risk factor for both diabetes and colon cancer.
According to Yuhara, people with diabetes should be aware of the link to colon cancer risk, and follow their doctors' advice on screening for the cancer.
It's estimated that just over 101,000 Americans will be diagnosed with colon cancer in 2011, while almost 40,000 will be told they have rectal cancer, according to the ACS. The average American has about a 1 in 20 lifetime chance of developing either cancer.
SOURCE: bit.ly/qJa6tK American Journal of Gastroenterology, online September 13, 2011.