FRIDAY, Dec. 2 (HealthDay News) -- People who have been diagnosed with colon cancer have a poorer prognosis if they're obese or have type 2 diabetes, new research suggests.

Two new studies that looked at the impact that body-mass index (BMI) and a diagnosis of type 2 diabetes had on survival rates after a colon cancer diagnosis found that both factors influence whether or not someone survives colorectal cancer. In addition, both studies found that deaths from any cause, including heart disease, were also increased in those who were obese or had type 2 diabetes.

Results of the studies were published online in the Journal of Clinical Oncology.

"The message here is to avoid obesity and type 2 diabetes because they have negative health outcomes. We don't know for sure that losing weight or increasing physical activity will help, but we know they're good for trying to avoid other diseases, like cardiovascular disease, that can come up down the road," said Dr. Jeffrey Meyerhardt, author of an accompanying editorial in the same journal, and an associate professor of medicine at the Dana-Farber Cancer Institute and Harvard Medical School in Boston.

There are more than 1 million people who've survived a colon cancer diagnosis living in the United States, and mortality from colorectal cancer has gone down over the past two decades, according to background information in the studies.

And, while previous research has linked a higher BMI and type 2 diabetes to the development of colorectal cancer in the first place, it hasn't been clear how these factors influence the course of colorectal cancer once someone has been diagnosed.

The first study included 2,303 people involved in an ongoing study that began in 1992. Between that time and 2007, the study participants had been diagnosed with colorectal cancer. Follow-up continued through December 2008.

During the study, 851 people with colorectal cancer died. Of those, 380 died as a result of colorectal cancer, while 153 died of heart disease, according to the study. The remaining 318 people died of other causes.

The risk of all-cause mortality was 30 percent higher for people who were considered obese at the start of the study vs. those of a normal weight. The risk of dying from colorectal cancer was 35 percent higher, and the risk of dying from heart disease was 68 percent higher.

Meyerhardt explained that the researchers tried to adjust the data for important factors, such as physical activity, red meat intake (a known risk factor for colorectal cancer), family history and blood pressure levels. Even after adjusting the data, obesity increased the risk of dying.

"In and of itself, obesity does seem to have some effect," Meyerhardt said.

In the second study, researchers recruited 2,278 people who'd been diagnosed with non-metastatic colon or rectal cancer between 1992 and 2007. In this group, there were 842 deaths. Of those, 377 were from colorectal cancer and 152 were from heart disease, according to the study.

People with type 2 diabetes had a 53 percent higher risk of dying from any cause and a 29 percent higher risk of dying from colorectal cancer compared to people without type 2 diabetes. The risk of dying from heart disease was 2.16 times higher in people with type 2 diabetes and nearly four times higher in people with type 2 diabetes who used insulin, compared with people without type 2 diabetes, the study found.

"Insulin use in type 2 diabetes usually indicates longer-standing diabetes, which is usually associated with worse outcomes," Meyerhardt noted.

Obesity, elevated body-mass index and diabetes are associated with worse disease states across the board," said Dr. David Bernstein, chief of gastroenterology at North Shore University Hospital in Manhasset, N.Y. "But, we don't know if you lose weight if that risk will go down. I don't counsel patients who've been diagnosed with colorectal cancer to lose weight, because weight loss tends to be a worry in people undergoing cancer treatment anyway. I do counsel my patients that haven't been diagnosed with cancer to lose weight. The medical community needs to spend more time counseling prevention."

Meyerhardt added that it's also not clear from these studies whether or not obese people or those with type 2 diabetes would benefit from changes in the treatment they receive for colorectal cancer.

More information

Learn more about preventing colorectal cancer from the American Cancer Society.