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A combined analysis of several studies shows evidence of a link between diabetes and colorectal cancer in both men and women, according to a report published in the Journal of the National Cancer Institute .

Colorectal cancer is the second leading cause of cancer-related deaths in the U.S. The disease develops in the large intestine, which includes the colon (the longest part of the large intestine) and the rectum (the last several inches).

Several factors that are known to increase the risk of colorectal cancer (including diet, physical inactivity, and obesity) also increase the risk of type 2 diabetes. The parallels between the two conditions have lead to suggestions that diabetes itself may increase the risk of colorectal cancer.

Diabetes could possibly affect risk of colorectal cancer through several mechanisms, including high insulin levels or factors related to insulin resistance, slower bowel transit time, and higher levels of fecal bile acids.

Several studies have evaluated the link between diabetes and colorectal cancer, but due to small sample sizes in these studies, researchers’ ability to draw firm conclusions has been limited. To overcome this limitation, researchers in Sweden pooled data from several previously published studies. The objectives of this pooled analysis were to provide a more precise estimate of the link between diabetes and colorectal cancer and to determine whether the association between the two conditions varied by sex or by cancer subsite (colon vs. rectum and proximal vs. distal colon).

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The primary study results are based on data from fifteen studies. These studies include a total of 2,593,935 participants. The combined analysis of these fifteen studies supports a link between diabetes and colorectal cancer:

  • Eight of the 15 studies report a “statistically significant” association between diabetes and colorectal cancer.
  • Taken together, the studies suggest a 30% increased risk of colorectal cancer among individuals with diabetes.
  • Diabetes increased risk of colorectal cancer in both men and women and for different cancer subsites.
  • The link between diabetes and colorectal cancer persisted after accounting for factors such as physical activity and body mass index.

A limitation of this study is that it was not able to distinguish between type 1 and type 2 diabetes.

The researchers conclude that these results “strongly support an association between diabetes and increased risk of colon and rectal cancer in both women and men.” The frequency of diabetes is expected to continue to increase in the U.S. in coming years, and this could contribute to additional colorectal cancer diagnoses and deaths.

Reference: Larsson SC, Orsini N, Wolk A. Diabetes Mellitus and Risk of Colorectal Cancer: A Meta-Analysis. Journal of the National Cancer Institute. 2005;97:1679-87.