Though the cause for the link between diabetes and cancer risk is not understood, diabetes is considered a risk factor for some types of cancer. Findings on the association between diabetes and cancer include:

• Colorectal cancer. Colorectal cancer is the No. 2 cancer killer in the United States, according to the American Cancer Society (ACS). It estimates that having diabetes increases a patient’s risk of developing cancer of the colon or rectum by 30 to 40 percent. The organization also reports that diabetic patients with colorectal cancer face an increased risk of relapse after surgery, and are more likely to die of the disease than nondiabetics. Some research has implicated insulin therapy, which may affect cells lining the intestinal tract.

Because most but not all studies on diabetes and colorectal cancer have found a link, scientists in 2005 conducted a meta-analysis of 15 studies involving more than 2.5 million people. This review, reported by the National Cancer Institute, found a strong relationship between diabetes and increased risk of colorectal cancer in men and women.

Furthermore, recent research including more than 226,000 Americans from the National Health Interview Survey found that people with diabetes were 1.4 times more likely than nondiabetics to have colon cancer. The connection remained even after accounting for other risk factors such as age, smoking and use of alcohol.

• Pancreatic cancer. Cancer of the pancreas, the glandular organ that secretes insulin (a hormone that helps regulate glucose). Though uncommon, pancreatic cancer is the fourth-leading cause of cancer deaths in the United States, according to the ACS. People with diabetes, particularly type 2 diabetes, are more likely to develop this condition than nondiabetics, the organization reports. Research has implicated hyperglycemia, hyperinsulinemia and insulin resistance as possible factors. Also, pancreatitis (inflammation of the pancreas) is a risk factor for diabetes and pancreatic cancer. 

For unknown reasons, the risk of developing pancreatic cancer is greatest within the first five years of being diagnosed with diabetes. Some scientists are even describing diabetes as potential marker for pancreatic cancer, which is difficult to identify in early, treatable stages. Recent research also links periodontal disease, which is common in diabetic patients, to increased risk of developing pancreatic cancer.

Insulinoma, a tumor of the insulin-producing pancreatic beta cells, is usually benign (noncancerous) and causes hyperinsulinemia. However, some insulinomas are malignant.

In nondiabetics, total pancreatectomy, a possible treatment for conditions including pancreatic cancer, will cause secondary diabetes and require treatment with insulin.

• Endometrial cancer. Cancer of the endometrium (lining of the uterus). Diabetic women may have up to four times the risk of endometrial cancer, according to the ACS. Obesity, a risk factor for this cancer and for type 2 diabetes, cannot explain away the link because women with type 1 diabetes are also more prone to developing endometrial cancer, according to the ACS.
  
  
• Liver cancer. Recent research has found diabetes to be a strong risk factor for liver cancer. The Surveillance, Epidemiology, and End-Results (SEER)-Medicare database indicated that diabetes approximately tripled the risk of liver cancer, even after excluding factors such as hemochromatosis, which raises the risk of both diseases.

The prevalence of fatty liver disease in people with type 2 diabetes may play a role in the higher risk of liver cancer among such individuals. Diabetes may also boost the risk of liver cancer in patients with hepatitis C.

• Esophageal cancer. The 10-year Korean Cancer Prevention Study (KCPS), involving nearly 1.3 million South Koreans, found a significant association between diabetes and cancer of the esophagus. This may be especially important because more Americans than Koreans are obese and eat a diet low in plant foods, two risk factors for this cancer.
  
  
• Cervical cancer. The KCPS revealed a significant association between diabetes and cancer of the cervix (lower uterus). As with esophageal cancer, this finding may be especially important because Americans are much more likely than Koreans to be obese and to eat insufficient amounts of vegetables and fruits, two risk factors for cervical cancer. Another study found that women with type 1 diabetes had almost twice the risk of nondiabetics for developing this cancer.
  
  
• Stomach cancer. People with type 1 diabetes have almost double the risk for developing stomach (gastric) cancer, one study showed. A type of stomach or intestinal cancer, gastrointestinal carcinoid tumors, can cause secondary diabetes by producing a hormone (ACTH) that causes the adrenal glands to overproduce cortisol.
  
  
• Oral cancers. Diabetes increases the risk of gum disease (see Dental Care & Diabetes). Some research has found that this inflammation, in turn, increases the likelihood of developing certain cancers of the mouth.
  
  
• Breast cancer. Some research has linked prediabetes and diabetes to increased risk of breast cancer, the second most common cancer in women after skin cancer. Other studies, though, have found no association.
  
  However, even if there is no causal connection, diabetes can be a factor in breast cancer. In Canada’s healthcare system, data involving more than 700,000 women showed that those with diabetes were much less likely to have a mammogram than nondiabetics despite having more healthcare visits. The difficulty of managing diabetes hindered screening for other conditions. In addition, diabetic women often do not have the option of some breast reconstruction surgeries that involve the blood vessels, according to the ACS.
  
  
• Prostate cancer. The relationship between diabetes and prostate cancer, the second most common and No. 2 cause of cancer deaths in men, is unclear. Among the approximately 72,000 men in the Cancer Prevention Study II, those diagnosed with diabetes within the previous three years had a slightly greater incidence of prostate cancer, but those who had diabetes for a longer period saw their rate of prostate cancer decline by one-third. The reason may be the level of insulin, which climbs early in the course of diabetes but then falls below normal.

Even if diabetes eventually lowers the risk of prostate cancer, some research has found that nondiabetics are more likely to survive prostate cancer. Obesity, which is common in people with type 2 diabetes, has been found to increase mortality rates in men with prostate cancer. Other research has linked hyperinsulinemia to increased mortality from prostate cancer.

Androgen deprivation therapy, one of the treatment options for prostate cancer, has been linked to insulin resistance and risk of developing diabetes.

• Leukemia (blood cancer). According to the National Cancer Institute, survivors of childhood acute lymphoblastic leukemia (ALL), the most common cancer in children, may face increased risk of metabolic syndrome, a major risk factor for diabetes. In addition, some data have linked ALL to risk for type 1 diabetes.
  
  
• Lung cancer. The association between diabetes and lung cancer, the leading cause of cancer deaths in the United States and worldwide, is unclear. One study found a slightly decreased risk of lung cancer in diabetic patients, but another found that prediabetes increased the risk of mortality from lung cancer by more than 50 percent. One type of lung cancer, lung carcinoid tumors, can cause secondary diabetes by producing the hormone ACTH, which causes the adrenal glands to overproduce cortisol.
  
  
• Skin cancer. Recent research in Sweden found that prediabetes doubled women’s risk of malignant melanoma and increased their risk of endometrial and breast cancers. It did not find an association between prediabetes and men’s cancer risk.

Some research has also linked diabetes, prediabetes and hyperglycemia to other cancers, including those of the kidneys, bile duct, brain, larynx and bladder. Further research is needed.