Cancer is a broad term used to describe more than 200 diseases characterized by abnormal cell division and the destruction of healthy body tissues. Individuals with diabetes are more likely than nondiabetics to develop certain types of cancer, according to authorities such as the National Institutes of Health.

Studies linking cancer and diabetes are often tentative and sometimes controversial. However, research most strongly associates diabetes to increased risk for developing cancers of the:

• Colon and rectum
• Pancreas
• Liver
• Endometrium (lining of the uterus)

For example, people with diabetes are up to 40 percent more likely to develop colorectal cancer, according to the American Cancer Society. There is also a possible association between diabetes and cancers of the mouth, esophagus, stomach, kidneys, bile duct, skin, breasts, cervix, ovaries, prostate and other organs.

Though the relationship between diabetes and cancer remains unclear, experts believe that hyperinsulinemia (elevated insulin levels) and obesity in people with diabetes and prediabetes are risk factors. In addition, some cancers (carcinoid tumors) and cancer treatments can cause secondary diabetes or hinder control of glucose (blood sugar).

Cancer is a general term that can be applied to more than 200 diseases. Characterized by abnormal cell division, cancer typically invades and destroys healthy tissues in the body.  According to authorities such as the American Medical Association, the American Cancer Society and the National Institutes of Health’s National Cancer Institute, individuals with diabetes and prediabetes are at increased risk for developing and dying from certain types of cancer.

Research has most strongly linked diabetes to:

• Colorectal cancer
• Pancreatic cancer
• Endometrial cancer
• Liver cancer

Some research also associates diabetes with greater risk of esophageal cancer, cervical cancer, oral cancer, stomach cancer and other tumors.

The relationship between diabetes and cancer remains unclear. Experts have postulated that obesity, a hallmark of type 2 diabetes, predisposes many diabetic patients to various cancers. Some researchers suggest that factors such as obesity, diet and use of alcohol explain higher diabetic rates of cancer. Other research indicates that insulin resistance (a condition in which the body produces insulin but is unable to use the hormone properly) and hyperinsulinemia (elevated levels of insulin in the blood) promote the growth of cancer cells.

However, obesity, insulin resistance and hyperinsulinemia are traits of type 2 diabetes, not type 1 diabetes, which has also been linked to increased risk of some cancers. For example, a 10-year study of births in Sweden, a nation with one of the highest rates of type 1 diabetes, found that children of women with type 1 diabetes had more than double the risk of childhood cancer.

Researchers are still trying to explain the connection. Some experts believe that certain hormonal changes associated with diabetes may boost the risk of tumors. For instance, scientists found in 2006 that a molecule called p110 alpha, a common pathway for cancer, also helps control insulin and other hormones involved in diabetes.

Another diabetic risk factor for cancer: immunosuppressive drugs, which patients take after an islet cell transplant, pancreas transplant or kidney transplant.

Even in the majority of cancers not caused by diabetes, having diabetes can complicate recovery. Corticosteroids, some chemotherapy drugs and radiation can cause hyperglycemia. Patients may need fast-acting insulin added to their insulin regimen. Even nondiabetic cancer patients may need to begin treatment with insulin and in some cases are at risk of secondary diabetes. Hormone therapy for prostate cancer (hormonal medication or surgical removal of the testicles) has also been linked to risk of developing secondary diabetes.

Diabetes treatments may one day be used to fight cancer. For example, scientists are exploring whether thiazolidinediones, a class of antidiabetic agents, have a role in treating malignant and benign tumors.

Cancer studies may also lead to treatments for diabetes. For instance, cancer researchers found that a gene mutation involved in an endocrine tumor condition called multiple endocrine neoplasia may lead to a way to grow pancreatic islet cells. And some immunosuppressives used to treat lymphoma are being studied as a possible treatment for type 1 diabetes.

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.

The lifestyle factors most often recommended for prevention and control of diabetes are also helpful in delaying and preventing certain forms of cancer. These factors include:

• Moderate exercise approved by a physician, typically 30 minutes a day, five days a week.
  
  
• Maintaining a healthy weight.
  
  
• Maintaining a good diet that includes foods high in fiber, such as whole grains, nuts, vegetables and fruits.
  
  
• Quitting smoking.
  
  
• Limiting alcohol.

Having regular physical examinations and keeping a complete medical history will also help.

In addition, people with diabetes should manage their glucose (blood sugar) carefully. Controlling diabetes and weight may, for example, reduce the risk of endometrial cancer, according to the American Cancer Society.

Patients are advised to ask their physician whether they should have some cancer screening tests, such as colonoscopy, earlier and more frequently than nondiabetics.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following  questions about cancer risk and diabetes:

1. Does my diabetes or prediabetes increase my risk of cancer?
   
   
2. Do any of my diabetes treatments or other treatments increase my risk of cancer?
   
   
3. Does my family medical history show anything significant about my risk of cancer?
   
   
4. Are there any particular cancers we should be on the watch for?
   
   
5. How often should I have a physical examination? Will it include checks for skin cancer, prostate cancer, breast cancer or other cancers?
   
   
6. Are there any cancer tests I should have at an earlier age or more often than recommended for the general population, such as colonoscopy?
   
   
7. How could cancer affect my diabetes or diabetic treatments?
   
   
8. Can I reduce my risk of cancer by improving management of my diabetes?
   
   
9. Can I reduce my risk of cancer and diabetic complications by exercising more, eating better or quitting smoking?
   
   
10. Does the latest research about the link between diabetes and cancer reveal anything that pertains to me?