Hsin-Chieh Yeh, PhD, is a co-author (with Drs. Asieh Golozar and Frederick L. Brancati) of the “Cancer and Diabetes” chapter in the NIDDK publication Diabetes in America, 3rd Edition. Here she summarizes the links between the two diseases and offers ideas for how health care professionals can help people with diabetes modify their cancer risks or manage comorbid conditions.

Q: Why should health care professionals address cancer risks when treating people with diabetes?

A: There are a couple of reasons. First, there is strong evidence from observational studies that diabetes is associated with certain types of cancers, including cancers of the pancreas, colon, rectum, kidney, and bladder. Diabetes has also been linked to postmenopausal breast cancer as well as uterine cancers. Cancers related to the digestive system seem to be the most implicated.

In addition to increased incidence of cancer, diabetes is associated with increased cancer mortality. There are also some, but fewer, studies that point to diabetes accelerating cancer progression, and there is some evidence to suggest that diabetes increases the risk for cancer complications such as infection.

Q: What might account for these associations between diabetes and cancer?

A: The two conditions share several risk factors. For example, people with obesity have a higher risk for developing diabetes as well as certain types of cancers. Other shared risk factors—smoking, physical inactivity, and alcohol consumption—have also been shown in the scientific literature.

Researchers are exploring two possible biological mechanisms for this association. One is hyperglycemia, or high blood sugar. People with diabetes are susceptible to high blood sugar, which can lead to organ damage, and both laboratory and human studies have found that a higher level of blood sugar also promotes tumor growth. Another possible mechanism is hyperinsulinemia, or high insulin levels in the blood. We know that a high insulin level is related to cardiovascular disease, not just diabetes, and also to cancers.

Q: What is known about the relationship between diabetes and cancer medications?

A: A hot topic for a while has been whether certain types of diabetes medications are related to cancer incidence. We don’t have conclusive evidence to say one way or the other. There were several observational studies¹ that suggested metformin, a first-line treatment for diabetes, may prevent certain cancers, but there were flaws in the study design, and better-designed studies that could give us more conclusive evidence have not yet been produced. There have also been questions about whether or not insulin treatment might increase the risk of cancer. Epidemiologists analyzing data from well-designed studies² have found conflicting results regarding insulin glargine and cancer risk, so there is no conclusive evidence. There are several newer lines of diabetes medications, such as glucagon-like peptide-1 (GLP-1) and dipeptidyl peptidase-4 (DPP4), for which we don’t have enough data to say whether they are related to cancer or not. This is an important area for future research.

Despite these unknowns, we have patients who say things like, “Doctor, I don’t want to take insulin, because I heard it will increase my risk for cancer, and my mom has cancer.” We have to advise our patients that concerns about cancer should not be the deciding factor in terms of choosing which diabetes treatment to use. Compared to cardiovascular complications, cancer is still a much rarer event. Diabetes management is still the top priority for patients with diabetes.

Q: How can providers help diabetes patients modify their cancer risk?

A: When health care providers talk to people with diabetes, we tend to inform them that they now face risks in terms of heart disease or stroke—these are traditionally well-known macrovascular complications. But we also need to point out that cancer is another possible consequence of diabetes, and they should be encouraged to get their regular cancer screenings. We clinicians need to be reminded of this ourselves—to bring it up with our diabetes patients, because cancer screenings should be a priority for them, just like it is for other populations. Current recommendations are for health care providers to refer patients for cancer screening, for example, mammograms, colonoscopies, pap smears, and PSA testing, based on the current guidelines by age and sex, whether or not they have diabetes.

And of course, providers should still advise their patient with diabetes to follow a healthy diet, be physically active, and maintain a healthy weight. We need to do whatever we can to promote healthy behaviors related to the four shared risk factors for cancer and diabetes—obesity, smoking, physical inactivity, and alcohol consumption.

Q: What research is being conducted on the association between cancer and diabetes?

A: There are a couple of different types of research we are working on. First is the basic science, or development of biomedical models. We need to understand the underlying mechanism for the relationship between diabetes and cancer. Does it occur through the insulin pathway and the glucose pathway or are other pathways and organs, like the liver, involved?

Another area of research is in clinical care. How do we provide better quality of care for a patient with both diabetes and cancer? This is important, because nowadays a lot of people survive cancer and it becomes a chronic disease. These people may also have diabetes. So, clinical care for people with comorbid conditions is an important area for research.

I can give you an example of this. Many women diagnosed with breast cancer will start chemotherapy, which often causes a significant increase in weight. This is not uncommon, but it’s not good for women who already have diabetes. And depending on their type of surgery, some women may have some difficulty in terms of physical activity, for example, moving their upper bodies or arms or performing exercise. In such cases, weight management becomes even more important, and we need more research on how to support this.

Another area that needs further study is how to support ongoing diabetes self-management behaviors in people with diabetes who receive a diagnosis of cancer. We don’t want these individuals to focus solely on their new cancer diagnosis and ignore important aspects of diabetes care, such as managing their blood glucose, blood pressure, and cholesterol, as well as managing their weight through meal planning and physical activity. We want these individuals to continue to do what they had been doing to manage their diabetes, before they got the cancer diagnosis. We need to do research to learn more about the “cancer effect” on diabetes care and how it can be improved.

What has been your experience addressing the connection between diabetes and cancer? Tell us below in the comments.