Most people with diabetes already know they have a higher risk of premature death, mainly due to their increased risk of cardiovascular disease (e.g., heart disease, stroke and peripheral vascular disease). There is also some evidence that people with diabetes have an increased risk of developing and dying from certain forms of cancer. However, it is unclear if diabetes actually causes cancer, or whether the association is confounded by other factors like overweight and obesity.
As mentioned in Food for Thought, it is not really feasible or ethical to conduct randomised controlled trials to determine if the association between type 2 diabetes and cancer is causal. However, a group of UK researchers recently undertook a systematic review and meta-analysis of observational studies (where large groups of people are observed via a thorough health assessment every 5 years), using a new method to minimise bias in these kinds of studies, to help determine whether type 2 diabetes is likely to cause certain forms of cancer.
Thorough searches of the scientific literature found 151 cohorts (groups of people) providing information from over 32 million people with type 2 diabetes, who developed 1.1 million cases of cancer, which caused 150,000 cancer deaths.
Overall, they found that people with type 2 diabetes have a 15-25% increase in the risk of developing any kind of cancer, compared to people who don’t have diabetes. The association between type 2 diabetes and cancer was thought to be causal for liver, gallbladder, pancreatic, and endometrial cancer, and pancreatic cancer mortality (deaths). Associations with kidney, colorectal, and thyroid cancer incidence were less robust to unmeasured confounding, and therefore it was difficult to draw strong conclusions, and therefore more research is needed for these forms of cancer. Finally, the association between type 2 diabetes and leukemia, prostate, breast, bladder, stomach, ovarian, non-Hodgkin’s lymphoma, melanoma, lung, and oesophageal cancer was found to be unlikely to be causal.
It’s thought that type 2 diabetes increases the risk of cancer via multiple biological mechanisms. While a direct effect of hyperglycemia (high blood glucose) is possible and supported by the evidence of an association between type 1 diabetes and cancer, the indirect effects of hyperinsulinemia (high blood insulin), insulin resistance, chronic inflammation, and hormone imbalance have also been suggested as contributing factors.
Finally, the authors noted that the stage at which cancer is diagnosed is a strong determinant of its outcome, with treatment options and long-term survival being much greater for early stage localized disease. Therefore, outcomes can be improved by detecting cancer earlier in people with type 2 diabetes.
- Ling and colleagues. Association of Type 2 Diabetes With Cancer – Meta-analysis With Bias Analysis for Unmeasured Confounding in 151 Cohorts Comprising 32 Million People. Diabetes Care. 2020
Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of nearly 40 scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).