Investigators for years have questioned whether diabetes is a risk factor for the development of cancer. There are theoretical reasons to suspect that there might be a link: insulin levels are increased in Type 2 (adult onset, non-insulin-requiring) diabetes because the body cannot utilize insulin properly to control blood sugar levels.
Insulin acts as a growth factor in certain situations and can theoretically stimulate tumor growth. Obesity and so-called oxidative stress, increased in diabetes, could also theoretically promote the development of cancer. In medicine things that should make sense don’t always, so there needs to be clinical confirmation of an association before one can be assumed based on scientific theory.
Does having diabetes make it more likely that you will get cancer?
Or, what is the likelihood that you will get a specific kind of cancer if you have or don’t have diabetes? If the ratio of these risks is significantly greater than one, the risk is statistically likely. For example, in any given year the risk of a 55 year-old woman getting breast cancer is about 0.55% (Thiébaut AC et al J Natl Cancer Inst. 2007;99(6):451.). If that risk in a diabetic turned out to be 2% that would probably be significant. Statisticians would have to tell you whether the size of the sample measured allowed you to be confident about the difference. What are the real-life numbers?
Recently the NIH-AARP Diet and Health Study released the results of its long-term analysis of this issue (recall its other pronouncements from past blogs). Overall the risk of dying of cancer was increased by 11% in women and 17% in men. This excess risk was made up mostly of diabetics with colon and rectal cancer. In women, there was an increase in stomach and uterine cancer as well; in men there was an increase in pancreatic and kidney cancers.
Why the gender differences?
There may be additional confounding variables created by differences in hormone levels, but no one really knows. There was no increase in breast, lung or prostate cancer (in the U.S, but there is in Taiwan for prostate for unknown reasons). These differences cannot be explained by what we currently know but arguably should be hypothesis-generating: that is, these results should result in theories which can now be tested in the laboratory under controlled conditions.
Elevated insulin levels are probably not the entire story, however. An elegant study from Taiwan (Yang et al, Diabetes 59:1254, 2010) looking at Type 2 diabetics concluded that insulin usage was associated with a major risk reduction of getting cancer. Their theory to explain this is that insulin users have lower blood sugars than non-users and it is the glucose level rather than the insulin level that creates the excess risk. Regardless, even in their hands diabetics have a higher incidence of cancer than the general population.
What can we make of all of this?
Most diabetes in this country is genetic but heavily influenced by obesity. Even if your parents were both diabetics you can have an impact on your diabetic risk by good diet and weight control. This will not only reduce your risk or ameliorate the severity of your diabetes but will help protect you from getting cancer.