Insulin and cancer

Clearly there are associations that are overwhelmingly indicative of a casual relationship. For example, if you take too much insulin, your blood glucose will go too low. The causal association is supported by the understanding that hypoglycemia is merely an exaggeration of the anticipated effect of lowering blood glucose, and the close time relationship: the hypoglycemic effect occurs within hours of the administration of the insulin.

And some associations slowly become evident: tobacco is now firmly linked with lung cancer, although for years the evidence was argued. The causal relationship was difficult to sort out in part because the time frame between using tobacco and getting lung cancer was so long, measured in years. On the other hand, the location of the cancer seems to fit with where cigarette smoke would be, so it's not too big of a leap of logic to conclude that tobacco is causative of lung cancer.

With respect to diabetes, it's now understood that several types of cancer are more common: colon, pancreas, and breast. But whether taking insulin increases the risk of these malignancies (or perhaps of others) is much less clear, and these studies do not, in my mind, clarify the risk. I do agree with the editorial comment in Diabetologia: "With respect to insulin glargine, it is in no one's interest to mount a witch-hunt against this popular and widely used insulin . . . but it is in everyone's interest for the truth to be known. The evidence presented in this set of papers is sufficient to establish that there is a case to answer, but is entirely insufficient to bring in a verdict."

What should the patient who's presently taking Lantus do? The EASD states that "The EASD does not recommend that you stop taking insulin glargine (Lantus) on the basis of the evidence presented here, particularly if you have found it helpful in the management of your own diabetes. People with diabetes do, however, have the option of using long-acting human insulin or a mixture of long- and short-acting human insulin twice a day instead of the once-daily analogue. You may wish to consider this option if you already have a cancer, or, for women, if there is a family history of breast cancer."

The American Diabetes Association weighs in with the comment that "the data within these studies and between these studies are conflicting and confusing. Until more information is available, the American Diabetes Association advises patients using insulin not to stop taking it. For patients using glargine and considering switching to another form of insulin, the data in these studies make it unclear as to whether any one type of insulin increases the risk of cancer more than other types of insulin."

My advice: if you are on insulin, stay on your present insulin program. If you have a family history of cancer, or been diagnosed with cancer, or have a high risk of cancer for other reasons, the information is still very unclear whether to switch from one form of insulin to another. Remember that glucose control decreases your risk of diabetic complications, and that's well-proven, established fact.