Dr. Irwin and her colleagues found that high levels of fasting C-peptide (a marker for insulin*) was associated with a higher incidence of death from breast cancer in women in their 40s whose cancer was at an early stage. That association decreased as women entered their 50s and 60s. The women did not have diabetes.
They analyzed data from 689 women with breast cancer who were enrolled in the Health, Eating, Activity, and Lifestyle (HEAL) Study. The women were followed for up to 9 years, until 2004.
Insulin has been shown in clinical studies to stimulate the growth of breast cancer cells. This epidemiological study supported that association.
When Do Insulin Levels Rise?
Often, before a diagnosis of diabetes, the body's cells become increasingly resistant to insulin. That sends a signal to the pancreas to secrete more. It isn't until the beta cells of the pancreas lose their ability to respond that insulin levels drop off. By that time, blood sugar levels are high and type 2 diabetes is well established.
The authors of this study recommended that younger women with breast cancer adopt lifestyle changes, such as exercise, that can moderate insulin levels.
* C-peptide is a protein fragment that splits from proinsulin after proinsulin is released from the pancreas. C-peptide levels reflect insulin levels.