A new prospective study has reported that higher C-peptide levels are associated with reduced survival after breast cancer. The study was designed to investigate the association between fasting C-peptide levels measured three years after breast cancer diagnosis and risk of death from breast cancer and other causes. Serum C-peptide is a marker of insulin production in the body by the pancreas. The normal range is approximately 0.8 to 3.1 ng/mL. High levels of C-peptide may be a response to high levels of blood sugar caused by high glucose intake and/or by insulin resistance. The study included 604 women enrolled in the Health, Eating, Activity, and Lifestyle (HEAL) Study who were diagnosed with local or regional breast cancer between 1995 and 1998. The women were followed until year-end 2006 or death, whichever came first.

Increasing fasting C-peptide levels were found to be associated with increasing risk of death from breast cancer and from all causes among women without type 2 diabetes. A 1 ng/mL increase in C-peptide level was associated with a 35% increase in risk of breast cancer-specific death and a 31% increase in risk of death from any cause. The associations between C-peptide levels and death from breast cancer were stronger among women with type 2 diabetes, women with a body mass index less than 25 kg/m2 (i.e., normal weight and underweight women), women diagnosed with higher stages of breast cancer, and women whose tumors were estrogen receptor positive (ER+). The authors conclude that treatment strategies to reduce C-peptide levels in patients with breast cancer should be explored, including dietary-induced weight loss, exercise and other physical activity, and/or the use of insulin-lowering medications.

Please see our article on type 2 diabetes for more information on the relationship between insulin and breast cancer.