Association Between Type 2 Diabetes and Breast Cancer
Type 2 diabetic women appear to have an increased risk of developing breast cancer, according to a report in the June issue of Diabetes Care.

"Hyperinsulinemia may promote mammary carcinogenesis," Dr. Karin B. Michels and colleagues from Harvard School of Public Health, Boston, Massachusetts, note. "Insulin resistance has been linked to an increased risk of breast cancer and is also characteristic of type 2 diabetes."

In a prospective analysis, the researchers examined the association between type 2 diabetes and the incidence of invasive breast cancer among 116,488 nurses enrolled in the Nurses' Health Study. The subjects were cancer-free in 1976 and were followed through 1996 for incident type 2 diabetes and through 1998 for breast cancer.
The investigators report that 6120 women developed type 2 diabetes and 5605 developed invasive breast cancer during 2.3 million person-years of follow-up. Compared with nondiabetic women, those with type 2 diabetes had a modestly increased incidence of breast cancer (hazard ratio [HR] = 1.17) after adjusting for various factors.

The association between type 2 diabetes and breast cancer was only observed among postmenopausal women (HR = 1.16), not premenopausal women (HR = 0.83), and was strongest among those with estrogen receptor-positive tumors (HR = 1.22).
"If hyperinsulinemia plays a role in breast cancer pathogenesis, interventions that improve insulin sensitivity such as exercise and dietary modifications may be expected to lower associated breast cancer incidence," Dr. Michels and colleagues point out, "but this assumption requires further study." Diabetes Care 2003; 26:1751-1758.

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