Insulin may increase mammographic density in women with diabetes

Taking insulin for diabetes appeared to increase mammographic density, a risk factor for breast cancer, according to results of a prospective cohort study of Danish women presented at the 10th European Breast Cancer Conference.

However, controlling diabetes through diet or pills decreased mammographic density, results also showed.

Although previous data suggest diabetes increases risk for breast cancer, the exact mechanisms of that risk are known.

Zorana Jovanovic Andersen, PhD, associate professor of epidemiology at University of Southern Denmark, and colleagues conducted the first analysis evaluating breast cancer risk with regard to diabetes treatment and insulin.

“One of the characteristics of cancer cells is their ability to grow rapidly and uncontrollably, and to resist the programmed death that occurs in noncancer cells,” Andersen said in a release. “Therefore, growth factors are critical to cancer development and progression. We know that insulin is an important growth factor for all body tissues, and even if we do not know exactly how it affects the development of cancer cells, it is also highly plausible that it increases breast density.”

The study included data from 5,703 women (mean age, 56 years) — 4,501 of whom were postmenopausal — who participated in the Danish Diet, Cancer and Health cohort between 1993 and 1997 and underwent mammographic screening in Copenhagen between 1993 and 2001.

Women self-reported their diabetes diagnosis and use of diabetes treatment.

Overall, 2.4% (n = 137) of women had diabetes and 56.3% (n = 3,180) had mixed or dense breasts. Forty-four women reported they controlled their diabetes by diet alone and 62 reported they took oral medication.

According to results of a crude model, having diabetes corresponded with an inverse association with having mixed or dense breasts (OR = 0.33; 95% CI, 0.23-0.48). The association persisted even after adjusting for adiposity and other risk factors (OR = 0.61; 95% CI, 0.40-0.92).

Researchers observed similar inverse associations among women who controlled their diabetes by diet only, without any medication (OR = 0.63; 95% CI, 0.29-1.36), as well as in women who reported only taking oral medication (OR = 0.59; 95% CI, 0.32-1.09).

Conversely, women with diabetes who took insulin were more likely to have mixed or dense breasts (OR = 2.08; 95% CI, 0.68-6.35). Menopause status or BMI did not impact this association.

Although breast density greater than 75% leads to a four- to six-fold increased risk for breast cancer compared with breast density less than 25%, the researchers noted the fact that insulin appears associated with greater breast density does not mean insulin increases breast cancer risk.

“Breast density is only one of many risk factors for developing breast cancer,” Andersen said.

Researchers are planning to extend their study by following their cohort for breast cancer and evaluating the effect of different diabetes treatments on breast cancer risk.

“If we find a relationship, we need to examine whether a high mammographic density is responsible, or whether other factors are involved,” Andersen said.

The researchers additionally plan to undertake a larger-scale study linking data on mammographic density from the Copenhagen Mammography Registry to data on the purchase of specific insulin treatments from the Danish Prescription Registry. – by Jennifer Southall

Reference:

Andersen ZJ, et al. Poster 158. Presented at: 10th European Breast Cancer Conference; March 9-11, 2016; Amsterdam.

Disclosure: The researchers report no relevant financial disclosures.