With commentary by Zorana Andersen, PhD, associate professor of epidemiology at the University of Southern Denmark; Gerald Bernstein, MD, former president of the American Diabetes Association and coordinator of the Friedman Diabetes Program at Lenox Hill Hospital, New York, and others.
Women who take insulin to control their diabetes are more likely to have dense breasts, a risk factor for breast cancer, than women who take non-insulin medications or follow lifestyle measures to control their diabetes, according to new research.
"Diabetic women taking insulin need their medication, but may possibly benefit from additional screening for breast cancer," said study leader Zorana Andersen, PhD, an associate professor of epidemiology at the University of Southern Denmark in Esjberg. She presented the results of her study March 8 at the 10th European Breast Cancer Conference. Studies presented at medical conferences are typically considered preliminary until the research is published in peer-reviewed medical journals.
"We know that diabetes is associated with an increased risk of breast cancer,'' Dr. Andersen says. However, she and other experts can't explain why that is so. "The role of insulin has been debated," she says.
Insulin is a ''growth promoting factor for all body tissues," Dr. Andersen says. So, it’s plausible that it can increase the amount of tissue in the breast known as epithelial or stromal tissue, and boost the overall breast density.
When breasts are dense, they have little fatty tissue. Doctors classify breast density into four categories, from nearly all fatty to extremely dense. In the middle are breasts that have some dense, some fatty tissues. On a mammogram, dense breast tissue shows up as white, and so do cancers, so it's more difficult to diagnose breast cancer in dense breasts.
Density is common, according to the American Cancer Society. Density can decline with age, but doesn't always. A woman can't determine if her breasts are dense by size or firmness. She can ask her doctor after a mammogram to inform her about her breast density.
Experts, including Dr. Andersen, say that women with the highest level of density (more than 75%) have a 4 to 6 times greater breast cancer risk than those with the least density, less than 25%.
Insulin, Metformin, Diet: Different Impact?
The Danish researchers looked at more than 5,600 women, most of whom were already past menopause. On average, they were 56 years old. They all participated in a large study, the Danish Diet, Cancer and Health Study Group. They received mammograms between 1993 and 2001. Just under 3%, or 137, had diabetes. Overall, women with diabetes were not likely to have dense or mixed breasts, versus fatty ones, she found.
Then, the researchers looked separately at the 44 who used diet to control their diabetes and the 62 who took pills (such as metformin) to control it, and found they were likely to have fatty, not dense, breasts. However, those on insulin were about two times as likely to have dense breasts, they found.
"We cannot conclude from this study whether taking metformin reduces breast density," she says. More research is needed, she says, to explain the links she found.
Two endocrinologists and a radiologist weighed in on the new research.
The study finding about women on insulin having denser breasts is expected, says Gerald Bernstein, MD, a former president of the American Diabetes Association and coordinator of the Friedman Diabetes Program at Lenox Hill Hospital, New York. "That's not surprising," he says, ''since insulin is a growth factor."
However, he cautions, the study is preliminary. Depending on how future research bears out, he says, one possible future strategy might be to add metformin to the medication schedule for women on insulin, ''not so much for its glucose benefit but for its offsetting of the potential effects of insulin." Meanwhile, he agrees with Andersen that regular breast screening is crucial.
Wei Feng, MD, an endocrinologist and assistant clinical professor at the City of Hope, Duarte, Ca., says the finding about insulin being linked with higher breast density is probably not common knowledge. She, too, would welcome more study about metformin and its effect on breast density.
Debra Monticciolo, MD, a professor of radiology at Texas A&M University, vice chair for research and section chief of breast imaging at Baylor Scott & White, said the finding is interesting, but ''it is too early to draw firm conclusions." If the insulin use turns out to be a significant risk factor, radiologists might consider adding it to the list of questions women are asked before their mammogram to evaluate risk, says Dr. Monticciolo, who also chairs the Commission on Breast Imaging for the American College of Radiology.
 Debra MOnticciolo, 254-724-2007, firstname.lastname@example.org
Wei Feng, MD, email@example.com, 626-476-7593 Cell for PR Letisia Marquez.
 ACS and breast density: http://www.cancer.org/acs/groups/content/@editorial/documents/document/acspc-039989.pdf