Women with recently diagnosed diabetes may be more likely to also get a breast cancer diagnosis than those without diabetes, suggests a new study from Canada.
It's not the first time diabetes has been linked to new cases of breast or other cancers. But the findings also hint that at least part of the reason why doctors find more breast cancer in diabetics is because they're looking harder -- and not necessarily because diabetes itself raises a woman's cancer risk.
"The relationship that we see (between diabetes and cancer), we wondered if it was something about the fact that people with diabetes go to the doctor's office more often," said Jeffrey Johnson, from the University of Alberta in Edmonton, who worked on the study.
"When a new diagnosis of diabetes is made, people undergo a lot of tests and general health exams." That may include breast cancer screening with mammograms, he added.
Previous studies found that people with diabetes have a higher risk of colorectal, liver and pancreatic cancers, along with breast cancer.
Researchers then suggested that certain behaviors might increase the risk of both types of diseases, including smoking, being sedentary and not eating well, and that those would explain the link.
It's also possible that changes in insulin and blood sugar levels that come with diabetes make it easier for breast tumors to grow, Johnson said.
While those explanations could still be partly behind the increase in breast cancer researchers have noticed, extra doctor's visits and tests "certainly seems to contribute to some extent."
Johnson and his colleagues consulted a database including about 170,000 women in British Columbia—half with a recent type 2 diabetes diagnosis and half without diabetes—and tracked them for the following four to five years. During that time, about 2,400 women, or 1.4 percent, were diagnosed with breast cancer.
Women in both groups had a similar chance of getting breast cancer. However, when the researchers broke them down by age and focused on the time shortly after the diabetes diagnosis, they found that older, post-menopausal women with diabetes were slightly more likely to be diagnosed with breast cancer than diabetes-free women.
Among those 55 and older, women with diabetes diagnosed in the last three months were about 30 percent more likely to also get a breast cancer diagnosis than those without diabetes. But even in these women, researchers couldn't tell for sure that the finding wasn't due to chance, according to results published in Diabetes Care.
After a few months—when the pace of appointments and tests after a diabetes diagnosis would have slowed—there was no difference in how often breast cancer was diagnosed in women with or without diabetes.
Johnson said the finding doesn't rule out other explanations, such as common risk factors for diabetes and cancer or a hormone-driven increase in tumor growth.
"I think there are so many things going on in the relationship that this is maybe only one part of it," he told Reuters Health. "We're really early on in understanding this relationship."
Dr. Christos Mantzoros, an hormone expert from Harvard Medical School in Boston, said the new findings could mean either that the higher rate of breast cancer diagnosis was due to more follow-up and screening in diabetics, or that the common roots of the diseases may lead both to develop within a short period of time.
"Women with diabetes need to be more vigilant and their doctors need to be screening them for malignancies associated with diabetes including, but not limited to breast cancer," Mantzoros, who was not involved in the new study, told Reuters Health in an email.
Johnson said the main message for women is still to cut out behaviors such as smoking that increase disease risks, and to get breast cancer screening as recommended.
The U.S. Preventive Services Task Force, a government-funded expert panel, calls for mammograms every other year for women between age 50 and 74.