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Breast cancer linked to 23% higher risk for new diabetes



Women with breast cancer faced an adjusted 23% higher risk of developing diabetes during the 5 years after their diagnosis, a new Danish study finds.

Reimar W. Thomsen, MD, PhD, of Aarhus University Hospital in Denmark Randy Dotinga/MDedge News

Dr. Reimar W. Thomsen

The findings are “quite a clear signal of increased diabetes following breast cancer,” said epidemiologist and study coauthor Reimar W. Thomsen, MD, PhD, of Aarhus (Denmark) University Hospital, in an interview. “It’s very important to tell [patients with breast cancer] what they may expect in the long term.”

He spoke at the annual scientific sessions of the American Diabetes Association, where he presented the study findings.

Much of the research into links between breast cancer and diabetes has focused on whether diabetes is a risk factor for breast cancer, and not the other way around. A 2018 meta-analysis of 18 studies found a slightly higher risk of breast cancer in women with diabetes (summary relative risk, 1.13; 95% confidence interval, 1.04-1.24). However, the researchers found evidence that the risk factor might be adiposity, and not diabetes itself (Diabetes. 2018 Jul;67[Supplement 1]. doi: 10.2337/db18-180-OR).

For the new study, researchers used health registries to track women in Denmark for up to 12 years, during 2005-2016. They compared 33,909 women who were older than 50 years and who had new-onset breast cancer with 313,998 women without breast cancer in a matched comparison cohort. The average age in both groups was 66 years; obesity was rare (4% vs. 3%, respectively), but statin therapy (21% in both groups) and hormone replacement therapy (36% vs. 32%) were more prevalent.

In the first year after a breast cancer diagnosis, the women in the breast cancer group were 15% more likely to develop diabetes (per use of diabetes medication or hospital-diagnosed diabetes) than those in the comparison group (adjusted hazard ratio, 1.15; 95% CI, 1.01-1.30) with adjustments for factors such as age, marital status, residence, medical history, medications, and comorbidity.

Over a median follow-up period of 5.2 years, the risk of diabetes was 23% higher in the breast cancer group, at 8.4 new cases per 1,000 women, compared with 6.8 new cases per 1,000 women in the comparison group (aHR, 1.23; 95% CI, 1.16-1.30). Unadjusted hazard ratios were similar.

Women in the breast cancer group who developed diabetes were more likely to use insulin-based therapy, suggesting they had more severe diabetes, compared with those in the control group (5% vs. 2%, respectively; P less than .00001). They were also more likely to be treated with insulin only (4% vs. 1%, P less than .00001).

It is not clear why patients with breast cancer face a higher risk of diabetes. Dr. Thomsen speculated that cancer drugs might play a role and he noted that cancer itself can cause inflammation and “lead to consequences.”

A 2018 study linked usage of hormone therapies, including tamoxifen (HR, 2.25; 95% CI, 1.19-4.26; P = .013) and aromatase inhibitors (HR, 4.27;95% CI, 1.42-12.84), in patients with breast cancer to higher levels of diabetes, compared with patients who did not use hormone therapy (J Clin Oncol. 2018;36[20]:2061-9).

Dr. Thomsen emphasized that physicians should monitor patients with breast cancer for diabetes. “It develops over time, and the risk is increasing, so you need to be aware of that.”

No study funding was reported. One of the researchers reported numerous ties to a range of drug companies. Dr. Thomsen and the other researchers reported no relevant disclosures.

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