A number of studies have suggested a link between diabetes and higher breast cancer risk. Research strongly suggests that women diagnosed with diabetes are more likely to be diagnosed with breast cancer than women who aren’t diabetic. This is likely due to a number of factors, including:
- obesity, which is a risk factor for both breast cancer and diabetes
- high glucose and insulin levels, which can happen in women whose diabetes isn’t well controlled, may lead to changes in growth and steroid hormones, which can increase breast cancer risk
Still, few studies have examined whether breast cancer treatments can increase the risk of diabetes.
An Israeli study suggests that women diagnosed with hormone-receptor-positive breast cancer treated with hormonal therapy were more likely to develop diabetes during the 6-year follow-up period.
The research was published online on April 24, 2018 by the Journal of Clinical Oncology. Read the abstract of “Diabetes After Hormone Therapy in Breast Cancer Survivors: A Case-Cohort Study.”
The study included 2,246 women who had no history of diabetes and were diagnosed with invasive, nonmetastatic breast cancer between 2002 and 2012. The women joined the study 1 year after being diagnosed.
During the roughly 6-year follow-up period, 324 of the women were diagnosed with diabetes.
The researchers looked to see how many of the women diagnosed with diabetes had been treated with hormonal therapy and which type of hormonal therapy they had taken.
After surgery, women diagnosed with hormone-receptor-positive breast cancer usually take hormonal therapy medicine to reduce the risk of the cancer coming back (recurrence). Hormonal therapy given after surgery is called adjuvant hormonal therapy.
Hormonal therapy medicines work in two ways:
- by lowering the amount of estrogen in the body
- by blocking the action of estrogen on breast cancer cells
There are several types of hormonal therapy medicines. Tamoxifen, a selective estrogen receptor modulator (SERM), is one of the most well-known. Tamoxifen can be used to treat both premenopausal and postmenopausal women. The aromatase inhibitors:
- Arimidex (chemical name: anastrozole)
- Aromasin (chemical name: exemestane)
- Femara (chemical name: letrozole)
have been shown to be more effective at reducing recurrence risk in postmenopausal women and are used more often than tamoxifen to treat women who’ve gone through menopause.
Overall, women treated with hormonal therapy had an almost 2.5 times higher risk of diabetes compared to women not treated with hormonal therapy.
This difference was statistically significant, which means that it was likely due to the difference in treatment and not just because of chance.
The researchers wanted to know if the type of hormonal therapy the women were treated with made a difference in diabetes risk:
- Women treated with tamoxifen had about a 2.2 times higher risk of diabetes compared to women not treated with tamoxifen.
- Women treated with an aromatase inhibitor had about a 4.3 times higher risk of diabetes than women not treated with an aromatase inhibitor.
The rate of diabetes diagnosed in women in the study rose from 6% in 2002 to 28% in 2015.
Compared to women who didn’t develop diabetes, women who did develop diabetes were:
- much more likely to be obese
- much less likely to exercise
- much more likely to eat an unhealthy diet
The researchers strongly emphasized that "breast cancer survivors should not be denied [hormonal therapy] treatment, because the survival benefits outweigh the risks." Still, the "dramatic increase" in rates of diabetes within a relatively short period of time strongly suggests that women treated with hormone therapy should be closely monitored for diabetes.
"We see a lot of breast cancer survivors in our clinics, and these are the kinds of issues they often struggle with," said Maryam Shafaee, M.D., of the Dan L Duncan Comprehensive Cancer Center of Baylor College of Medicine, in an interview. Dr. Shafaee was not part of the study, but was asked to comment on it. "First, because treatment with an AI often causes joint pain, making exercise difficult, and second, because treatment with tamoxifen often causes hot flushes, making it unpleasant to exercise. Life after diagnosis and treatment of breast cancer is not the same as the way it was before, so a lot of things change in these patients' lives -- in their work as well as what they are able or willing to do after the diagnosis.
"But given that not only obesity but diabetes are huge risk factors for morbidity in breast cancer survivors, these issues need to be better addressed, and lifestyle modification and education are something our society as a whole needs to be more aware of and implement."
If you’ve been treated with hormonal therapy for breast cancer, you might want to talk to your doctor about this study. Ask about the symptoms of diabetes and how your care team will be monitoring you for any signs of the condition.
At the same time, there are lifestyle choices you can make to keep your risk of diabetes as low as it can be. Many people are diagnosed with diabetes because of an unhealthy diet and lifestyle: too much sugar and too many simple carbohydrates, combined with not enough exercise. One of the best ways to avoid diabetes and help keep the risk of breast cancer recurrence as low as it can be is to have a healthy diet and lifestyle:
- eat a diet low in added sugar and processed foods and rich in fruits and vegetables
- maintain a healthy weight
- exercise regularly -- every day if you can -- at a moderate intensity
- avoid excess alcohol
- don’t smoke
Visit the Breastcancer.org Lower Your Risk section to learn more about breast cancer risk and steps you can take to make yours as low as it can be.
To connect with others taking hormonal therapy for breast cancer, join the Breastcancer.org Discussion Board forum Hormonal Therapy - Before, During, and After. Also, the forums Healthy Recipes for Everyday Living and Fitness and Getting Back in Shape offer suggestions for keeping your diet healthy and your lifestyle active.