A number of studies have suggested a link between diabetes, diabetes treatments, and cancer risk, including breast cancer risk. Sorting out these links can be challenging since some factors linked to diabetes -- poor diet or being overweight/obese, for example -- also are linked to higher breast cancer risk.
A large study has found that diabetic women taking the medicine metformin (brand names: Fortamet, Glucophage, Glumetza, Riomet), which is commonly used to treat type 2 diabetes, had a lower-than-average risk of breast cancer.
The study was published online June 11, 2012 in the Journal of Clinical Oncology. Read the abstract of “Diabetes, Metformin, and Breast Cancer in Postmenopausal Women.”
Type 2 diabetes is also known as adult-onset diabetes.
Women taking other medicines for diabetes had a slightly higher-than-average risk of breast cancer. The researchers think that the lower-than-average breast cancer risk in women taking metformin suggests how diabetes affects breast cancer risk.
The researchers looked at 12 years’ worth of medical records of more than 68,000 women in the very large Women’s Health Initiative (WHI) study. The WHI looked at many aspects of women's health over time, including bone health, diabetes, cardiovascular health, and cancer risk. About 3,400 women were diagnosed with diabetes when they enrolled in the WHI.
During the 12 years, 3,273 cases of breast cancer were diagnosed. The researchers compared breast cancer risk in diabetic women on different diabetes medicines to breast cancer risk in non-diabetic women.
- Diabetic women treated with diabetes medicines other than metformin were 16% more likely to be diagnosed with breast cancer compared to non-diabetic women.
- Diabetic women treated with metformin were 25% less likely to be diagnosed with breast cancer compared to non-diabetic women.
Doctors aren't sure why having diabetes may increase breast cancer risk. They think that higher insulin levels may be part of the reason. The hormone insulin helps our bodies regulate blood sugar. Insulin also helps cells grow. Many people who have adult-onset diabetes tend to have higher-than-normal insulin levels (hyperinsulinemia). This is partially because their bodies no longer respond to normal insulin levels. Some experts think that higher insulin levels in diabetics may help breast cancer cells develop and grow, which increases breast cancer risk.
Metformin is often used by itself as a first treatment for adult-onset diabetes, or later in combination with other diabetes medicines. Metformin helps make diabetics less resistant to insulin and lowers their insulin levels. This could explain, at least in part, why the women treated with metformin had a lower-than-average risk of breast cancer. Still, it’s likely that the link between metformin and lower risk is more complex than just insulin levels. The link does give scientists a clue for future research on the role of diabetes and insulin in the development and growth of breast cancer.
While your genes play a role in the development of diabetes, an unhealthy diet and lifestyle are major causes in many people: too much sugar and too many simple carbohydrates, combined with not enough exercise. Even when diabetes doesn’t develop, these factors are linked to obesity -- and to higher breast cancer risk. No matter how old you are, one of the best ways to avoid obesity and diabetes -- and keep your risk of heart problems and breast cancer risk as low as they 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 at a moderate intensity level
- avoid 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.