Overweight and obese women -- defined as having a BMI (body mass index) higher than 25 -- have a higher risk of being diagnosed with breast cancer compared to women who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence) in women who’ve been diagnosed with the disease.
This higher risk is partially because fat cells make estrogen; extra fat cells mean more estrogen in the body and estrogen can make hormone-receptor-positive breast cancers develop and grow. Scientists also have recently found that extra fat cells can trigger long-term, low-grade inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence; the proteins secreted by the immune system seem to stimulate breast cancer cells to grow, especially estrogen-receptor-positive breast cancer in postmenopausal women.
Statistics show that more than 66% of U.S. women are overweight or obese, which puts them at higher risk for breast cancer.
Research has suggested that excess fat in the stomach area, also called belly fat, raises the risk of several conditions, including heart disease, diabetes, and colorectal cancer. But it hasn’t been clear if belly fat was worse than fat in other parts of the body for increasing breast cancer risk.
A Chinese study adds another layer of complexity to the discussion. The study found that obese women with more belly fat had a higher risk of hormone-receptor-negative breast cancer compared to women with less belly fat. Women with more fat around their thighs, hips, and buttocks had a higher risk of hormone-receptor-positive breast cancer compared to women with less fat in these locations.
The research was published online on Sept. 14, 2017 in The Oncologist. Read the abstract of "Distinct Effects of Body Mass Index and Waist/Hip Ratio on Risk of Breast Cancer by Joint Estrogen and Progestogen Receptor Status: Results from a Case-Control Study in Northern and Eastern China and Implications for Chemoprevention."
The study included 1,316 Chinese women who were newly diagnosed with breast cancer. The women were age 25 to 70. The researchers collected information on the women’s:
- body measurements, including BMI and waist-to-hip ratio
- reproductive history
- medical history
The researchers also recorded the characteristics of the cancer with which each woman had been diagnosed.
The researchers then compared this information with similar information from 1,316 healthy women who visited the same hospitals for regular physical exams.
The researchers found that women with more subcutaneous fat -- fat around the thighs, hips, and buttocks -- measured by BMI, were more likely to be diagnosed with hormone-receptor-positive breast cancer, especially if they were premenopausal.
Women with more visceral fat -- fat around the internal organs -- measured by hip-to-waist ratio/belly fat, were more likely to be diagnosed hormone-receptor-negative breast cancer, especially if they were postmenopausal. This higher risk of developing hormone-receptor-negative breast cancer was seen in women with a high waist-to-hip ratio, even if they had a low BMI.
"A possible reason is that subcutaneous fat is involved in estrogen production, which may promote [hormone-receptor-positive] breast cancer," said Zhigang Yu, of Shandong University and one of the study’s authors. "Visceral fat is more closely related to insulin resistance and may be more likely to promote [hormone-receptor-negative] breast cancer."
"The results of this study suggest that general and central obesity may play different roles in different breast cancer subtypes," the researchers wrote, "supporting the hypothesis that obesity affects breast carcinogenesis via complex molecular interconnections, beyond the impact of estrogens."
The authors also suggested that tamoxifen, a type of hormonal therapy medicine used to reduce breast cancer risk in women who haven’t been diagnosed but are at high risk for the disease, may not be effective in women with more belly fat because tamoxifen does not work in hormone-receptor-negative disease.
It’s important to know that the study only included body measurements that were taken when the women were diagnosed with breast cancer, so did not include information on weight gain over time, which may affect breast cancer risk. The study also didn’t include information on whether the women used hormone replacement therapy, which also can affect breast cancer risk.
Losing weight can be harder as you age, but it can be done with careful changes to your diet and daily exercise. The first thing to do is talk to your doctor about a healthy weight for you based on your age, height, body type, and activity level. Next, talk to your doctor about a safe and sensible plan to lose weight designed specifically for you and your needs.
For more information on breast cancer risk and weight, as well as steps you can take to lose weight, visit the Breast Cancer Risk Factors: Being Overweight page in the Breastcancer.org Lower Your risk section.