Research Suggests Risk Continues to Increase as Weight Goes Up

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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.

A new analysis of information from the Women’s Health Initiative (WHI) suggests that the more obese a postmenopausal woman is, the higher her risk of breast cancer.

The study was published in the August 2015 issue of JAMA Oncology. Read the abstract of “Overweight, Obesity, and Postmenopausal Invasive Breast Cancer Risk: A Secondary Analysis of the Women’s Health Initiative Randomized Clinical Trials.”

Cyndi Thomson, Ph.D., R.D., associate professor of nutritional sciences at the University of Arizona and member of the Breastcancer.org Professional Advisory Board, is one of the study’s authors.

The National Institutes of Health started the WHI to look at the most common causes of death, disability, and factors that decrease quality of life in postmenopausal women, including heart disease, cancer, and osteoporosis. There were three major parts of the WHI:

  • a randomized study of promising but unproven prevention strategies
  • an observational study looking for factors that could predict which women would develop specific diseases
  • a study looking at different community-based approaches to help women develop healthy behaviors

More than 67,000 postmenopausal women aged 50 to 79 were included in this analysis. The researchers measured all the women’s height and weight at the beginning of the study in 1993 and then each year after that. The researchers also collected other information:

  • routine mammogram results and whether the women were screened yearly or every other year
  • personal and family health histories
  • smoking habits
  • exercise habits
  • demographic information (where the women lived, their income and education levels, etc.)

After about 13 years of follow-up, 3,388 invasive breast cancers had been diagnosed in the women.

Echoing the results of earlier studies, the researchers found that women who were overweight and obese had a higher risk of breast cancer compared to women at a healthy weight.

Women who had a BMI higher than 35 had a much higher risk of developing hormone-receptor-positive breast cancer; their risk was more than 80% higher than the average woman’s.

Women with a BMI higher than 35 also were more likely to be diagnosed with more advanced disease, including:

  • cancers that were larger at diagnosis
  • cancers that had spread to the lymph nodes
  • cancers that had spread away from the breast area to other parts of the body

Women with a BMI lower than 25 at the beginning of the study (meaning they were at a healthy weight) who gained more than 5% of their body weight during the follow-up period also had a higher risk of breast cancer compared to women who stayed at a healthy weight; their risk was about 36% higher than average. Still, this increase in risk wasn’t as high as it was for the women with a BMI higher than 35.

The results also showed that even if postmenopausal women who were overweight or obese at the beginning of the study lost weight during the study, it didn’t seem to lower their higher risk of breast cancer.

In an accompanying editorial, Clifford Hudis, M.D., chief of the Breast Cancer Medicine Service at Memorial Sloan-Kettering Cancer Center and member of the Breastcancer.org Professional Advisory Board, and a colleague wrote: “In additional to identifying an increased risk for women who are overweight or obese, [the] data suggest a dose-response relationship with higher BMI categories associated with greater relative risk. …[The researchers] help refine our understanding of the risk of overweight and obesity; it is a particular concern for the most common form of breast cancer, hormone-receptor-positive postmenopausal disease.

“The investigators also made a frustrating observation with regard to weight loss: it was not protective, whereas weight gain (among women who were in the nonoverweight/nonobese category at baseline) raised risk. This challenges the simple suggestion that patients who are overweight or obese should just lose weight to reduce their cancer risk. Weight control (when achieved) may be very effective for many weight-associated illnesses and ailments, but the data suggesting that it will reduce an already elevated risk of breast cancer are limited. We need clinical trials to determine whether weight loss and body composition changes in overweight and obesity will reduce breast cancer risk.”

Losing weight can be harder as you get older, 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 health 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.

While this study didn’t find any breast cancer protective benefits when overweight and obese postmenopausal women lost weight, we do know that losing weight can help lower any inflammation in your body and also reduce your risk of heart disease. More research is needed to figure out if losing weight can lower breast cancer risk in overweight women.

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.



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