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June 12, 2015 | 5 minute read

Study: Obesity Increases Breast Cancer Risk, Preventing Weight Gain Key

Postmenopausal women who are overweight — and especially obese — have a greater risk of developing breast cancers, finds a new study that highlights the importance of preventing weight gain, as it also raises questions about whether losing weight necessarily reduces that risk.

The study adds to a consistent body of research showing that overweight and obesity increases women’s risk of postmenopausal breast cancers. It was published yesterday in JAMA Oncology.

AICR estimates that a third of US breast cancers could be prevented if women were at a healthy weight throughout life, were active and did not drink alcohol.

In this study, as other research has seen, the heavier the women, the greater the risk. Women categorized as the most obese were at almost double the risk of the most common type of breast cancer, ER-positive, along with PR-positive tumors. These cancers are fueled by the hormones estrogen and progesterone, respectively.

The study used data from approximately 67,000 women who were all part of the Women’s Health Initiative trials. That study focused on preventing certain cancers, heart disease and osteoporosis. When the women entered the study in the mid 1990s they were 50 to 79 years old, and they were weighed. They also answered questions about their lifestyle habits, medical history and other health risk factors. After that, they were weighed annually and had regular mammograms.

Women were placed into weight categories based on their BMI (body mass index), a rough measure of body fat that takes height and weight into account. A BMI of 25 to 30, for example, is considered overweight; a BMI of 30 and above is obese.

After an average followup of 13 years, 3,388 of the women had been diagnosed with invasive breast cancers.

Compared to women at a healthy BMI (18.5 up to 25), women who were overweight had a 17 percent increased risk and that risk increased to 58 percent among women who were in the highest categories of obesity having a BMI over 35). Breast cancer deaths were also more than 2-fold higher among women who the most obese compared with those with normal BMI.

Women with who started the study with a healthy BMI but who gained more than 5% of their body weight over the 13 years also were at increased breast cancer risk.

For the women who started the study overweight or obese, losing —or gaining —weight did not affect breast cancer risk. These results may be clouded because it’s unknown whether some of that weight loss was intentional or unintentional, notes lead author Marian L. Neuhouser, PhD, RD, at the Fred Hutchinson Cancer Research Center. “For the overweight and obese women, it could be that if they had been overweight or obese for a while that the ‘damage may have been done.'”

And while “we can’t tell definitively from these data whether weight loss would be helpful or effective for women already overweight or obese…. This finding underscores the importance of prevention and of achieving and maintaining a healthy weight throughout the life cycle,” says Neuhouser.


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This study is one of the largest to add to a growing body of research on how weight change may affect women’s risk of breast cancers. Other studies are mixed and many focus on weight over the entire lifespan, having women recall their weight as teens.

But in short-term intervention trials, which don’t run long enough to see clinical cancer outcomes, we do see beneficial changes in estrogen, insulin and markers of inflammation, says AICR Nutrition Advisor Karen Collins, MS, RDN. “Since these are the mechanisms through which research suggests obesity increases cancer risk, these changes suggest potential to reduce risk.”

“However, though 5% weight loss is enough to show beneficial effects on markers of heart disease risk, these benefits are greater with weight loss of 10%… so perhaps for some women, it takes slightly greater loss than could be seen in the WHI study grouping all women with 5% or more loss together.”

Two-thirds of US women are currently overweight or obese and for these women, weight loss and maintaining that healthy weight has numerous health benefits, including lowering risk of heart disease and type 2 diabetes. Overweight and obesity also links to 9 other cancers, aside from breast cancer, many of which are common among women.

“Since we know that weight loss based on healthy eating and physical activity clearly brings other health benefits, for women who are overweight or obese, these findings should not discourage efforts to reach and maintain a moderate weight loss,” says Collins.

Study funding/disclosures: The study notes no specific funding source. One author (Dr. Chlebowski) has received speaker’s fees and honorarium from Novartis; honorarium for advisory boards and consulting for Novartis, Novo Nordisk, Pfizer, Genetech, and Amgen.

BMI is a common method of finding out if you are a healthy weight (there are others); we have a BMI calculator here.

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