Is Higher BMI Linked to Lower Breast Cancer Risk for Premenopausal Women?

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Research has shown that 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 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.

An observational study seems to suggest that premenopausal women with a higher BMI (body mass index) may have a lower risk of breast cancer.

The research was published online on June 21, 2018 by JAMA Oncology. Read “Association of Body Mass Index and Age With Subsequent Breast Cancer Risk in Premenopausal Women.”

While the results may sound promising for overweight premenopausal women, there are several important points to keep in mind when considering this study:

  • This was an observational study. This means that the women in the study were simply observed, or followed, and certain outcomes, such as how many women developed breast cancer, were noted. The researchers made no attempt to affect the outcome with a treatment or intervention. The researchers also could not totally control for all the differences that could affect the study results. The results of an observational study are not as strong as the results of a randomized, controlled study.
  • The study used BMI as a measure of the women’s body fat. Still, women with the same BMI can have different levels of body fat, as well as different body fat distribution.
  • The weight of the women in the study often was self-reported and self-measured. It’s likely that some women reported their weight as either higher or lower than it actually was. This could dramatically affect the results.

To do the study, the researchers looked at information on 758,592 premenopausal women from 19 studies: 9 in North America, 7 in Europe, 2 in Asia, and 1 in Australia. The women joined the studies between 1963 and 2013 and each was followed for about 5 to 14 years. The researchers analyzed the number of women diagnosed with breast cancer with BMI measurements at four5 age periods:

  • 18 to 24 years
  • 25 to 34 years
  • 35 to 44 years
  • 45 to 54 years

BMI is calculated by dividing a person’s height by weight. In the studies, the women often self-reported their height and weight, and in some cases, the women measured their own height and weight. If any errors were made in reporting height and weight, the results of the study would likely be affected.

During the course of the study, 13,082 cases of breast cancer were diagnosed.

The researchers’ analysis suggested that there was an association between higher BMI and lower breast cancer risk among the premenopausal women. This association was particularly strong in women ages 18 to 24.

“Obesity is linked with a higher risk of breast cancer in older women and is one of the leading causes of cancer worldwide,” said Minouk Schoemaker, Ph.D., of the Institute of Cancer Research in London, in a statement. “But our study shows that the link with breast cancer is more complicated than we thought, and that younger women with higher BMIs are at lower risk of the disease before the menopause.”

“Obesity has many adverse effects on general health, and we do not advocate weight gain as a preventative measure against premenopausal breast cancer,” the researchers wrote. “However, understanding the mechanistic action underlying the inverse association of premenopausal adiposity with breast cancer risk could potentially identify modifiable pathways. Because the association with BMI at ages 18 to 24 is significant for ER-positive and ER-negative tumors, hormonal and non-hormonal mechanisms might be involved.

“After the menopause, obese women have an increased risk of breast cancer, which is likely due to estrogen hormones produced by fat cells,” the researchers continued. “We now need follow-up research to understand why this effect seems to be reversed in younger women.”

Stay tuned to Breastcancer.org Research News for the latest information on the relationship between BMI and breast cancer risk in younger women.

For more information on breast cancer risk factors, visit the Breastcancer.org Breast Cancer Risk Factors section.

To talk with others who have an increased risk of breast cancer, join the Breastcancer.org Discussion Board forum High Risk for Breast Cancer.


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