Most menopausal women will tell you how hard it is to lose weight. Yet there’s one very good reason why they should. Postmenopausal women who are overweight and obese have an increased risk of invasive breast cancer compared to women of normal weight, finds a new analysis of data from the Women’s Health Initiative clinical trials.

Even more, compared to normal weight women, obese women were more likely to have large tumors, poorly differentiated tumors (these tend to grow faster), and disease spreading into the lymph nodes — all factors indicating a worse prognosis.

For some time now, public health officials have warned of a relationship between obesity and breast cancer. Meanwhile, the number of obese people climbs, particularly in the United States. The percentages are nothing short of shocking:

“The number of Americans in the highest weight category has risen from 10 percent to 14 percent to more than 30 percent in many regions of the country since 1990,” noted the authors of an accompanying commentary. “It is predicted that more than 60 percent of the adult citizens of many states will be in the highest body mass index (BMI) category by 2030.”

Dr. Marian L. Neuhouser of the Fred Hutchison Cancer Research Center in Seattle and her colleagues looked at invasive breast cancer in postmenopausal women participating in the Women's Health Initiative. Normal weight women, as defined by the initiative, weighed in at a BMI of less than 25. Meanwhile, a BMI of 25 to 29 indicated overweight; BMI 30 to 34 indicated obese grade 1; and a BMI over 35 indicated obese grade 2. For a total of 67,142 postmenopausal women, the researchers measured and recorded heights and weights, baseline and subsequent mammograms, and whether or not the women developed breast cancer during the follow-up period, which lasted, on average, for 13 years.

Sadly, 3,388 invasive breast cancers were discovered in that time.

The analysis also showed:

  • Overweight women, obese grade 1 women, and obese grade 2 women had an increased risk of invasive breast cancer compared to normal weight women.
  • The risk was greatest for women with obese grade 2 women; they had a 58 percent increased risk of invasive breast cancer when compared to their normal weight peers.
  • A BMI of 35 or higher was linked to an increased risk of estrogen and progesterone receptor-positive breast cancer but not estrogen receptor-negative cancers.
  • Obesity was associated with large tumors, evidence of lymph node involvement, and poorly differentiated tumors, all markers of poor prognosis.
  • Normal weight women who gained more than five percent of their body weight during follow-up had an increased risk of breast cancer.
  • Hormone therapy did not influence the BMI-breast cancer relationship.

Source: Neuhouser ML, Aragaki AK, Prentice RL, et al. Overweight, Obesity, and Postmenopausal Invasive Breast Cancer RiskA Secondary Analysis of the Women’s Health Initiative Randomized Clinical Trials. JAMA Oncology. 2015.