The BMI/Breast Cancer Paradox

Approximately 1 in 8 women will be diagnosed with breast cancer in their lifetime and 2.7% of all women will die from it. While the average woman is 61 at the time of diagnosis, meaning they are post-menopausal, breast cancer that occurs prior to menopause is far less common than postmenopausal breast cancer. We know, for example, that a higher Body Mass Index after menopause is related to a higher risk of breast cancer. What about Body Mass Index before menopause?

A team of scientists known as The Premenopausal Breast Cancer Collaborative Group focuses on researching the causes of premenopausal breast cancer. Their research pools individual-level data from multiple studies performed by institutions participating in the United States National Cancer Institute Cohort Consortium (JAMA Oncol doi:10.1001/jamaoncol.2018.1771).

For today's article the team pooled the data from 19 prospective studies: 9 from North America, 7 from Europe, 2 from Asia, and 1 from Australia, with recruiting starting as early as January of 1963 and ending in December of 2013. Each study gathered data from the participating women that included either professionally-measured or self-reported height and weight from multiple questionnaire responses over the years, allowing the researchers to calculate each participants' Body Mass Index for the age ranges of 25-34, 35-44, and 45-54.

For the purposes of this study, the authors classified Body Mass Index according to the World Health Organization's definitions, with 18.5-24.9 being normal weight, 25.0-29.9 being overweight, 30.0-34.9 considered class I obese, and 35.0-39.9 considered class II obese.

After excluding all women who were already in menopause, as well as those with a personal history of breast cancer at the start of the study, the authors could then compare the BMI at various stages of life for those women who developed breast cancer prior to menopause with the BMI at similar stages of life with those women who did not develop breast cancer prior to menopause.

Their pooled results included over 758,000 women, of whom over 13,000 developed in situ or invasive breast cancer during an average of over 9 years of follow-up.

In contrast with the consensus that a higher Body Mass Index is linked with a greater risk of cancers in general, the authors of this study found that those with a higher BMI between 18 and 24 years old were 23% less likely to develop breast cancer than those women with a BMI 5 points lower during the same period of their lives (that is, a woman with a BMI of 25-30 would be 23% less likely to develop breast cancer than a woman with a BMI of 20-25).

When comparing women with a BMI over 35 with those women whose BMI was under 17 (clinically underweight), the underweight women were over 4 times as likely to develop breast cancer as those who were clinically obese.

The authors dove deeper into the results, comparing in situ with invasive breast cancers, finding that a 5-point increase in BMI between the ages of 25-34 meant a 24% increase in risk for in situ breast cancers as opposed to a 12% increased risk for invasive breast cancers. That said, the risks for estrogen-receptor-positive or -negative breast cancers were inconsistent across ages and Body Mass Indices.

What this means for you

This should not be taken to mean that women should seek to be overweight or obese during their early adulthood in order to avoid the risk of breast cancer. Quite simply, the known risks of overweight and obesity far outweigh the possible risks of breast cancer when there are also multiple factors affecting breast cancer risk that have little to do with weight (see our Breast Cancer Overview article).

This is a case where you (the patient) and your physician need to look at the bigger picture of your overall health and look at what are the most significant risks for you in light of your family history as well as your personal health history. Following a healthy, Mediterranean-style diet, maintaining a healthy weight (taking body Body Mass Index and Waist to Hip Ratio into account), and getting enough exercise should be your focus, not simply your Body Mass Index at a particular time in your life.

First posted: June 27, 2018