What’s your weight got to do with your breast cancer risk?

By GeneType

What’s your weight got to do with your breast cancer risk?

Not to sound like a broken record, but yes, it is important to maintain a healthy body mass index (BMI). We are going to focus on breast cancer risk associated with BMI, but having a high BMI can also increase your risk of other cancers, heart disease, and diabetes, just to name a few.

Your BMI is a basic measurement that considers your weight and height to classify adults into underweight, normal, overweight and obese categories. Here is the CDC’s BMI calculator if you don’t know your BMI. 


The basic take-home message is that you should strive to maintain your BMI between 18.5 and 25 (the “normal” range).

The take-home message may be boring and expected, but let’s dive into the complicated yet interesting piece to the story: the BMI breast cancer risk paradox

It turns out that premenopausal women with high BMI are actually at lower risk of developing premenopausal breast cancer compared to those women with low BMI.1 However, the tables turn with menopause. Postmenopausal women with a high BMI, have an increased risk of developing breast cancer—particularly women who had a BMI less than 25 but then gained weight post-menopause.2

The crazy thing is, we don’t fully understand why this paradox exists, but we definitely have some clues.

Premenopausal breast cancer risk and BMI:

Scientists believe that one reason young obese women are at lower risk of developing breast cancer is that they have lower progesterone levels during the menstrual cycle that can result in longer cycles and more anovulatory cycles (periods where you don’t ovulate). This disruption in hormone levels ultimately influences the ability of the breast cells to grow. Likewise, fat cells in the breast tissue (called adipocytes) play a role in mammary gland development by sending out signals to the other breast cells. This may be one reason very thin young women are at increased risk of developing breast cancer. If they don’t have enough fat cells to send out signals to the breast tissue, the breast tissue may not develop “normally.” 

Postmenopausal breast cancer risk and BMI:

Postmenopausal women who are overweight (BMI greater than 25) are at higher risk of developing breast cancer. Having an increased number of fat cells (adipocytes) means higher levels of estrogen production due to higher aromatase activity. [This is part of the reason aromatase inhibitors are so successful at reducing risk of postmenopausal breast cancer incidence (eventually link to risk reduction article).]

There are many different types of breast cancer risk factors. Risk factors all have down-stream consequence or “chain-reactions” associated with that particular risk. When you modify one risk factor, the downstream events shift, and the complex network of events changes. 

Defined by Oxford’s English dictionary, the “perfect storm” refers to “a particularly violent storm arising from a rare combination of adverse meteorological factors.” (If you are currently picturing George Clooney and/or Mark Wahlberg, you’re welcome.)

We like to use this term when talking about breast cancer; its development is a perfect storm. There is still a lot of work being done to better understand the role of all the known breast cancer risk factors. It is a complex disease, and there are a lot of different ways that breast cancer can develop. But like a “perfect storm,” breast cancer risk does not rely on a single factor, but a multitude of factors that each drive a different set of chain-reactions. Abnormal BMI (too high or too low) can interfere with your body’s natural hormone regulation and can set off a chain of events that, if combined with the right risk factors, could produce breast cancer.

  1. The Premenopausal Breast Cancer Collaborative Group. Association of Body Mass Index and Age With Subsequent Breast Cancer Risk in Premenopausal Women. JAMA Oncol. 2018;4(11):e181771. doi:10.1001/jamaoncol.2018.1771
  2. Neuhouser ML, Aragaki AK, Prentice RL, et al. Overweight, Obesity, and Postmenopausal Invasive Breast Cancer Risk: A Secondary Analysis of the Women's Health Initiative Randomized Clinical Trials. JAMA Oncol. 2015;1(5):611-621. doi:10.1001/jamaoncol.2015.1546
  3. Dowsett M, Folkerd E. Reduced progesterone levels explain the reduced risk of breast cancer in obese premenopausal women: a new hypothesis. Breast Cancer Res Treat. 2015 Jan;149(1):1-4. doi: 10.1007/s10549-014-3211-4. Epub 2014 Nov 21. PMID: 25414027.


Tags: breastcancer, cancer, riskassessment, genetics, geneticrisk, breasthealth, cancerscreening, cancerprevention, preventativehealthcare, womenshealth

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