Are you at high risk of developing breast cancer? Cancer of the breast is a disease that many women fear, but, fortunately, if you catch it early, it has a good prognosis. Breast cancer is the most common cancer diagnosed in American women, but only about 10% of women diagnosed with breast cancer have a family history of the disease. The other 90% can be blamed on lifestyle and environmental exposures. The lifetime risk of being diagnosed with the disease is currently about 1 in 8, and the risk increases with age.
Body Composition and Breast Cancer Risk
One factor that contributes to breast cancer risk is body weight. Studies show that being overweight or obese increases the odds of developing breast cancer after menopause, although the incidence of pre-menopausal breast cancer is actually lower in women who are obese or overweight. But, overall, the rate of breast cancer is highest after menopause. That’s why it’s so important to avoid obesity as you age.
Health care professionals use BMI, or body mass index, to measure obesity. But body mass index only looks at a person’s height relative to their total body weight. It doesn’t distinguish between body fat and muscle. Because athletes have more muscle than the average person, they sometimes fall into the “obese” category. So, BMI is not a good measure of body composition or health risks. What matters more is body composition. So, we should also look at body fat percentage and waist circumference as a measure of whether a person has a healthy body composition.
In fact, studies show that waist circumference is a better marker of future health risks than BMI or total body weight. Studies suggest that people who have a waist size larger than half their height are at greater risk of health problems related to obesity, including cardiovascular disease and type 2 diabetes.
But what role does body composition, as opposed to total body weight, play in the risk of developing breast cancer? According to a recent study published in the JAMA Oncology, body composition is a better marker of breast cancer risk than BMI or body weight.
In the study, researchers followed 161,800 post-menopausal women between the ages of 50 and 79 years of age. Prior to the study, they collected data about their menstrual history, reproductive history, family history, medical history, and lifestyle. Each participant underwent a DXA scan to look at their body fat percentage as well as the distribution of that body fat. As you might expect, some women carried more fat around their belly while others had more weight around the thighs.
After following a portion of the women for 16 years, they discovered that those with higher body fat were more likely to develop estrogen receptor-positive breast cancer (ER-positive), the most common type. ER-positive breast cancer is characterized by estrogen receptors on the surface of the tumor cells. When estrogen binds to these receptors, it fuels the growth of the breast tumor. One of the treatments for estrogen-receptor positive breast cancer is anti-estrogen therapies. These treatments either block the activity of estrogen on the breast or lower estrogen levels in the body.
Waist Fat is Particularly Risky
In the study, researchers also found that women who carried more of their body fat around the trunk had a higher risk of invasive breast cancer. So, it wasn’t just having more body fat that was linked with a higher risk but having more trunk or belly fat. In fact, they found that with every 5-kilogram increase in trunk fat, the risk of developing estrogen receptor-positive breast cancer rose by 56%. This was true even among the women who had a normal BMI.
What does this suggest? Carrying too much body fat in general, irrespective of BMI or body weight, is a risk factor for breast cancer and belly fat is a particularly dangerous kind of fat in terms of breast cancer risk and health in general. We already know that people with a large waistline and greater quantities of belly fat are metabolically less healthy and often have insulin resistance. Belly fat also produces inflammatory chemicals called cytokines that cause low-grade inflammation.
It makes sense that breast cancer risk would be higher in people who have higher circulating insulin levels due to insulin resistance. Insulin acts as a growth factor that can fuel the growth of tumor cells as well, both directly and by boosting the levels of IGF-1 in the area of the tumor. Plus, low-grade inflammation is a contributor to cancer as well.
Studies also show that inflamed fat cells in breast tissue, indirectly related to insulin resistance, produce more of an enzyme called aromatase. Aromatase increases the conversion of androgens to estrogens and estrogens fuel the growth of estrogen receptor positive breast tumors. It’s all making sense now, doesn’t it? We need to maintain a healthy percentage of body fat, not just a normal BMI, and avoid abdominal obesity and insulin resistance.
The timing of weight gain and weight loss matters too. The Nurse’s Health Study found that women who gained weight after menopause were at higher risk of breast cancer relative to women who maintained their weight or lost weight. In fact, losing weight after menopause was associated with a reduction in risk in women who never used hormonal therapy.
These data suggest that weight gain during adult life, specifically after menopause, increases the risk of breast cancer among postmenopausal women, whereas weight loss after menopause is associated with a decreased risk of breast cancer. Thus, in addition to other known benefits of healthy weight, these results provide another reason for women approaching menopause to maintain or lose weight, as appropriate.
The Take-Home Message
This study suggests that having excess body fat and belly fat, irrespective of BMI, is associated with a higher risk of estrogen receptor-positive breast cancer. To lower the risk, we should monitor body fat percentage and waist circumference closely over a lifetime and avoid gaining body fat as we age. Excess body fat, particularly around the waistline, creates a hormonal environment that supports the growth of estrogen receptor positive breast tumors.
To further reduce your risk, eat a whole food diet with lots of fruits and vegetables and exercise regularly. Studies show that exercise is linked with a modest reduction in the risk of developing breast cancer. Also, don’t smoke and avoid alcohol.
JAMA Oncol. Published online December 6, 2018. doi:10.1001/jamaoncol.2018.5327.
Up-to-Date. “Factors that modify breast cancer risk in women”
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Breast Cancer Res Treat. 2013 Jun;139(3):851-61. Epub 2013 Jun 15.