Maintaining a healthy weight is one of the best things you can do for your health. However, the percentage of Americans who are overweight or obese is higher than ever. Currently, about 72 percent of men and 64 percent of women in the U.S. are overweight or obese (defined as a body mass index (BMI) of 25 or higher).1
What is BMI?
BMI is a ratio of weight (in kilograms) and height (in meters):
Because BMI includes a measure of height, it gives better comparisons of body weight status among a group of people than weight alone. For example, two people may weigh the same amount, but one may be tall and lean and the other may be short and stout. Even though they have the same weight, they would have different BMI and you would be able to compare their body weight status more easily. For this reason, BMI is the measure most commonly used in research studies on weight and health.
Calculate your BMI.
Body weight and its effects on breast cancer depend upon whether a woman has gone through menopause.
Before menopause, women who are heavier have a 20 to 38 percent lower risk of developing breast cancer compared to leaner women.3,5,7 This benefit is not well understood and may be limited to estrogen receptor-positive (ER+) breast cancers.8
Although being overweight seems to offer modest protection against breast cancer in premenopausal women, weight gain should be avoided. Most breast cancers occur after menopause, and any weight you gain before menopause you will likely carry into your postmenopausal years.
Women who are heavier have a 20 to 60 percent higher risk of postmenopausal breast cancer compared to leaner women.2-6
Similar to premenopausal weight, body weight after menopause may have the most impact on the risk of ER+ breast cancers. A meta-analysis that combined the results of four studies found postmenopausal women who were heavy had a 70 percent greater risk of ER+ breast cancer compared to those who were lean. However, these heavier women did not have an increased risk of estrogen receptor-negative (ER-) breast cancers.9
Although we do not fully understand why breast cancer risk differs in pre- and postmenopausal women, the hormone estrogen likely plays a role. Women with higher estrogen levels have an increased risk of breast cancer (learn more).10
In premenopausal women, most of the estrogen in the body is produced in the ovaries. Weight has little effect on this estrogen. In postmenopausal women, the main source of estrogen is fat which contains an enzyme that makes estrogens. So, heavier postmenopausal women tend to have higher estrogen levels compared to leaner postmenopausal women. This extra estrogen likely explains the increased breast cancer risk among postmenopausal women who are overweight.11
Women who are heavier also tend to have higher levels of insulin in their bodies compared to leaner women.12 Some studies have shown an increased risk of breast cancer among postmenopausal women with high levels of insulin including women with type 2 diabetes.13-16 Among premenopausal women, findings on a possible link between insulin levels and breast cancer risk are less clear.17-18 These topics are still under study.
Weight gain after age 18 appears to increase the risk of breast cancer in postmenopausal women. Study findings show the more weight women gain, the more their breast cancer risk tends to increase.6,19-21 For example, findings from the Nurses Health Study showed women who gained about 10 to 45 pounds after age 18 had a 15 percent greater breast cancer risk compared to women with little weight change. Women who gained more than 55 pounds after age 18 had a 45 percent increase in postmenopausal breast cancer risk.21
It’s not just the weight gained from age 18 that seems to be important to risk. Even among women who gain weight after menopause, there appears to be an increased risk of breast cancer. The National Institutes of Health-American Association of Retired Persons Health Study found women who gained more than 66 pounds after age 50 had an 89 percent increased risk of breast cancer.6
To date, only a few studies have addressed weight gain and premenopausal breast cancer risk. Most have shown no link between the two, but this topic is still under study.22-24
The good news is that for women who are overweight, losing weight may help lower breast cancer risk. And, it’s never too late to start. One large study found women who lost four to 11 pounds after menopause had more than a 20 percent lower risk of breast cancer compared to women whose weight did not change.21
According to Dr. Jennifer Ligibel, assistant professor of medicine at Harvard Medical School, “Many women ask how they can lower their risk of breast cancer. One of the important things about identifying excess weight as a risk factor for breast cancer is that, unlike many of the other risk factors for breast cancer that women can't do anything about (like being a woman or getting older), weight is something that a woman can modify to potentially lower her risk of developing breast cancer. Although losing weight isn't easy, it might help a woman avoid breast cancer. We also know that keeping your weight in a healthy range helps to lower the risk of many other health problems such as heart disease and diabetes. A healthy diet and moderate exercise should be part of a healthy lifestyle for all women.”
During adolescence, girls enter puberty and their breasts begin to mature. Because this is an important time of breast growth and development, body weight during adolescence may play an early role in breast cancer risk. Some study findings suggest girls who are heavier than average during adolescence have a decreased risk of pre- and postmenopausal breast cancer.23,25-26 And, some findings suggest girls who are leaner than average also have a decreased risk of breast cancer.25 These relationships are under active study and reflect the complex links between body weight throughout life and breast cancer risk.
Avoiding weight gain not only helps protect against breast cancer, it also has many other health benefits. Compared to women who are overweight or obese, women with a healthy weight have a lower risk of pancreatic, endometrial, ovarian, kidney and other types of cancer.5,27-30 And, they are less likely to develop other health problems, including:31-32
With so many risk factors for breast cancer out of our hands (like age, being female and genetic factors), body weight is one of the few things most of us can try to control. And, making healthy lifestyle choices has benefits at any age. Getting regular physical activity and eating a balanced diet are the best ways to avoid weight gain and maintain a healthy weight at any point in life.
In our busy lives, making healthy choices can be easier said than done. Fortunately, there are many resources available to help you plan a healthy diet and make simple changes to your daily life to increase activity. Your health care provider is a good place to start. And, many websites offer tips on adopting a healthier lifestyle, including:
Centers for Disease Control and Prevention (CDC)
Find tips on healthy ways to prevent weight gain or lose weight.
Department of Agriculture’s ChooseMyPlate.govCreate a personalized daily food plan (designed to maintain or lose weight) and find tips on balancing calories and making healthy food choices.
National Heart, Lung, and Blood Institute (NHLBI)
Find a menu planner, tips on being physically active and other tools for maintaining a healthy weight.http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/tools.htm
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