Body mass index (BMI) is a measure used to define body weight status (normal, overweight and obese).
BMI includes a measure of height and weight, so it is a better comparison of body weight status than weight alone. Calculate your BMI or find your BMI in a table.
For people ages 20 and older, weight status categories are defined as:
Body weight status
18.5 to 24.9
25.0 to 29.9
30.0 and greater
Many studies link BMI to breast cancer risk. However, BMI affects risk differently before and after menopause.
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Women who are overweight or obese before menopause have a 20 to 40 percent lower risk of breast cancer than those who are lean [12,54-56]. This benefit is not well understood and may be limited to estrogen receptor-positive breast cancers .
Although being overweight or obese may lower breast cancer risk before menopause, weight gain should be avoided. Most breast cancers occur after menopause. Any weight you gain before menopause may carry into your postmenopausal years.
Women who are overweight or obese after menopause have a 30 to 60 percent higher breast cancer risk than those who are lean [54-56].
Being overweight after menopause may increase the risk of estrogen receptor-positive breast cancers more than estrogen receptor-negative cancers.
A meta-analysis that combined the results of four studies found postmenopausal women who were heavy had a 70 percent greater risk of estrogen receptor-positive breast cancer compared to those who were lean. However, these heavier women did not have an increased risk of estrogen receptor-negative breast cancers .
For a summary of research studies on body weight and breast cancer, visit the Breast Cancer Research Studies section.
Before menopause, most estrogens in the body are produced in the ovaries. After menopause, the ovaries no longer produce much estrogen and estrogens mainly come from fat tissue.
Fat tissue contains an enzyme called aromatase that converts hormones called androgens (made mostly in the adrenal glands) to estrogens. So, heavier women have higher blood estrogen levels than leaner women .
This extra estrogen likely explains at least some of the increased breast cancer risk among postmenopausal women who are overweight. Women with higher estrogen levels have an increased risk of breast cancer compared to women with lower estrogen levels .
Learn more about estrogen and breast cancer risk.
Women who are heavier also tend to have higher levels of insulin in their bodies compared to leaner women .
Some studies have shown an increased risk of breast cancer among postmenopausal women with high levels of insulin including women with type 2 diabetes [60-62].
Among premenopausal women, findings on a possible link between insulin levels and breast cancer risk are less clear [63-64].
Learn more about insulin and breast cancer risk.
These topics are still under study.
Gaining weight in adulthood appears to increase the risk of breast cancer before and after menopause [65-68].
One large study found that women who gained about 20 pounds after age 18 had a 15 percent higher risk of breast cancer compared to women who gained little or no weight . Women who gained 55 pounds or more had a 45 percent higher risk .
It’s not just the weight a woman gains after age 18 that seems to be important to risk. The weight a woman gains after menopause also appears to increase their risk of breast cancer [65,69].
Findings from the same large study above showed that women who gained 20 pounds or more after menopause had an 18 percent higher risk of breast cancer compared to those who gained little or no weight after menopause .
For a summary of research studies on weight gain and breast cancer, visit the Breast Cancer Research Studies section.
Losing weight after menopause may help lower risk of breast cancer .
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 . Not all studies have shown this benefit .
Weight loss in adulthood and breast cancer risk before menopause is under study .
Body shape may also affect breast cancer risk.
Some findings show women who put on extra weight around their middle sections (sometimes called "apple-shaped"), as opposed to their hips and thighs (sometimes called "pear-shaped"), have a small to moderate increased risk of breast cancer [72-75].
Other findings show that after BMI is taken into account, body shape does not increase breast cancer risk .
This topic is still under study.
Maintaining a healthy weight is important for breast cancer survivors.
Learn about body weight and breast cancer recurrence and survival.
For a summary of research studies on body weight and breast cancer survival, visit the Breast Cancer Research Studies section.
Making healthy lifestyle choices has benefits at any age. Being more active, eating a balanced diet and becoming more aware of your health can be physically and mentally rewarding at any point in life.
The U.S. Department of Agriculture’s website www.choosemyplate.gov has free tools to help you set weight and activity goals for healthy living. For example, their tool Supertracker can help you plan, track and analyze your diet and exercise.
Susan G. Komen's breast self-awareness messages
1. Know your risk
2. Get screened
3. Know what is normal for you and see a health care provider if you notice any of these breast changes (see images):
4. Make healthy lifestyle choices
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date at this time.
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