Body mass index (BMI) is a measure used to help show whether or not a person has a healthy weight.
BMI includes a measure of height and weight. So, BMI is better than weight alone when making comparisons. Calculate your BMI or find your BMI in a table.
For people ages 20 and older, weight status categories are:
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.
Women who are overweight or obese before menopause have a 20-40 percent lower risk of breast cancer than those who are lean [10,65-68].
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 you may carry into your postmenopausal years.
Some findings suggest being overweight or obese may increase the risk of estrogen receptor-negative breast cancers before menopause, including triple negative breast cancers [69-70].
Women who are overweight or obese after menopause have a 30-60 percent higher breast cancer risk than those who are lean [65-67].
Being overweight after menopause may increase the risk of estrogen receptor-positive breast cancers more than estrogen receptor-negative cancers [70-71].
A meta-analysis that combined the results of 4 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 .
Women with higher estrogen levels have an increased risk of breast cancer compared to women with lower estrogen levels . So, the extra estrogen likely explains at least some of the increased breast cancer risk among postmenopausal women who are overweight.
Learn more about estrogen and breast cancer risk.
Women who are heavier 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 [73-75].
Among premenopausal women, a possible link between insulin levels and breast cancer risk is less clear [76-77].
Learn more about insulin and breast cancer risk.
These topics are under study.
Gaining weight in adulthood increases the risk of breast cancer before and after menopause [78-81].
One large study found 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 in the study 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 the risk of breast cancer [78,82].
One large study above showed 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 the risk of breast cancer [78,83].
One large study found women who lost 4-11 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 the risk of breast cancer 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 [86-89].
Other findings show that after BMI is taken into account, body shape does not increase breast cancer risk .
This topic is under study.
BMI estimates body weight status, but it has some limits. For example, it doesn’t include measures of fat and muscle in the body. So, a person with a lot of muscle and little fat can have the same BMI as a person with a lot of fat and little muscle.
Body composition, including the amount of fat versus the amount of muscle in a person’s body, can be measured with a CT scan. Body composition may be an important measure in studies of body weight status and breast cancer .
This topic is under study.
Maintaining a healthy weight lowers the risk of breast cancer after menopause and is one of the best things you can do for your health overall.
Being overweight or obese causes [92-93]:
Being overweight or obese may also increase this risk of :
Maintaining a healthy weight is important after a breast cancer diagnosis.
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 active, eating a balanced diet and making other healthy lifestyle choices can be physically and mentally rewarding at any point in life.
Learn more about healthy lifestyle choices and breast cancer risk.
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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