Illustration by Kim Carney / Fred Hutch
What are breast cancer’s main causes?
We don’t know what causes breast cancer, but we know what increases or decreases risk.
The risk factors include:
- Getting older, as breast cancer incidence and death rates generally increase with age; the majority of new cases and deaths from breast cancer occur in women 50 and older.
- Having a strong family history of breast cancer.
- Having breasts that show up as “dense” on mammograms.
- Having a history of certain benign breast diseases.
- Carrying a particular gene mutation that greatly increases risk (such as in one of the BRCA genes).
- Drinking more than an average of one alcoholic drink per day.
- Starting menstrual periods before age 11.
- Never getting pregnant or having a first pregnancy after age 30.
Are most cases of breast cancer potentially preventable?
Most researchers would agree that at least 80 percent of breast cancers are caused by lifestyle or environmental factors, not by a genetic predisposition inherited from your parents. However, scientists estimate that at least 25 percent of new breast cancer cases could be avoided if women kept their weight at normal levels and maintained a physically active lifestyle throughout their lives.
What lifestyle factors help prevent breast cancer?
There are a number of things women can do to reduce their risk of breast cancer:
- Stay at a healthy weight – try to keep your body-mass index under 25 (use an online calculator to find your BMI).
- Eat a diet that’s high in vegetables and fruits, high in whole grains, lower in fat – especially saturated fats – and low in alcohol.
- Keep active – strive to at least follow the U.S. Surgeon General’s guidelines of 30 minutes a day of moderate-intensity physical activity at least five days per week. More exercise is associated with an even lower risk.
- Drink little or no alcohol – keep to an average of one drink per day or fewer.
- Don’t smoke.
- Childbearing women should breast-feed for as long as possible – ideally six months or longer.
Photo by Robert Hood / Fred Hutch
Is it ever too late to try to reduce your risk?
It’s never too late to make positive lifestyle changes! We’ve shown in our studies that women who lose weight or take up an exercise program, even when they are in their 50s, 60s or 70s, can reduce their breast cancer risk.
What are the mechanisms behind the link between overweight/obesity and increased risk of breast cancer?
Women who are overweight or obese make excessive amounts of some hormones and other blood factors. This includes estrogens, which are produced in fat tissue after menopause. Fat tissue contains inflammation-producing cells and promotes excessive production of insulin and glucose, all of which can increase cancer risk. Losing as little as 5 percent of one’s body weight reduces these risk factors substantially.
What should a woman do if she is at high risk of breast cancer due to family history or genetic factors?
She should talk to her doctor or health care provider. High-risk women may need more frequent screening so that if a breast tumor develops, it can be caught at an earlier stage. Those at very high risk, especially women with the BRCA gene mutation, may decide to undergo removal of both breasts as a way to reduce risk.
Are there medications that reduce a woman’s odds of breast cancer?
Some medications have been shown to reduce breast cancer risk. They include tamoxifen, raloxifene and the aromatase inhibitors exemestane and anastrozole. There are some risks to these medications, so women should talk with their doctor to determine which one might be appropriate.
What about vitamins or supplements?
There are no data showing that any vitamin or supplement will reduce breast cancer risk.
Are there medications women should avoid?
Women should avoid hormone-replacement therapy to manage menopause symptoms. The Women’s Health Initiative, a landmark nationwide study coordinated here at Fred Hutch, found hormone-replacement therapy that included progesterone dramatically boosted the risk of breast cancer. As a result, millions of women stopped taking the therapy and U.S. breast cancer rates started to drop. An estimated 20,000 U.S. women per year since 2002 have been spared from developing breast cancer – with tens of thousands of additional lives spared in other countries.