To learn more about the role of the environment in breast cancer, Komen sponsored a study from the Institute of Medicine, "Breast Cancer and the Environment, a Life Course Approach." A culturally appropriate, related Question and Answer booklet on breast cancer and the environment is also available in Spanish.
Many possible risk factors for breast cancer are under study. Though the findings to date are not strong enough to say whether these factors are truly related to breast cancer, research suggests they call for further study.
The factors below are listed alphabetically, not by level of risk or strength of evidence.
French fries and other foods cooked at high temperatures contain a chemical called acrylamide. Five large cohort studies have found no link between eating foods that contain acrylamide, like French fries, and breast cancer risk [293-297].
The time in life when a person eats foods containing acrylamide may be important. One study found an increased risk of breast cancer among women who often ate French fries when they were preschool-aged (three to five years old) . These topics are under study.
Few studies have looked at a potential link between antibiotics and breast cancer risk. Some studies have found the use of antibiotics increases risk, while others have found no link between the two [299-304]. The authors of one of the studies that found an increase in risk stated the link to breast cancer could be explained by other factors . The one cohort study found a weak increase in risk only among premenopausal women using long-term antibiotics to treat urinary tract infections .
At this time, it is unclear whether antibiotics are linked to breast cancer risk. Current research shows this topic deserves further study.
Study findings on a potential link between aspirin use and breast cancer have been mixed. Some studies, including two meta-analyses, have found a slight decrease in risk with regular aspirin use [305-307]. However, other cohort studies have found no difference in risk between women who took aspirin regularly compared to those who did not [308-311]. And, findings from the Women’s Health Study, a randomized controlled trial, found no difference in breast cancer risk among women who took aspirin every other day for 10 years compared to those who took a placebo .
More research is needed to draw conclusions on a possible link between the use of aspirin, as well as other non-steroidal anti-inflammatory drugs (NSAID), and breast cancer risk.
Some studies, including a meta-analysis, have found that women with a higher birthweight (a woman’s weight at her birth) have an increased risk of breast cancer (especially before menopause) [313-315]. Other studies have found no link between birthweight and breast cancer risk [316-318].
Reasons for a possible link between birthweight and breast cancer risk are unclear. A higher birthweight does not appear to be linked to higher estrogen levels in adulthood or breast density [317-318]. Birthweight may be a marker of factors related to growth (such as adult height) that affect breast cancer risk [318-319].
More research is needed to learn whether or not birthweight is linked to breast cancer risk.
Study findings on a potential link between breast size and breast cancer have been mixed. Some studies have found having a larger breast size increases risk, while others have found no link between breast size and risk [320-325].
Two studies have found that breast size increases risk among thin women, but not among heavy women [323,325]. In contrast, one small study found women with smaller breasts may have a higher risk of breast cancer due to tissue density patterns that are more common in smaller breasts .
More research is needed to learn whether or not breast size is linked to breast cancer risk.
Researchers are studying whether there is a possible link between having been breastfed as an infant and a lower risk of breast cancer. Most large studies to date have found no difference in breast cancer risk between women who were breastfed as an infant and women who were not breastfed [326-330].
At this time, there are too few studies on this topic to draw conclusions. Data on other early life exposures are also limited.
Learn about breastfeeding and the risk of breast cancer in mothers.
Dairy products are under study as a factor that may:
Some researchers have suggested that the high fat content of many dairy products or traces of pesticides or growth hormones in milk may increase risk [331-332]. Others have studied whether the calcium and vitamin D in dairy products may protect against breast cancer [331,333].
Large cohort studies and a pooled analysis of data from more than 20 studies have found no link between consuming dairy products (including milk, cheese and yogurt) and breast cancer risk after menopause [334-337].
At this time, it appears unlikely that consuming dairy products is related to breast cancer after menopause.
Most studies have found no link between consuming dairy products and breast cancer in premenopausal women [334-336]. However, data from the Nurses’ Health Study II found women who ate a lot of high-fat dairy products (like whole milk or butter) had an increased risk of breast cancer before menopause . More research is needed to confirm these findings.
For a summary of research studies on dairy/milk products and breast cancer, visit the Breast Cancer Research Studies section.
Exposures during pregnancy (in utero) have been suggested as risk factors for breast cancer among offspring. Diethylstilbestrol (DES) was a drug with estrogen-like traits that was given to women to prevent pregnancy problems until about 1970. DES was taken off the market because it was shown to increase the risk of vaginal cancers in daughters of women who took the drug.
There is some concern DES might also increase the risk of breast cancer among daughters of women who took the drug during pregnancy. Studies to date have had mixed findings [339-345]. This topic is under active study.
Data on other in utero exposures are limited at this time.
Women who have type 2 diabetes may have an increased risk of breast cancer after menopause [173-175]. For premenopausal women, there does not appear to be an increased risk .
The reasons behind a possible link between diabetes and breast cancer are unclear, but may be related to high levels of insulin in women with diabetes. Insulin is a hormone released by the pancreas. It helps the body's cells take up sugar that enters the bloodstream after a meal with carbohydrates. Women with other conditions related to high levels of insulin (such as hyperinsulinemia) may also have an increased risk of breast cancer after menopause [346-349]. These issues are under active study.
Most studies have shown no link between a high-fat diet in adulthood and an increased risk of breast cancer [350-356]. However, the type of fat, rather than the total amount of fat, may be important to breast cancer risk [350-351,354,356]. This topic is under study.
Eating a high-fat diet during the teenage years (adolescence) may also play a role in breast cancer. Findings from the Nurses’ Health Study II showed that women who ate a diet high in total fat when they were teens had an increased risk of premenopausal breast cancer compared to women who ate a diet low in fat as teens .
For a summary of research studies on dietary fat and breast cancer, visit the Breast Cancer Research Studies section.
Fertility drugs stimulate the ovaries, causing estrogen levels in the body to increase. Because high estrogen levels are linked to breast cancer risk, it has been suggested that the use of fertility drugs might also increase risk.
Most studies, including a meta-analysis of 12 studies on the fertility drug clomiphene (Clomid) and 22 studies on other types of fertility drugs have found no link to breast cancer [358-364].
At this time, the use of fertility drugs does not appear to increase breast cancer risk, but studies with longer-term data are needed to confirm these findings.
Learn more about estrogen and breast cancer risk.
Folic acid (also called folate) is a vitamin found in many fruits and green leafy vegetables. It is also added to some breakfast cereals, breads and many multivitamins.
Folic acid is important in copying and repairing DNA. Getting enough folic acid may make it more likely that DNA is correctly copied when cells divide. This in turn may make it less likely that cells become cancerous. For this reason, folic acid and multivitamins have been studied to see if they might lower the risk of breast cancer.
Most studies, including three meta-analyses have found no link between blood levels of folic acid, eating a diet high in folic acid, taking a folic acid supplement or taking multivitamins and breast cancer risk [365-373]. However, given the role of folic acid in DNA repair, more research is needed to draw conclusions about a possible benefit from folic acid or multivitamin use on breast cancer risk.
Insulin-like growth factor 1 (IGF-1) is a hormone that plays a role in the growth and development of the body.
Some studies have found that higher levels of IGF-1 in the blood may increase the risk of breast cancer [374-378].
Though more studies are needed to confirm possible links to breast cancer, measures of blood levels of this hormone may one day help estimate risk.
For a summary of research studies on IGF-1 and breast cancer, visit the Breast Cancer Research Studies section.
Some have suggested eating meat may increase the risk of breast cancer. Possible reasons include increased fat intake, exposure to chemicals formed from cooking meat at high temperatures and exposure to hormones in meat [332,379]. However, results from two pooled analyses have found no link between meat intake and breast cancer risk [336,380].
The timing of eating a diet high in meat may be important. Some findings suggest eating a diet high in red meat during the teenage years may increase breast cancer risk, but further study is needed .
Studies have looked at whether eating well done or overcooked meat is linked to an increased risk of breast cancer. Some studies have found a link, while others have not [379,381-384].
Read our perspective on meat consumption and breast cancer risk(July 2010).*
For a summary of research studies on meat consumption and breast cancer, visit the Breast Cancer Research Studies section.
*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.
Some early findings suggest that women who have a lot of moles on their bodies have a higher risk of breast cancer than women with few moles [389-390].
The reasons behind a possible link are not yet clear. However, a high number of moles may be a marker of increased levels of estrogen and testosterone in the body . Increased levels of these hormones are linked to an increased risk of breast cancer .
More research is needed before conclusions can be made about a possible link between moles and breast cancer risk.
Learn more about estrogen and breast cancer risk.
Learn more about testosterone (and other androgens) and breast cancer risk.
Parabens are chemical preservatives found in many cosmetics and body care products (including make-up, lotion and shampoo), as well as foods and medicines. (Most deodorants and antiperspirants do not contain parabens.)
Parabens have been shown to have very weak estrogen activity. Animal studies have found some health effects of parabens, but only in very large doses [391-392].
In 2008, the Cosmetics Ingredient Review Expert Panel, which conducts research on product safety for the U.S. Food and Drug Administration (FDA), first concluded that paraben exposure from cosmetics and body care products was safe . In 2012, they looked at newer data and again concluded that paraben exposure from these products was safe .
The Panel found the estrogen effects from parabens are so weak it is highly unlikely they can be harmful to health, especially in the small amounts used in cosmetics and body care products [392-394]. The Panel also noted parabens do not build up in the body over time and therefore are unlikely to cause harm [392-393].
More research in this area is needed though before conclusions can be made about a possible link between products containing parabens and breast cancer risk.
Find more information from the FDA on the safety of products containing parabens.
Read our perspective on body care products containing parabens andbreast cancer risk (April 2010).*
Links between plastics and cancer are often reported by the media and in e-mail hoaxes (one e-mail hoax falsely claims to be a study from Johns Hopkins University). However, there is no scientific evidence supporting a link between using plastic items, such as drinking water from a plastic bottle, and the risk of breast cancer.
Bisphenol A (BPA) is a chemical found in some plastic containers for food and beverages. Small amounts of BPA from the containers can get into the food and beverages inside. As a result, we can be exposed to low levels of BPA. At this time, there is no evidence to suggest a link between BPA and the risk of breast cancer [395-396].
Read our perspective on plastics containing bisphenol A (BPA) and breast cancer risk (April 2010).*
Most large cohort studies have found no link between breathing secondhand smoke (the smoke from other people's cigarettes), also called passive smoking, and breast cancer risk [397-401]. However, findings from some case-control studies and one large cohort study have shown a slightly increased risk among women with long-term or high levels of exposure [399-400].
More research is needed to draw conclusions about a potential link between passive smoking and breast cancer risk.
For a summary of research studies on secondhand smoke exposure and breast cancer, visit the Breast Cancer Research Studies section.
Smoking increases the risk of many cancers, but its effect on breast cancer is still under study. Although findings on a possible link to breast cancer remain mixed, there is growing evidence that smoking may slightly increase the risk of breast cancer [19,397,402-403].
Some studies show that smoking long-term, starting early in life, especially before a first pregnancy, may increase risk later in life [402-405]. Others have found no link between the two .
Women who smoke are more likely than non-smokers to be thin, have lower blood estrogen levels and be younger at menopause, all of which can lower risk of breast cancer . On the other hand, data show that smoking may be linked to factors that can increase risk of breast cancer, such as having fewer children and alcohol use . Certain genes may also affect whether smoking increases breast cancer risk in some people [408-409]. These factors may explain some of the mixed study findings.
Although smoking may only slightly increase breast cancer risk, stopping smoking, or never starting to smoke, is one of the best things you can do for your health. Smoking causes :
Read our perspective on smoking and breast cancer risk(November 2012).*
For a summary of research studies on smoking and breast cancer, visit the Breast Cancer Research Studies section.
For breast cancer survivors, the risks from smoking are clearer.
A large pooled analysis of data from about 10,000 breast cancer survivors found that smoking increased the risk of recurrence and death from breast cancer . The more women smoked, the higher these risks . Moreover, smoking doubled the risk of death from any cause for survivors .
Learn more about smoking and breast cancer survival.
For a summary of research studies on smoking and breast cancer survival, visit the Breast Cancer Research Studies section.
Soybeans, as well as other plants like flaxseed, certain grains, beans, fruits and vegetables, contain chemicals called phytoestrogens that mimic estrogen in the body. Some cell studies have shown soy can increase cancer cell growth [412-414]. However, many Asian countries (where people eat a lot of soy) have low rates of breast cancer. This seeming conflict has caused debate about the role of soy in breast cancer risk.
Some studies have found eating a diet high in soy and other phytoestrogens may lower the risk of breast cancer, while others have found no benefit [415-418].
Studies in Asian American and Asian women tend to show that eating a lot of soy lowers breast cancer risk. However, studies in non-Asian women have found no benefit of soy on breast cancer risk . These findings may differ because Asian women tend to eat more soy throughout their lives (starting early in life) compared to other women .
Although eating soy does not appear to increase breast cancer risk, the potential benefit of soy on risk remains unclear. This topic is under study.
Learn more about estrogen and breast cancer risk.
Read our perspective on soy and breast cancer(May 2015).*
Read our perspective on the role of soy in breast cancer recurrence and survival(February 2010).*
Learn more about soy and health.
For a summary of research studies on soy and breast cancer, visit the Breast Cancer Research Studies section.
For a summary of research studies on soy and relief of menopausal symptoms, visit the Breast Cancer Research Studies section.
Although one large study found a weak link between life events and breast cancer risk, most studies do not support a link between stress and breast cancer [419-426].
At this time, there are too few studies on stress and breast cancer risk to draw conclusions about a possible link between the two.
A possible link between vitamin D and breast cancer is under study.
Most of the vitamin D a person gets comes from the sun and a small amount comes from diet.
A few studies have found vitamin D exposure (through diet alone or diet plus sunlight) lowers breast cancer risk [334,427-428]. However, most studies, including a randomized controlled trial and a meta-analysis of 11 studies, have found no link between vitamin D and breast cancer risk [429-431].
Studying vitamin D presents some challenges. It is difficult to measure sunlight exposure and because so many foods that contain vitamin D also contain calcium, it is hard to tease out the effects of vitamin D alone. By studying blood levels of vitamin D, researchers can avoid these issues.
A few studies have found women with higher blood levels of vitamin D had a lower risk of breast cancer compared to women with lower levels [428,432]. Most studies, however, have found no link between blood levels of vitamin D and breast cancer risk [433-436].
Further study is needed to know whether vitamin D affects breast cancer risk.
Read our perspective on the vitamin D and breast cancer risk (January 2011)*.
Learn more about vitamin D.
The data in this section come from two main types of research studies:
The goal of these studies is to give information that helps support or disprove an idea about a possible link between an exposure (like alcohol use) and an outcome (like breast cancer) in people. Although they have the same goal, observational studies and randomized controlled trials differ in the way they are conducted and in the strength of the conclusions they reach.
Learn more about different types of research studies.
Animal studies add to our understanding of how and why some factors cause cancer in people. However, there are many differences between animals and people that make it hard to translate findings directly from one to the other. Animal studies are also designed differently than human studies. They often look at exposures in larger doses and for shorter durations than are suitable for people. Thus, animal studies can lay the groundwork for research in people, but in order to draw conclusions for human populations, we need human studies.
All data presented within the About Breast Cancer section of this website come from human studies unless otherwise noted.
Several organizations conduct research and/or prepare summary reports of research on certain exposures shown to have a link (or no link) to breast and other types of cancer. These agencies are a good place to find detailed, up-to-date information (for example, if you have concerns over a news item on cancer).
IARC is a part of the World Health Organization. The CDC, NTP and FDA are all part of the U.S. Department of Health and Human Services.
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