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 they are truly related to breast cancer, research suggests they call for further study. The factors below are listed alphabetically, not by level or 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 [294-298].
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 the potential link between antibiotics and breast cancer risk. Some studies found a higher risk with increased use of antibiotics, while others found no link between the two [300-305]. The authors of one of the studies that found a weak 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 to date 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 [306-308]. However, other cohort studies have found no difference in risk between women who took aspirin regularly compared to those who did not [309-312]. 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.
Few large studies have looked at a potential link between birthweight (a woman’s weight at her birth) and breast cancer risk. A meta-analysis and a large cohort study found women with a higher birthweight had an increased risk of breast cancer [314-315]. Another large cohort study found a higher birthweight increased the risk of premenopausal, but not postmenopausal, breast cancer . A higher birthweight does not appear to be linked to higher estrogen levels in adulthood, so the reasons for a possible link are unclear at this time .
More research is needed to draw conclusions about a possible link between birthweight and 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 [321-326].
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 [327-333].
At this time, there are too few studies on this topic to draw solid conclusions. Data on other early life exposures are also limited.
However, breastfeeding lowers the risk of breast cancer in mothers (learn more).
Dairy products are under study as a factor that may:
Some researchers have suggested the high fat content of many dairy products or traces of pesticides or growth hormones in milk may increase risk [334-335]. Others have studied whether the calcium and vitamin D in dairy products may protect against breast cancer [334,336].
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 postmenopausal breast cancer risk [337-340]. At this time, it appears unlikely that consuming dairy products is related to postmenopausal breast cancer.
Most studies have found no link between consuming dairy products and breast cancer in premenopausal women [337-339]. However, data from the Nurses’ Health Study II found premenopausal women who ate a lot of high-fat dairy products (like whole milk or butter) had an increased risk of breast cancer . 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 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 [342-348]. This topic is under active study.
Data on other in utero exposures are limited at this time.
Most studies have shown no link between a high-fat diet in adulthood and an increased risk of breast cancer [349-355]. However, the type of fat, rather than the total amount of fat, may be important to breast cancer risk [349-350]. This topic is under study.
Eating a high-fat diet during 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 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. However, most studies, including a meta-analysis of 12 studies on the fertility drug clomiphene (Clomid) and 22 studies on other types of fertility drugs found no link to breast cancer [357-364].
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, making 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 large prospective cohort studies to date have found no link between eating a diet high in folic acid, taking a folic acid supplement or taking multivitamins and breast cancer risk [365-372]. 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.
There is growing interest in the link between insulin and breast cancer risk. Insulin is a hormone released by the pancreas. Insulin helps the body's cells take up sugar that enters the bloodstream after a meal with carbohydrates. Some studies have shown an increased risk of breast cancer among postmenopausal women with high levels of insulin including:Women who produce high levels of insulin or who have high levels of insulin in their blood (a condition called hyperinsulinemia) [373-376]Women who have type 2 diabetes [168-171]Among premenopausal women, findings on a possible link between insulin levels and breast cancer risk are less clear [172-173].
One way to estimate how a food affects insulin levels is by measuring glycemic index. Glycemic index shows how fast and how strongly a food increases blood sugar levels. This determines how much insulin the body needs to release in a normal response. A high glycemic index means a food quickly increases blood sugar levels, while a low glycemic index means a food increases blood sugar levels more slowly. A person who eats a lot of high glycemic index foods will need to produce a lot of insulin to try to clear the high levels of sugar in the blood. Although some studies have found a high glycemic index diet increases breast cancer risk, most cohort studies and meta-analyses have not shown this link (or have shown a link only among certain groups of women) [377-388]. There is still much that is not known about the link between insulin levels, glycemic index and breast cancer risk. These issues are under active study. What is better established is the link between a diet rich in high glycemic foods and an increased risk of heart disease and diabetes . For this reason, choosing foods with a low or moderate glycemic index is good for your health. Foods without processed carbohydrates (such as whole wheat, whole oats and brown rice) have a lower glycemic index than foods made with highly refined grains (such as white bread, sugary breakfast cereals and white rice). Fruit and legumes are also good, low glycemic choices.
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 higher levels of IGF-1 in the blood may increase the risk of breast cancer [389-393]. 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 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 [335,394]. However, results from two pooled analyses have found no link between meat intake and breast cancer risk [339,395].
Eating meat may play a role only in certain types of breast cancer. One study found women who ate five or more servings of red meat each week increased their risk of premenopausal hormone receptor-positive breast cancer . These findings need to be confirmed in future studies.
The timing of eating a diet high in meat also may be important. Early findings suggest eating a diet high in red meat during adolescence 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 [394,398-401].
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 section.
Migraine headaches are related to falling estrogen levels in a woman’s body and estrogen is related to breast cancer risk. Some studies have found women with a history of migraines have a lower risk of breast cancer than other women [402-404]. Other studies have found no link between the two . More research is needed before conclusions can be made about a possible link between migraines 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 [318-319].
In 2008, the Cosmetics Ingredient Review Expert Panel, which conducts research on product safety for the U.S. Food and Drug Administration (FDA), concluded that paraben exposure from cosmetics and body care 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 [319-320]. The Panel also noted parabens do not build up in the body over time and therefore are unlikely to cause harm .
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 [406-407].
Read our perspective on plastics containing bisphenol A (BPA) and breast cancer risk (April 2010).*
Large prospective 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 [408-412]. However, findings from some case-control studies have shown a slightly increased risk among women with long-term or high levels of exposure [410,413-414].
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 section.
Smoking increases the risk of many cancers, but its effect on breast cancer is still under study. Although there is growing evidence that smoking may slightly increase the risk of breast cancer, overall, study findings remain mixed [408,415-417].
Some studies show that smoking long-term, starting early in life, especially before a first pregnancy, may increase risk later in life [418-420]. 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 [422-423]. 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 :
For breast cancer survivors, the risks of 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.
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 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 [425-427]. However, lower rates of breast cancer are seen in many Asian countries where people eat a lot of soy. 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 [428-431]. Studies among Asian American and Asian women tend to show that eating a lot of soy lowers breast cancer risk. However, studies among 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 .
At this time, it remains unclear whether eating soy foods lowers the risk of breast cancer. This is an area under active study.
Learn more about estrogen and breast cancer risk.
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 section.
For a summary of research studies on soy and relief of menopausal symptoms, visit the Breast Cancer Research 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 [432-439].
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 active study.
Most of the vitamin D a person gets comes from the sun and a small amount comes from diet.
Some studies have found vitamin D exposure (through diet alone or diet plus sunlight) lowers breast cancer risk [337,440-443]. Other studies, including a meta-analysis of 11 studies, have found no link between vitamin D and breast cancer risk [444-451]. Further study is needed to know whether vitamin D affects breast cancer risk.
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. Findings from large studies that have looked at possible links between blood levels of vitamin D and breast cancer risk are mixed. Some studies have found women with higher blood levels of vitamin D had a lower risk of breast cancer compared to women with lower levels, while the others have found no link [452-457].
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 Understanding 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.
*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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