Factors Under Study

Many possible risk factors for breast cancer are under study. Though the findings to date are not strong enough to say whether or not they are truly related to breast cancer, research suggests they call for further study.

Dairy products

Dairy products are under study as a factor that may:

• Increase the risk of breast cancer
• Decrease the risk of breast cancer

Some researchers have suggested the high fat content of many dairy products or traces of pesticides or growth hormones in milk may increase risk [277-278]. Others have studied whether the calcium and vitamin D in dairy products may protect against breast cancer [277,279].

Postmenopausal breast cancer

Large cohort studies and a pooled analysis of more than 20 studies have found no link between dairy product intake (including milk, cheese and yogurt) and postmenopausal breast cancer risk [280-283]. At this time, it appears unlikely consuming dairy products is related to postmenopausal breast cancer.

Premenopausal breast cancer

Most studies have found no link between consuming dairy products and breast cancer in premenopausal women [280-282]. However, data from the Nurses’ Health Study II found women who ate a lot of high-fat dairy products (like whole milk or butter) were at higher risk of premenopausal breast cancer [284]. More research is needed to confirm these findings.  

Dietary fat

Most studies have shown no link between a high-fat diet in adulthood and an increased risk of breast cancer [285-290].

Dietary fat intake may still play a role in breast cancer. The type of fat, rather than the total amount of fat, may be important to breast cancer risk [291-293]. Some studies have found a high intake of some types of fat, such as monounsaturated fat (found in olive and canola oils), may lower breast cancer risk [292-293]. However, these results need to be confirmed by further 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 increase in premenopausal breast cancer risk compared to women who ate a diet low in fat as teens [294].

Fruits, vegetables and carotenoids

Fruits and vegetables

Although some studies show a high total intake of fruits and vegetables may lower breast cancer risk, most have not found a link between the two [295-299]. A pooled analysis that combined data from eight large studies found higher total intakes of fruits and vegetables did not lower breast cancer risk [300].

While total fruit and vegetable intake does not appear to lower risk, certain fruits and vegetables may have some benefit for certain types of breast cancer. Findings from two large cohort studies found women with the highest intake of vegetables compared to those with the lowest had a reduced risk of estrogen receptor-negative (ER-) breast cancers, but not estrogen receptor-positive (ER+) cancers. This benefit was not seen in women with a high intake of fruit [296,298-300].

Carotenoids

Carotenoids are natural orange-red food pigments found in fruits and vegetables (like melons, carrots, sweet potatoes and squash).

Many carotenoids, such as beta-carotene, are antioxidants and can be converted into vitamin A in the body. Researchers can study carotenoids through a person’s diet or by measuring levels of carotenoids in the blood.

Although a few studies show eating a diet high in foods that contain carotenoids may lower breast cancer risk, most studies have not found such a link [295,301-303].

Some findings suggest women with higher blood levels of carotenoids have a reduced risk of breast cancer compared to women with lower levels [304]. At this time, however, data are limited and study findings are mixed [305-307].

Note of caution on carotenoid supplements

Eating too much of certain carotenoids may have some health risks. A few studies have found that taking a daily supplement of the carotenoid beta-carotene may increase the risk of lung cancer and premature death in smokers [308-310].

In general, fruits and vegetables are the best sources of carotenoids (rather than supplements) and are part of a healthy diet.

Glycemic index and insulin

Insulin

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) [311-314]
• Women who have type 2 diabetes [315-319]

Among premenopausal women, findings on a possible link between insulin levels and breast cancer risk are less clear [320-321].

Glycemic index

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) [322-331].

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 [328]. 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.  

Meat consumption

It has been proposed that 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 [278,332]. However, results from two pooled analyses have found no link between meat intake and breast cancer risk [282,333].

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 estrogen receptor-positive/progesterone receptor-positive (ER+/PR+) breast cancer [334]. These findings need to be confirmed in future studies.

The timing of eating a diet high in meat 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 [335].

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 [332,336-339].

Read our perspective on meat consumption and breast cancer risk.

Soy and phytoestrogens

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 [340-342]. 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 [343-347]. Studies in Asian and Asian American women tend to show that women who eat the most soy have a lower breast cancer risk than women who eat the least soy. However, studies among non-Asian women have found no benefit of soy on breast cancer risk [344]. These findings may differ because Asian women tend to eat more soy throughout their lives (starting early in life) compared other women [344].

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 soy and health.

Learn more about estrogen and breast cancer risk.  

Read our perspective on the role of soy in breast cancer recurrence and survival.  

Vitamin D

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 [280,348-351]. Other studies, including a meta-analysis of six studies, have found no link between vitamin D and breast cancer risk [352-356]. 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. At this time, only a few large studies have looked at possible links between blood levels of vitamin D and breast cancer risk. One study found women with higher blood levels of vitamin D had a lower risk of breast cancer compared to women with lower levels, while the other two studies found no link [357-359].

Read our perspective on the vitamin D and breast cancer risk.

Learn more about vitamin D.

For a summary of research studies on soy and breast cancer, visit the Breast Cancer Research section.

Healthy diet

The diet outlined below promotes overall health and may help protect against different types of cancer and other diseases [173,200,360].

Protein hormones (prolactin and IGF-1)

Like insulin, other protein hormones —may be linked to breast cancer risk. These include:

• Prolactin
• Insulin-like growth factor 1 (IGF-1)

Though more studies are needed to confirm such links, measures of blood levels of these hormones may one day help estimate breast cancer risk.

Prolactin

Prolactin plays a role in breast growth and the production of milk during breastfeeding. One large study found high levels of prolactin in the blood can increase the risk of breast cancer [361].

IGF-1

IGF-1 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 in pre- and postmenopausal women [362-366].

Smoking

Smoking increases the risk of many cancers, but its effect on breast cancer is not yet known. Some studies show smoking increases breast cancer risk, while others do not [367-372].

There is some evidence that smoking long-term, starting early in life, especially before a first pregnancy, may increase risk later in life [368,371-375]. However, a meta-analysis of 11 studies that looked at smoking before the birth of a first child and breast cancer risk, found no link between the two [369].

Women who smoke are more likely than non-smokers to be lean, have lower blood estrogen levels and be younger at menopause, all of which can lower risk of breast cancer [370]. And, some data suggest smoking is linked to factors that can increase risk of breast cancer, such as having fewer children and alcohol use [374]. Certain genes may also affect whether smoking increases breast cancer risk in some people [376-377]. These factors may explain some of the mixed study findings.

Although there is no strong evidence to date that smoking causes breast cancer, stopping smoking, or never starting to smoke, is one of the best things you can do for your health. Smoking causes [370]:

• Heart disease
• Stroke
• Bladder cancer
• Cervical cancer
• Esophageal cancer
• Kidney cancer
• Larynx cancer
• Lung cancer
• Pancreatic cancer
• Stomach cancer
• Throat and mouth cancers

Secondhand smoke exposure (passive smoking)

Large, prospective cohort studies have found no link between breathing secondhand smoke (the smoke from other people's cigarettes) and breast cancer risk [372,374,378-380]. However, findings from some case-control studies have shown a slightly increased risk among women with long-term or high levels of exposure [379,381-382].

More data are needed to draw conclusions about a potential link between passive smoking and breast cancer risk.

Factors with limited data

Researchers continue to look for possible sources of breast cancer risk. The factors described below have been the focus of only a few studies, so little is known about them. However, these topics will likely continue to be studied until conclusions can be made.  

Antibiotic use

Few studies have looked at the potential link between antibiotic use and breast cancer risk. Some studies found a higher risk with increased use of antibiotics, while others found no link between the two [383-388]. 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 [388]. The one cohort study found a weak increase in risk only among premenopausal women using long-term antibiotics to treat urinary tract infections [383].

At this time, it is unclear whether antibiotics are linked to breast cancer risk. Current research shows this topic deserves further study.  

Aspirin use

Studies to date on a potential link between aspirin use and breast cancer have shown mixed results. A meta-analysis and one cohort study found a slight decrease in risk with regular aspirin use [389-390]. However, other cohort studies have found no difference in risk between women who took aspirin regularly compared to those who did not [391-393]. And, the randomized controlled trial Women's Health Study, 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 [394].

More research is needed to draw conclusions on whether there is a link between the use of aspirin, as well as other non-steroidal anti-inflammatory drugs (NSAID), and breast cancer risk.  

Body care cosmetics containing parabens

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. And, animal studies have found some health effects of parabens, but only in very large doses [395-396].

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 [396-397]. In addition, the Panel noted parabens do not build up in the body over time and therefore are unlikely to cause harm [396].

More research in this area is needed before conclusions can be made about a link between body care products that have 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 and breast cancer risk.  

Breast size

Study findings on a potential link between breast size and breast cancer are mixed. Some studies have found larger breast size increases risk, while others have found no link between breast size and risk [398-403].

Two studies have found breast size increases risk among lean women, but not among heavy women [401,403]. 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 [404].

More research is needed to learn whether or not breast size is linked to breast cancer risk.  

Caffeine

Some data suggest that caffeine may affect blood hormone levels [405]. However, the four largest cohort studies to date on caffeine and breast cancer risk found no link between drinking either coffee or tea and the risk of breast cancer [406-409].

Although findings to date suggest there is no link between caffeine and breast cancer risk, more research is needed to draw conclusions.  

Cell phones

Because cell phones have not been a part of our daily lives for many years, there are few data on a possible link between cell phone use and cancer. Current evidence, though, shows no increase in risk of breast or other types of cancer from cell phone use [410-411].  

DES, in utero exposure and breast cancer risk among offspring

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 [412-418]. This topic is under active study.

Data on other in utero exposures are limited at this time.  

Folic acid (folate) and multivitamin use

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 [419-423]. 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.  

French fry consumption (foods containing acrylamide)

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) [424]. Because this is the first study to look at preschool diet and breast cancer risk, further studies are needed to confirm these findings.

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, like French fries, that contain acrylamide and breast cancer risk [425-429].  

Hair relaxers

The largest study to date found no link between use of hair relaxers and risk of breast cancer among African American women [430]. Because this is the first study to look at these hair care products and breast cancer risk, further research is needed to confirm these findings.

In utero exposure to DES and breast cancer risk among offspring (see DES. in utero exposure)

Left-handedness

Although it is not clear why, breast cancer is five percent more likely to occur in the left breast compared to the right breast [431]. This difference prompted a theory that being left-handed may increase breast cancer risk.

The largest cohort study to date found no link between being left-handed and an increased risk of breast cancer [432]. Smaller studies have shown both increased and decreased risks of breast cancer among left-handed women [433-435]. Studies have also found no link between being left-handed and reproductive factors that increase breast cancer risk (such as age at first period or age at menopause) [433-434].

Women who are left-handed do not appear to have an increased risk of breast cancer. However, there are too few studies at this time to draw strong conclusions.  

Migraine headaches

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 [436-438]. At this time, though, data are limited and more research is needed before conclusions can be made about a possible link between migraines and breast cancer risk.

Learn more about estrogen and breast cancer risk.  

Stress

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 [439-445].

At this time, there are too few studies on stress and breast cancer risk to know whether there may be a link between the two. More studies are needed to draw conclusions.

Where do the data come from?

Human studies

The data in this section come from two main types of research studies: observational studies (prospective cohort or case-control) and randomized controlled trials. 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

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. And, animal studies are 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.

Finding information on risk factors for breast cancer

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). The International Agency for Research on Cancer (IARC) is a part of the World Health Organization. The Centers for Disease Control and Prevention (CDC), the National Toxicology Program and the U.S. Food and Drug Administration (FDA) are all part of the U.S. Department of Health and Human Services.

Updated 01/23/12