Many factors are being studied in relation to breast cancer. 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.

Dietary fat

Most studies have shown no link between a high-fat diet in adulthood and an increased risk of breast cancer [237-242].

Dietary fat, though, may still play a role in breast cancer. There are data that suggest that the type of fat, rather than the overall amount of fat, may be important to breast cancer risk [239,243,244]. Some studies have found that a high intake of some types of fat, such as monounsaturated fat (found in olive and canola oils), may lower breast cancer risk [245,246]. However, these results need to be confirmed by further study.

Fruits, vegetables and carotenoids

It is unclear if there is a link between eating fruits and vegetables and breast cancer risk. Some data show that a high intake of fruits and vegetables—as well as the carotenoids (an antioxidant) found in them—may lower breast cancer risk [134,247-250]. However, little if any link was found between the two in a pooled analysis that combined data from eight large studies [251].

A few studies have found blood levels of carotenoids to be linked to a lower breast cancer risk [252,253]. However, data are limited and study findings are mixed [254].  

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) [255-258] 
• Women who have type 2 diabetes [259-261]

Among premenopausal women, findings are less clear on a possible link between insulin levels and breast cancer risk [262,263].

Glycemic index

One way to estimate how a food affects insulin levels is with a measure called the glycemic index. The glycemic index shows how fast and how strongly a food raises blood sugar levels. This controls 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. There is some early evidence from case-control studies that such a diet modestly increases risk [264-266]. However, large prospective studies and a meta-analysis of six such studies have found no link between high glycemic index diet and breast cancer risk or have found a link only among certain groups of women [267-271].

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 diets rich in high glycemic foods and an increased risk of heart disease and diabetes. For this reason alone, it is best to eat a diet high in low or moderate glycemic index foods. One way to do this is to choose foods without processed carbohydrates. Foods like whole wheat, whole oats and brown rice have a lower glycemic index than foods made with highly refined grains, like white bread, sugary breakfast cereals and white rice. Whole fruit and legumes are also good, low glycemic choices.

Soy and phytoestrogens

Soybeans—as well as many other plants like flaxseed, certain grains, beans, fruits and vegetables—contain chemicals called phytoestrogens that mimic estrogen in the body. These estrogen-like traits, coupled with the lower rates of breast cancer seen in countries with high soy intake, have caused debate about the role of soy in breast cancer risk and prevention. Learn more about estrogen and breast cancer risk.

While some studies have found that soy and other phytoestrogens may lower the risk of breast cancer, other studies have found no benefit [272-274]. And there are some findings from laboratory studies that soy can be harmful [275-277]. When all the data are looked at together, it remains unclear how soy affects the risk of breast cancer. This is an area under active study. Learn more about soy and health.  

Dairy products

Dairy products have been studied both as a factor that might increase breast cancer risk and as a factor that might decrease risk. It has been suggested that the high fat content of many dairy products or traces of pesticides or growth hormones in milk may increase risk [278,279]. And, some researchers are studying whether calcium and vitamin D protects against breast cancer [278,280]. A pooled analysis of more than 20 studies found no link between dairy product intake (including milk, cheese and yogurt) and breast cancer risk [281].

At this time, it appears unlikely that consuming dairy products is related to postmenopausal breast cancer. However, data from the Nurses Health Study II found that women who ate a lot of high-fat dairy products (like whole milk or butter) were at higher risk of premenopausal breast cancer [282]. More research is needed to draw conclusions about the possible link between dairy products and premenopausal breast cancer.

Vitamin D  

The possible link between vitamin D and breast cancer is under active study. It is somewhat hard to study vitamin D because levels are determined by both sunlight exposure and diet. Some studies have found that vitamin D exposure (through diet alone or diet plus sunlight) lowers breast cancer risk [283-285]. Other studies, including a meta-analysis of six studies, have found no link between vitamin D and breast cancer risk [286-288]. Further study is needed to know whether vitamin D affects breast cancer risk.

Protein hormones (prolactin and IGF-1)

Like insulin, other protein hormones—such as prolactin and insulin-like growth factor 1 (IGF-1)—may be linked to breast cancer risk. 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 that high levels of prolactin in the blood can modestly increase the risk of breast cancer [290].

IGF-1

IGF-1 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 in pre- and possibly postmenopausal women [291-295].

Smoking

Smoking increases the risk of many cancers, but its effect on breast cancer is not yet known. Most studies do not support a link between smoking and breast cancer among women or men [296-308]. There is some evidence that smoking early in life, especially before a first pregnancy, may increase risk later [297,299-303]. However, this link has not been shown in all studies [304]. 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 [298]. Some data suggest that smoking is linked to certain risk factors that can increase risk of breast cancer, such as having fewer children and alcohol use [305]. Some genes may also affect whether or not smoking increases breast cancer risk in certain people [306]. 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 heart disease, stroke, as well as cancers of the lung, larynx, oral cavity, kidney, esophagus, stomach, bladder, cervix and pancreas [298].

Secondhand smoke exposure (passive smoking)

There is some evidence that breathing secondhand smoke (the smoke from other people's cigarettes) may slightly increase the risk of breast cancer [307-310]. However, this risk may be limited to those with long-term or high levels of exposure [311,312]. At this time, there is not enough evidence to draw conclusions about a potential link between passive smoking and breast cancer risk [299].

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 [279]. However, results from a pooled analysis of more than 20 studies found no link between meat intake and breast cancer risk [281]. 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 [313]. There are mixed findings on whether eating well done or overcooked meat may be linked with an increase in breast cancer risk. Some studies have found a link, while others have not [314-316].  

Factors with limited data

Research continues to find new 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. Two studies found a higher risk with increased use of antibiotics, while three others found no link between the two [316-321]. However, the authors of one of the studies that found a weak increase in risk stated that the link to breast cancer could be explained by other factors [321]. The one cohort study that looked at this topic found a weak increase in risk among premenopausal women using long-term antibiotics to treat urinary tract infections [322]. While it's not clear that antibiotics are linked to breast cancer risk, current research shows that it deserves further study.

Aspirin use

Aspirin deceases swelling and may have some effects on estrogen in the body. For these reasons, some propose that it may decrease breast cancer risk. Studies to date on a potential link between aspirin use and breast cancer have shown mixed results. A meta-analysis of 38 smaller studies found a small decrease in risk with regular aspirin use [323]. However, large cohort studies have found no difference in risk between women who took aspirin regularly compared to those who did not [324-326]. 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 [327]. More research is needed to draw conclusions on whether there is a link between aspirin use and breast cancer risk. Findings for other non-steroidal anti-inflammatory (NSAID) drugs and breast cancer risk are also not conclusive at this time.

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. These chemicals have been shown to act a bit like estrogen in cellular and animal studies. For this reason, it has been proposed that cosmetics with parabens may be absorbed into the skin and increase the risk of breast cancer [328]. However, one study found the estrogen-like effects of parabens to be so weak that it was not likely that they would be linked to breast cancer risk [329]. 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.

Breast size

Although the idea that breast size might be linked to breast cancer has been studied, the findings are mixed. Some studies have found no link between breast size and breast cancer risk, while others have found larger breast size increases risk [330-335]. Two studies have found breast size increases risk among lean women, but not among heavy women [333,335]. In contrast, one small study found that women with smaller breasts may have a higher risk of breast cancer due to tissue density patterns that are more common in smaller breasts [336]. More research is needed to learn whether or not breast size is linked to breast cancer risk.

Caffeine

The two 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 [337,338]. However, at this time, there are too few studies on caffeine and breast cancer to comment on a potential link between the two.

Cell phones

Because cell phones have not been a part of people's daily lives for very long, there are few data on a possible link between cell phone use and cancer. Current evidence, though, shows no overall cancer risk or breast cancer risk from cell phone use [338,339].

Fertility drugs

Fertility drugs stimulate the ovaries, causing estrogen levels in the body to increase. Because high estrogen levels are linked to breast cancer risk (learn more), it has been suggested that fertility drug use might also increase risk. While some small studies have shown that fertility drugs increase breast cancer risk, overall, the evidence does not support a link between the two [341-346]. Because most studies to date have been small, more research is needed.

French fry consumption

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

Hair relaxers

The largest study to date found no link between use of hair relaxers and risk of breast cancer among African American women [348]. 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.

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 [349]. This slight difference has 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 [350]. Smaller studies have shown both increased and decreased risks of breast cancer among left-handed women [351-353]. Although a possible link between being left-handed and some reproductive risk factors for breast cancer (like age at first period or age at menopause) has been suggested, two studies that have looked at these relationships found no such link [351,352]. It does not appear likely that women who are left-handed are at increased risk of breast cancer. However, there are too few studies at this time to draw strong conclusions.  

Migraine headaches

Two case-control studies have found that women with a history of migraines have a lower risk of breast cancer than other women [354,355]. Migraines are related to falling estrogen levels in a woman’s body and estrogen is related to breast cancer risk. (Learn more about estrogen and breast cancer risk.) 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.

Stress

Although one large prospective study found a weak link between life events and breast cancer risk, overall studies do not support a link between stress and breast cancer [356-361]. More prospective studies are needed to draw conclusions.

Where do the data come from?

The data in this chapter 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 the link between a factor (like smoking) 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. For more on research studies, visit the Breast Cancer Research section. 

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 and often look at exposures in larger doses and for shorter times than are suitable for humans. Thus, animal studies can lay the groundwork for human research, but in order to draw conclusions for people, we need human studies. All data presented on this Web site come from human studies unless otherwise noted.

Some organizations do research and/or prepare summary reports of research on certain exposures that have been linked to breast and other types of cancer. If you have concerns over a news item on cancer, these agencies are a good place to find detailed, up-to-date information. The International Agency for Research on Cancer (IARC) is a part of the World Health Organization. The National Toxicology Program is part of the U.S. Department of Health and Human Services and the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) are other federal agencies.

Updated 11/11/09

Healthy Behaviors and Breast Cancer Risk

Factors Not Related to Breast Cancer Risk