- What is breast cancer?
- The importance of finding breast cancer early
- What are the risk factors for breast cancer?
- Breast cancer risk factors you cannot change
- Lifestyle-related risk factors for breast cancer
- Factors with unclear effects on breast cancer risk
- Disproven or controversial breast cancer cancer risk factors
- Can breast cancer be prevented?
- Signs and symptoms of breast cancer
- American Cancer Society recommendations for early breast cancer detection in women without breast symptoms
- Breast magnetic resonance imaging
- Clinical breast exam
- Breast awareness and self-exam
- Breast ultrasound
- Other breast cancer screening tests
- Paying for breast cancer screening
- To learn more about breast cancer early detection
- References: Breast cancer early detection
Factors with unclear effects on breast cancer risk
Diet and vitamin intake
Many studies have looked for a link between certain diets and breast cancer risk, but so far the results have been conflicting. Some studies have indicated that diet may play a role, while others found no evidence that diet influences breast cancer risk. For example, a recent study found a higher risk of breast cancer in women who ate more red meat.
Studies have also looked at vitamin levels, again with inconsistent results. Some studies actually found an increased risk of breast cancer in women with higher levels of certain nutrients. So far, no study has shown that taking vitamins reduces breast cancer risk. This is not to say that there is no point in eating a healthy diet. A diet low in fat, low in red meat and processed meat, and high in fruits and vegetables may have other health benefits.
Most studies have found that breast cancer is less common in countries where the typical diet is low in total fat, low in polyunsaturated fat, and low in saturated fat. But many studies of women in the United States have not linked breast cancer risk to fat in the diet. Researchers are still not sure how to explain this apparent disagreement. It may be at least partly due to the effect of diet on body weight (see below). Also, studies comparing diet and breast cancer risk in different countries are complicated by other differences (such as activity level, intake of other nutrients, and genetic factors) that might also alter breast cancer risk.
More research is needed to better understand the effect of the types of fat eaten on breast cancer risk. But it is clear that calories do count, and fat is a major source of calories. High-fat diets can lead to being overweight or obese, which is a breast cancer risk factor. A diet high in fat has also been shown to influence the risk of developing several other types of cancer, and intake of certain types of fat is clearly related to heart disease risk.
Chemicals in the environment
A great deal of research has been reported and more is being done to understand possible environmental influences on breast cancer risk.
Compounds in the environment that have estrogen-like properties are of special interest. For example, substances found in some plastics, certain cosmetics and personal care products, pesticides, and PCBs (polychlorinated biphenyls) seem to have such properties. These could in theory affect breast cancer risk.
This issue understandably invokes a great deal of public concern, but at this time research does not show a clear link between breast cancer risk and exposure to these substances. Unfortunately, studying such effects in humans is difficult. More research is needed to better define the possible health effects of these and similar substances.
For a long time, studies found no link between cigarette smoking and breast cancer. In recent years though, more studies have found that long-term heavy smoking is linked to a higher risk of breast cancer. Some studies have found that the risk is highest in certain groups, such as women who started smoking before they had their first child. The 2014 US Surgeon General’s report on smoking concluded that there is “suggestive but not sufficient” evidence that smoking increases the risk of breast cancer.
An active focus of research is whether secondhand smoke increases the risk of breast cancer. Both mainstream and secondhand smoke contain chemicals that, in high concentrations, cause breast cancer in rodents. Chemicals in tobacco smoke reach breast tissue and are found in breast milk.
The evidence on secondhand smoke and breast cancer risk in human studies is controversial, at least in part because the link between smoking and breast cancer is also not clear. One possible explanation for this is that tobacco smoke may have different effects on breast cancer risk in smokers compared to those who are just exposed to secondhand smoke.
A report from the California Environmental Protection Agency in 2005 concluded that the evidence about secondhand smoke and breast cancer is “consistent with a causal association” in younger, mainly pre-menopausal women. The 2014 US Surgeon General’s report concluded that there is “suggestive but not sufficient” evidence of a link at this point. In any case, this possible link to breast cancer is yet another reason to avoid secondhand smoke.
Several studies have suggested that women who work at night, such as nurses on a night shift, may have an increased risk of developing breast cancer. This is a fairly recent finding, and more studies are looking at this issue. Some researchers think the effect may be due to changes in levels of melatonin, a hormone whose production is affected by the body’s exposure to light, but other hormones are also being studied.
Last Medical Review: 09/10/2014
Last Revised: 09/10/2014