Factors with Unclear Effects on Breast Cancer Risk

There are some things that might be risk factors for breast cancer, but the research is not yet clear about whether there really is a link. They include things like tobacco smoke and working at night. Learn more about these factors.

Diet and vitamins

Many studies have been done looking for a link between certain diets and breast cancer risk, but so far the results have been conflicting. Results of some studies have shown that diet may play a role, while others showed no evidence that diet influences breast cancer risk.

Studies looking at vitamin levels have had inconsistent results. And some studies have found that higher levels of certain nutrients ; increased the risk for breast cancer in women. So far, no study has shown that taking vitamins reduces breast cancer risk. This is not to say that there’s 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 can have other health benefits.

Many studies of women in the United States have not linked breast cancer risk to fat in the diet. Still, 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. Researchers are still not sure how to explain this. It may be at least partly due to the effect of diet on body weight. 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 affect breast cancer risk.

More research is needed to better understand the effect of the types of fat eaten on breast cancer risk. It’s 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 known breast cancer risk factor. A diet high in fat is also a risk factor for some other types of cancer. And intake of certain types of fat is clearly related to higher risk of heart disease.

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. In theory, these could affect breast cancer risk.

This issue raises 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. Studying such effects in humans is hard to do. More research is needed to better define the possible health effects of these substances and others like them.

Tobacco smoke

For a long time, studies showed no link between cigarette smoking and breast cancer. But in recent years, more studies have shown that heavy smoking over a long-time is linked to a higher risk of breast cancer. In some studies, the risk was 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.

Researchers are also looking at 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. Studies have shown that chemicals in tobacco smoke reach breast tissue and are found in breast milk of rodents.

In human studies, the evidence on secondhand smoke and breast cancer risk is not clear, at least in part because the link between smoking and breast cancer is also not clear. One reason for this might be that tobacco smoke may have different effects on breast cancer risk in smokers compared with 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.

Night work

Some studies have suggested that women who work at night, such as nurses on a night shift, might have an increased risk of breast cancer. This is a fairly recent finding, and more studies are looking at this. Some researchers think the effect may be due to changes in levels of melatonin, a hormone that’s affected by the body’s exposure to light, but other hormones are also being studied.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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US Department of Health and Human Services. The Health Consequences of Involuntary Exposure to Tobacco Smoke: A Report of the Surgeon General. 2006. Accessed at www.surgeongeneral.gov/library/secondhandsmoke/ on November 3, 2011.

Last Medical Review: June 1, 2016 Last Revised: August 18, 2016

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