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There are some things that might be risk factors for breast cancer, but the research is not yet clear about whether they really affect breast cancer risk.

For information on other known and possible breast cancer risk factors, see:

• Lifestyle-related Breast Cancer Risk Factors
• Breast Cancer Risk Factors You Cannot Change
• Disproven or Controversial Breast Cancer Risk Factors

Diet and vitamins

While being overweight or obese and not being physically active have been linked to breast cancer risk, the possible link between diet and breast cancer risk is less clear.  Results of some studies have suggested that diet may play a role, while others have not found that diet influences breast cancer risk.

Studies of women in the United States have not found a consistent link between high-fat diets and getting breast cancer, although some studies have found a possible link between high-fat diets and a higher risk of dying from breast cancer. Studies have also 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. 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.

We do know that 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 eating certain types of fat is clearly linked to a higher risk of heart disease.

Some studies have also suggested that diets high in fruits and vegetables and calcium-rich dairy products, but low in red and processed meats might lower the risk of breast cancer. This remains an active area of research.

Several studies looking at women in Asian countries have found that diets high in soy products might lower breast cancer risk. But this link has not been as clear in studies looking at women in Western countries. This might be because Asian women generally eat more soy products (and start at an earlier age) than Western women.

Studies looking at vitamin levels in the body have had inconsistent results. So far, there’s no strong evidence that taking vitamins (or any other type of dietary supplement) reduces the risk of breast cancer. 

The lack of a strong link between diet and breast cancer in studies so far doesn’t mean that there’s no point in eating a healthy diet. A diet low in fat, red meat, and processed meat and high in fruits and vegetables can have many health benefits, including lowering the risk of some other types of cancer.

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.

Chemicals 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.

At this time research does not show a clear link between breast cancer risk and exposure to these substances, but it is hard to study such effects in humans. More studies are needed in this area.

Tobacco smoke

Some studies have found that heavy smoking over a long time might be linked to a slightly higher risk of breast cancer. In some studies, the risk has been 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 in rodents have shown that chemicals in tobacco smoke reach breast tissue and are found in breast milk. In human studies, the evidence on secondhand smoke and breast cancer risk is not clear. Most studies have not found a link, but some studies have suggested it might increase risk, particularly in premenopausal 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 shift work

Women who work at night, such as nurses on a night shift, might have an increased risk of breast cancer. This is an active area of research. This 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.