Mushrooms have been shown to have anti-inflammatory, antiviral, cholesterol-reducing, and immune-enhancing properties, as well helping to reduce blood pressure and blood sugar levels. Generally speaking, mushrooms are a good source of dietary niacin (vitamin B3) and riboflavin (vitamin B2).
Mushrooms contain numerous compounds with anti-cancer activities, including various β-glucans and lectins, butein, ganodermanontriol, grifolin, lentinan, marmorin, and nebrodeolysin. White button mushrooms are also a good source of conjugated linoleic acid (CLA). Portobello mushrooms that have been exposed to UV light are a good source of vitamin D. Various mushroom extracts have been shown to reduce proliferation and/or induce apoptosis of human bladder, cervical, colorectal, esophageal, head and neck, kidney, liver, lymphoma, multiple myeloma, non-small cell lung, ovarian, pancreatic, prostate, spleen, and stomach cancer cells, as well as malignant glioma and leukemia cells. Mushroom extracts also have been shown to prolong survival of mice inoculated with melanoma cells.

Breast cancer-related effects of eating mushrooms

Mushroom consumption is associated with reduced breast cancer risk

Many types of mushroom appear to have beneficial effects with respect to breast cancer prevention (although note that some mushrooms are poisonous and others have been shown to strongly promote cancer in the laboratory).
Asian population studies have reported links between mushroom consumption and reduced breast cancer risk. A case-control study of 1,009 Chinese breast cancer patients found that mushroom consumption was inversely related to breast cancer risk. A study of Korean breast cancer patients also found consumption of mushrooms to be associated with a significantly decreased risk of breast cancer. Another Korean study found that both high daily intake and high consumption frequency of mushrooms was associated with reduced risk of breast cancer among postmenopausal women.
The information in the remainder of this section concerns mushrooms commonly available in the U.S. Generally speaking, we would be cautious about consuming mushroom supplements because safe and effective dosages have not been established.

White button mushrooms

White button mushrooms (Agaricus bisporous) are the most common mushrooms sold in supermarkets. They are also known as button mushrooms, white mushrooms, or table mushrooms. Portobello and crimini mushrooms are more mature forms of Agaricus bisporous—as they age, white button mushrooms mature into cremini mushrooms (which are brown) and then into portobellos (also brown and much larger).
Of the mushrooms studied, white button mushrooms have among the most powerful breast cancer chemopreventive properties. Several studies have concluded that diets high in white button mushrooms may lower the risk of estrogen receptor positive (ER+) breast cancer, especially in postmenopausal women, by reducing aromatase activity, a process by which androgens are converted to estrogens in the body. White button mushrooms have also been shown to inhibit proliferation of ER+ breast cancer cells. However, they should be eaten cooked, not raw (see Additional comments below).

Maitake mushrooms

Maitake mushrooms (Grifola frondosa) are sold dried or fresh in some specialty markets and supermarkets. Maitake mushrooms have been shown to reduce growth, inhibit angiogenesis, and induce apoptosis of human breast cancer cells in the laboratory. In particular, maitake extract has been shown to reduce triple negative (ER-/PR-/HER2-) breast cancer cell viability and reduced their metastatic potential. However, a human trial of maitake mushroom supplementation in postmenopausal breast cancer survivors found that, contrary to expectations, the maitake extract had both immune enhancing and immune suppressant effects at various doses.

Shiitake mushrooms

Shiitake mushrooms (Lentinus edodes) are also available fresh or dried in grocery stores and specialty markets. Shiitake mushrooms have been found to inhibit increases in tumor volume of human breast cancer cells implanted in mice. Dried shiitake mushrooms are also a good source of dietary selenium, which is associated with reduced risk of breast cancer.

Reishi mushroom

Reishi mushroom (Ganoderma lucidum) usually is sold only in powder form (typically in capsules), as a liquid extract, or as a tea, since reishi mushrooms are too bitter and tough to be eaten as food. Reishi mushroom extracts have been shown to inhibit the proliferation, adhesion, angiogenesis, migration, and invasion of several types of breast cancer cells, including triple negative cells.
The addition of green tea extract to reishi extract has been shown to have a synergistic effect in the inhibiting adhesion, migration and invasion of hormone receptor negative (ER-/PR-) breast cancer cells.
Reishi extract should not be used during chemotherapy. One study reported that reishi extract reduced the cytotoxic effects of Adriamycin in ER+/PR+ breast cancer cells. The same study reported that reishi increased the effectiveness of tamoxifen. However, reishi extract has been shown to have estrogenic effects, significantly increasing uterine weight in young rats in one study and promoting ER+/PR+ cell proliferation in another. Therefore, on balance, reishi probably should be avoided by those with ER+ breast cancer.
Reishi extract has been shown to be effective in inhibiting inflammatory breast cancer (IBC) progression, one of the few foods to have demonstrated beneficial impact on this highly aggressive form of breast cancer. Even those with ER+ IBC might benefit from reishi.

Additional comments

White button and related mushrooms should be eaten cooked. Raw white button mushrooms have been shown to cause various cancers when consumed by mice, presumably due to their agaritine and related content. Agaritine, a type of mycotoxin, is neutralized by heat exposure.
A 2005 study found that the risk of breast cancer for Latina mushroom agricultural workers in California was sharply higher than the risk for any other type of agriculture, for reasons that are not clear. This high risk may have been the result of pesticide use in enclosed areas or contaminants in the compost in which such mushrooms typically are grown.
In addition to neutralizing the relatively small fraction of carcinogens in raw white button and related mushrooms, cooking would serve to eradicate any pathogenic bacteria on their surface. It makes sense to thoroughly clean and cook any such mushrooms before consuming them.
Below are links to recent studies concerning mushrooms and their components. For a more complete list, including less recent studies, please click on mushrooms.