Salmon is an excellent source of astaxanthin and choline, and a very good source of marine omega-3 fatty acids, vitamin B6 and vitamin D. Salmon is also a good source of selenium and contains some CoQ10. Most of these compounds have been associated with lower risk of cancer, including breast cancer.
Consumption of fatty fish such as salmon has been found to reduce the risk of coronary heart disease. Fatty fish consumption has been found to be associated with reduced risks of leukemia, multiple myeloma, and non-Hodgkin lymphoma, as well as renal cell, endometrial and prostate cancer.

Mercury and other potential salmon contaminants

Generally speaking, the benefits of consuming fatty fish, including salmon, are thought to outweigh the potentially detrimental effects of the toxins from pollution and other sources that tend to accumulate in their adipose tissue. Salmon is considered a low mercury fish.
However, depending on its location, salmon can accumulate levels of polychlorinated biphenyls (PCBs), dioxin-like PCBs, polychlorinated dibenzo-p-dioxins, dibenzo-p-furans, chlorinated pesticides, mercury, and methylmercury high enough to be detrimental to human health. Exposure to PCBs has been associated with increased risk of developing breast, prostate, testicular, ovarian and uterine cancers.

Wild caught compared to farmed salmon

Wild salmon caught in the open ocean have been shown to incorporate lower levels of contaminants than farmed salmon. Farmed salmon feed consists of a concentrated mix of fishmeal and fish oil that tends to be high in contaminants. Some of the feed now used is plant-based, partly in response to the contaminant problem and partly because it is less costly than fish oil.
Wild salmon normally has a higher omega-3 to omega-6 polyunsaturated fatty acid ratio than farmed salmon. Substituting vegetable oils in the farmed salmon diet reduces the omega-3 to omega-6 fatty acid content, giving such fish an even less favorable profile for breast cancer prevention.
Food coloring is often added to farmed salmon feed because otherwise the salmon would not have the brilliant color of wild salmon. In addition, farmed salmon are treated with antibiotics, pesticides and hormones in the struggle to keep them growing and healthy in the massively crowded conditions of the pens in which they are raised. Therefore, wild salmon is a better choice than farmed salmon.
Generally speaking, farmed salmon from the North Atlantic (including near Scandinavia) tend to have the highest levels of contaminants, Pacific North American farmed salmon have moderate levels, and Pacific South American farmed salmon have the lowest levels. However all farmed salmon have higher levels than the levels found in wild salmon. Based on fairly stringent toxin allowances, people can safely eat up to two servings of farmed salmon (up to 12 ounces, total) per month and up to eight servings of wild salmon.
Removing the skin from salmon is recommended to reduce the level of contaminants ingested. Farmed salmon consumption should be avoided by pregnant women and nursing mothers due to its contaminant content.

Breast cancer-related effects of eating salmon

Human studies

Relatively high fatty fish intake has been shown to be associated with reduced risk of breast cancer and its recurrence. A number of epidemiological studies have found convincing evidence of a negative association between the intake of the marine omega-3 fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) or fatty fish consumption and the risk of breast cancer, although not all are in agreement. One study reported that DHA increased survival times for almost half of a group of stage IV breast cancer patients on FEC (5-FU, epirubicin and cyclophosphamide) chemotherapy.
In addition, several studies have found that higher omega-3 compared to omega-6 fatty acid intakes are associated with reduced risk of breast cancer; consuming wild salmon would tend to improve this ratio for most women.

Astaxanthin

Wild caught salmon, with its red or pink flesh, is a source of astaxanthin, a red-colored carotenoid derived from algal phytoplankton consumed by the fish. Astaxanthin has been found to reduce the incidence of palpable mammary tumors in a mouse model of carcinogen-induced breast cancer. Astaxanthin has also been demonstrated to inhibit the growth and proliferation of ER+/PR+ and ER-/PR-/HER2+ breast cancer cells in several studies.
In addition, pretreatment with astaxanthin was found to increase the toxicity of Adriamycin (doxorubicin) and 5-FU by 40% to 50% in triple negative (ER-/PR-/HER2-) breast cancer cells in one study. Other studies have reported that astaxanthin reduces Adriamycin-induced heart damage in rats.
Note that synthetic astaxanthin has significantly less antioxidant properties than algal-based astaxanthin and may be be unsuitable as a dietary supplement, according to one study.

Marine omega-3 fats

EPA and DHA have been shown to inhibit proliferation of breast cancer cells in the laboratory. In one experiment, higher omega-3 to omega-6 fatty acid diets reduced mammary gland density in mice, which in turn reduced carcinogen-induced mammary tumor development.
Fish oil has been shown to inhibit early stages of mammary tumor development in a mouse model of HER-2/neu overexpressing (HER2+) breast cancer. DHA has been demonstrated to reduce bone metastasis in a mouse model of breast cancer. In addition, fish protein has been shown to have antiproliferative activity against human breast cancer cell lines.
Marine fatty acids have been found to enhance the therapeutic effects of tamoxifen and chemotherapy drugs such as Adriamycin and Taxol.

Additional comments

GMO salmon

The U.S. Food and Drug Administration (FDA) has approved genetically modified (GMO) farmed salmon to be sold in the U.S. (without any labeling requirements in restaurants). The salmon grows to market size in as little as half the time as non-engineered salmon.
The GMO salmon contains a growth hormone gene from the Chinook salmon and a genetic switch from the ocean pout (an eel-like sea creature), that ensures that the transplanted gene is continuously active. Normal salmon growth hormone gene is active only parts of the year. Although the FDA has declared the fish safe for human consumption, it will take years to gain meaningful information on the consequences to human health of consuming it.

Raw salmon

Salmon sashimi, nigiri and other raw salmon preparations should be avoided since they have the potential to cause infection with parasites. In early 2017, it was reported that some wild Pacific salmon are infested with tapeworm larvae.

Pan-fried salmon

Pan frying fatty fish has been shown to release carcinogenic heterocyclic amines (HCAs) in concentrations high enough to affect human health. Population studies have found that consumption of fried fish is associated with increased risk of breast cancer.

Smoked salmon

Smoked salmon is also a potential source of HCAs, as well as carcinogenic polycyclic aromatic hydrocarbons (PAHs) and nitrites and nitrates, and typically has high levels of sodium. This depends on the type. Lox, which is brined salmon that is not smoked, has the highest levels of salt, but is not a source of HCAs and PAHs since it is not smoked. Cold-smoked salmon, which is cured in salt and then smoked at low temperatures, also is high in sodium, but typically has low levels of HCAs and PAHs. Hot smoked salmon, which is smoked at high temperatures, has the most HCAs and PAHs.

Arctic char

Arctic char is a high-omega-3 fatty fish related to salmon and is likely to have a similar health profile. However, note that most of the arctic char available in the U.S. is farm raised.
Below are links to recent studies concerning this food and its components. For a more complete list, please click on the salmon tag. For a list of fatty fish, see recommended fatty fish.