Sweet potatoes (Ipomoea batatas) have anti-inflammatory properties and are a rich source of vitamin A through their high beta-carotene content. This makes them a candidate for possible prevention of breast cancer. Sweet potatoes are also a source of alpha-lipoic acid, copper, fiber, manganese, pantothenic acid and vitamin B6.
Sweet potato consumption was reported to be associated with decreased risk of kidney cancer in one Japanese study and decreased risk of gallbladder cancer in an Indian study. Vitamin A-related components of sweet potatoes have been found to reduce cancerous and precancerous conditions of the cervix.

Breast cancer-related effects of eating sweet potatoes

Sweet potatoes contain a glycoprotein (SPG-56) which has been demonstrated to reduce proliferation and promote apoptosis of hormone receptor positive (ER+/PR+) breast cancer cells and to inhibit metastasis in a mouse model of breast cancer. In addition, β-sitosterol-D-glucoside, a phytosterol found in sweet potatoes, has been shown to reduce tumor growth in mice bearing ER+/PR+ tumors. Sweet potatoes also contain daucosterol linolenate, which has been shown to inhibit tumor growth in a dose dependent manner in another mouse model of ER+/PR+ breast cancer. However, the most well-studied micronutrient in sweet potatoes is beta-carotene.

Beta-carotene and breast cancer

Sweet potatoes are an excellent source of beta-carotene and vitamin A. The intensity of a sweet potato's yellow or orange flesh color is correlated with its beta-carotene content. Women with substantial intake of carotenoids such as beta-carotene have been reported to have lower risks of breast cancer and its recurrence than those with low intake. For example, consuming foods high in carotenoids has been shown to reduce the risk of breast cancer among women with high breast density, although not all studies are in agreement.
A Scandinavian study found that dietary (but not supplemental) beta-carotene had a protective effect against lobular breast cancer in postmenopausal women. Another European study reported that high intake of beta-carotene was protective against breast cancer in postmenopausal women using hormone replacement therapy (HRT). The same study also found that dietary beta-carotene was associated with lowered risk of breast cancer in postmenopausal women with relatively high alcohol consumption.
Beta-carotene was shown to enhance the cytotoxicity of Adriamycin (doxorubicin) in both hormone receptor positive (ER+/PR+) and triple negative (ER-/PR-/HER2-) breast cancer cells in one study. Higher levels of carotenoids and vitamin A (retinol) in the blood of breast cancer survivors have both been found to be associated with greater likelihood of breast cancer-free survival. Beta-carotene has also been demonstrated to reduce multidrug resistance in cancer cells.

Purple sweet potatoes are rich in anthocyanins

Purple sweet potatoes and yams (most commonly consumed in Hawaii and parts of Asia) are also a good source of beta-carotene. In addition, they have high levels of anthocyanins, including peonidins, cyanidins, and some delphinidin. These are closely-related plant pigments with chemopreventive properties that give these root vegetables their rich purple color. Women in the highest quartile (fourth) of anthocyanin intake were found to have significantly lower risk of breast cancer than those in the lowest quartile in one study.
Cyanidins such as cyandin-3-glucoside have been reported to suppress angiogenesis by inhibiting tumor cell proliferation and migration. Cyandin-3-glucoside has been shown to reduce HER2+ cell proliferation and interfere with the viability and metastatic potential of triple negative breast cancer cells. Cyandin-3-glucoside has also been shown to enhance the treatment effects of Herceptin in HER2+ breast cancer cells. Peonidins derived from purple sweet potatoes have also been demonstrated to suppress the overexpression of the HER2.
Delphinidin has also been found to induce apoptosis (programmed cell death) in HER2+ breast cancer cells and to enhance the treatment effects of Herceptin. In addition, purple sweet potato anthocyanins have been found to reduce the heart damage caused by Adriamycin in a mouse model of chemotherapy.

Additional comments

Sweet potatoes are often prepared with in ways (fried, or with butter and/or brown sugar) that counteract their chemopreventive properties. While sweet potatoes have a lower glycemic index than white potatoes, adding sugar, honey, or sweet toppings in preparing them can easily drive up the glycemic index of the prepared food.
The "yams" sold in U.S. supermarkets normally are orange-colored sweet potatoes. True yams are African and Asian root vegetables belonging to the Dioscoreae family and are only distantly related to sweet potatoes. Sweet potatoes are also not closely related to white potatoes (Solanum tuberosum). The antioxidant activity of a sweet potato's skin is much higher than that of its flesh. However, since the exterior may be treated with dye or wax (which cannot be washed off), the skin of sweet potatoes should be consumed only if they are organically grown.

Wild yam preparations are not recommended

Wild yam (Dioscorea villosa) or wild Mexican yam, is a perennial vine that is used in some women's herbal treatments. Used as an alternative to HRT to alleviate menopausal symptoms, wild yam contains diosgenin, a plant steroid which has been described as natural progesterone. Scientists can manipulate diosgenin to produce steroids such as progesterone, estrogen, and cortisone, which are then used for medical purposes.
However, it is not clear how well the body itself can convert diosgenin into these steroids, suggesting that wild yam itself might not be very effective in alleviating menopausal symptoms. Some wild yam creams labeled as containing "natural progesterone" actually contain the pharmaceutical drug progesterone, which might increase breast cancer risk. In short, wild yam preparations with the greatest ability to offer menopausal symptom relief may be the least safe.
Wild yam root extract also contains compounds with direct anti-cancer activities that do not appear to rely on estrogenic and anti-estrogenic effects. For example, dioscin, another steroidal compound, has been shown to reduce both ER+ and ER- breast cancer cell in the laboratory. However, safe and effective dosages of wild yam preparations containing dioscin or other bioactive compounds have not been determined.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Note that while we are continually searching for new evidence concerning sweet potato and breast cancer, there is not much interest in it among cancer researchers so few studies that focus on this food available.