Kale (Brassica oleracea var. acephala), like broccoli and cabbage, is a cruciferous vegetable. Kale has been shown to have antioxidant, antigenotoxic, anti-inflammatory and antimicrobial properties, and to be cardioprotective. Kale contains significant levels of beneficial micronutrients associated with reduced breast cancer risk.
Kale is a source of chemopreventive isothiocyanates, including allyl isothiocyanate (AITC), indole-3-carbinol (I3C) and its metabolic product 3,3'-diindolylmethane (DIM), and sulforaphane. In addition, kale is a good source of the carotenoids beta-carotene and lutein, as well as kaempferol, quercetin, vitamin C and vitamin K.

Breast cancer-related effects of eating kale

Kale consumption is associated with reduced breast cancer risk. Components of kale have been found to down-regulate hormone receptor expression, promote apoptosis, suppress cell cycle progression and inhibit angiogenesis of human breast cancer cells. Consumption of cruciferous vegetables has been shown to reduce the estrogen metabolite 16alpha-hydroxyestrone, which is a breast cancer promoter. Kale can increase the beneficial effects of some breast cancer treatments. Heavily cooking kale greatly reduces its potential effectiveness in preventing breast cancer or its relapse.

Epidemiological studies

Kale has been reported to be associated with reduced breast cancer risk in population studies. A Korean study found an association between kale consumption and lower incidence of breast cancer. A large U.S. prospective study reported that women with high kale intake in early adulthood had reduced risk of breast cancer in middle age and later years.

Kale can increase treatment effectiveness

DIM, I3C and lutein have been shown to enhance the therapeutic efficacy of the chemotherapy drugs Adriamycin (doxorubicin), Taxol (paclitaxel) and Taxotere (docetaxel) in breast cancer cells. For example, the combination of DIM plus Taxol has been reported to increase the death of HER2 overexpressing (HER2+) breast cancer cells more than Taxol alone in one study.
Kale component sulforaphane has been shown increase the anti-cancer effects of Adriamycin, cisplatin, 5-Fluorouracil (5-FU), Taxol and Taxotere. For example, the combination of sulforaphane plus cisplatin has been shown to synergistically inhibit key steps of metastatic cellular growth in triple negative (ER-/PR-/HER2-) breast cancer cells.
AITC has been reported to reduce Adriamycin-induced heart damage (cardiomyopathy) in a rat model of chemotherapy. Kale compound kaempferol has also been shown to reduce the degree of heart and kidney damage caused by Adriamycin in rat models of chemotherapy.
Beta-carotene enhanced the cytotoxicity of Adriamycin in both hormone receptor positive (ER+/PR+) and triple negative (ER-/PR-/HER2-) breast cancer cells in one study. Beta-carotene has also been demonstrated to reduce multidrug resistance in cancer cells.
Cruciferous vegetables are recommended during aromatase inhibitor treatment in part because several isothiocyanates have been shown to reduce aromatase expression, thereby helping to block the production of estrogens from androgens within the body. Breast cancer patients taking tamoxifen who also had relatively high cruciferous vegetable intake were less likely to experience a recurrence than tamoxifen users with low consumption in one study.

Important micronutrients in kale

Kale isothiocyanates

Urinary isothiocyanate levels have been found to be related to lower breast cancer risk among both premenopausal and postmenopausal women.
AITC
Oral administration of AITC has been demonstrated to restore levels of proliferation and aromatase activity to near normal levels in a rat model of breast cancer. Another rat study found that AITC prevented angiogenesis and invasion. A 2021 study reported that AITC induced ER+/PR+ breast cancer cell death in a dose-dependent manner by causing DNA damage and altering DNA damage repair proteins.
I3C and its metabolic product DIM
Numerous studies have reported that I3C and DIM reduce the growth, proliferation and migration of various types of breast cancer cells. For example, I3C has been shown to inhibit proliferation of estrogen receptor positive (ER+) breast cancer cells. I3C/DIM has also been shown to inhibit aromatase (the conversion of androgens to estrogens) in both normal and ER+/PR+ breast cancer cells at concentrations in the range of that observed in human plasma.
DIM has been shown to inhibit the growth of transplanted human breast cancer cells in mice. In a separate study, DIM administered to mice injected with cancer cells caused a marked reduction in the number of lung metastases.
Sulforaphane
Sulforaphane has been shown to reduce triple negative breast cancer growth and metastasis. A relatively low concentration of sulforaphane has been demonstrated to preferentially eliminate breast cancer stem cells from triple negative cells.
Mice were implanted with breast cancer tumors and the tumors were treated directly with sulforaphane in one experiment. Daily injection with sulforaphane for two weeks was found to reduce the number of stem cells by more than 50% in the xenograft tumors.
Sulforaphane has also been shown to inhibit aromatase in both normal and ER+/PR+ breast cancer cells at concentrations in the range of that observed in human plasma.
Sulforaphane has also been demonstrated to inhibit Herceptin-resistant HER2+ breast cancer.

Carotenoids

Women with substantial intake of carotenoids such as beta-carotene and lutein have been reported to have lower risks of breast cancer and its recurrence than those with low intake, although not all studies are in agreement.
Beta-carotene
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.
Lutein
Lutein consumption and circulating lutein levels have also been found to be associated with reduced risk of breast cancer in several epidemiological studies. Lutein has been shown to inhibit the progression of both ER+/PR+ and triple negative breast cancer cells under hypoxia, a low-oxygen condition in which solid breast tumors can thrive.

Kaempferol

Diets abundant in kaempferol have been found to be associated with reduced breast cancer risk. Kaempferol has been shown to inhibit the growth of ER+/PR+ breast cancer cells, in part through inhibition of glucose uptake. Kaempferol has also been found to inhibit both primary tumor growth and lung metastasis in a mouse model of breast cancer. In addition, kaempferol may protect against the cancer-promoting effects of triclosan, an antibacterial chemical to which most people are routinely exposed which is an endocrine disruptor.

Quercetin

Kale flavonol quercetin has been shown to increase the effectiveness of both Adriamycin (doxorubicin) and Taxol (paclitaxel) chemotherapy in multidrug resistant ER+/PR+ breast cancer cells, in part by eliminating cancer stem cells. In addition, quercetin has been reported to increase the sensitivity of ER+/PR+ cells to 5-fluorouracil (5-FU), thereby increasing its treatment effects. Quercetin has also been found to inhibit the migration and adhesion of triple negative (ER-/PR-/HER2-) breast cancer cells and to significantly inhibit tumor progression in a mouse model of triple negative breast cancer. Finally, quercetin also acts as an iron chelator, which can help reduce iron's breast cancer-promoting effects in some women.

Additional comments

Kale is most chemopreventive when prepared first by chopping, and then lightly cooking (steaming, stir‐frying, blanching or microwaving), not heavily cooking (boiling, stewing, or chip‐baking). Non-organic kale must be washed very thoroughly to remove pesticide residue.
We recommend consuming kale and other cruciferous vegetables as food and against consuming components of them as pills (e.g. DIM pills). There is some evidence that concentrated cruciferous vegetable extracts can act as estrogen agonists and promote breast cancer cell proliferation. For example, one study reported that while physiologically achievable doses of I3C reduced aromatase expression of ER+/PR+ breast cancer cells, higher doses induced its expression. Also, the anticancer properties of kale are likely to be the result of synergistic interaction of its various chemical components - isolated components have successfully inhibited proliferation in the laboratory, but their efficacy and safety in humans needs to be evaluated in large scale clinical trials.
Some cruciferous vegetables such as Russian kale, but not most kale sold in the U.S., contain thioglucoside compounds in sufficient amounts to potentially interfere with the formation of thyroid hormone in women with iodine deficiency.

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.
Below are links to 20 recent studies concerning this food and its components. For a more complete list, including less recent studies, please click on kale.