Onions, garlic and other members of the allium genus such as leeks, chives, scallions and shallots have been shown to have antimicrobial, radioprotective, antithrombotic, hypolipidaemic, anti-inflammatory, antiarthritic and hypoglycemic effects, as well as improving immune function. Allium vegetables contain various substances reported to have anti-cancer effects, including quercetin, apigenin, fisetin, myricetin, kaempferol, ajoene, diallyl disulfide and related chemicals, S-allylcysteine, dipropyl, and various thiosulfinates. Garlic is a good dietary source of the lignan enterolactone. Garlic has been shown to decrease DNA strand breaks induced by carcinogens, inhibit DNA and RNA synthesis in human cancer cells, retard the growth of cancer cells by causing cell cycle arrest and apoptosis, and suppress angiogenesis. Epidemiological studies have demonstrated an association between increased consumption of allium vegetables and a reduction in many different types of cancer, including cancer of the prostate, lung, endometrium, stomach, colon, bladder, esophagus, larynx, mouth, ovary, and liver, as well as melanoma, acute myeloid leukemia, and childhood acute leukemia.
Breast cancer-related effects of eating onions and garlic
Numerous studies have shown that onions, garlic and their components inhibit proliferation, reduces migration and invasiveness, and induces apoptosis of cultured human breast cancer cells. One large European study found that eating onions and garlic was associated with lower risk of breast cancer. Another large Italian population-based study found a relationship between the consumption of increasing intake of flavones and flavonols found in allium vegetables and a reduction in the risk of breast cancer. A study of women in Mexico City found that consuming more than one slice of onion per day was associated with reduced risk of breast cancer. A Korean study found an association between onion and garlic consumption and lower incidence of breast cancer. However, there were no clear associations found between breast cancer risk and the consumption of onions or of individual flavonols in a study of premenopausal women in the Nurses Health Study II.
The anti-carcinogenic effect of allium vegetables is attributed in part to organosulfur compounds, which are generated upon cutting or chewing of these vegetables. Although some of the anticancer benefits of garlic are retained after cooking or processing it, raw garlic appears to have the most benefits.
Leeks and yellow onions are a very good source of quercetin, which has been shown increase the anti-cancer effects of the chemotherapy drug Taxol (paclitaxel). Garlic is a good source of apigenin, which also has been shown to increase the effectiveness of Taxol. Garlic has also been shown to have protective effects against Adriamycin (doxorubicin)-induced heart damage. However, garlic supplementation has been shown to reduce the clearance of the chemotherapy drug Taxotere (docetaxel) in some (typically, African-American) women, a result that might also hold for Taxol.