Onions clearly have cancer prevention properties. Numerous epidemiological studies have been done on the possible medicinal properties of onions in protecting against cancer. Several studies have shown that eating onions regularly significantly reduces the risk of cancers, such as stomach, prostate, kidney, oral (mouth), breast, and colon cancer. In one study, onion consumption at a level of at least half an onion per day was associated with a 50% decrease in stomach cancer risk. In another epidemiological study, a higher intake of onions was correlated with a lower risk of breast cancer. The strongest evidence points to protective effects of onions (as well as garlic) against cancers of the digestive tract. Factors that may affect the relationship of onions and cancer prevention are the interactions of onions and their bioactive components with the oral and gut microbiota. All this does not mean that onions can cure cancer without a holistic approach to diet and lifestyle. However, eating onions on a daily basis may counteract the chance of developing cancer.