January 2015
Holly L. Nicastro, Sharon A. Ross, and John A. Milner
Abstract
The Allium genus includes garlic, onions, shallots, leeks, and chives. These vegetables are popular in cuisines worldwide and are valued for their potential medicinal properties. Epidemiological studies, while limited in their abilities to assess Allium consumption, indicate some associations of Allium vegetable consumption with decreased risk of cancer, particularly cancers of the gastrointestinal tract. Limited intervention studies have been conducted to support these associations.
The majority of supportive evidence on Allium vegetables cancer preventive effects comes from mechanistic studies. These studies highlight potential mechanisms of individual sulfur-containing compounds and of various preparations and extracts of these vegetables, including decreased bioactivation of carcinogens, antimicrobial activities, and redox modification.
Allium vegetables and their components have effects at each stage of carcinogenesis and affect many biological processes that modify cancer risk. This review discusses the cancer preventive effects of Allium vegetables, particularly garlic and onions, and their bioactive sulfur compounds and highlights research gaps.
Conclusion
For centuries, Allium vegetables have been used in a wide variety of cuisines worldwide and are valued for their potential medicinal properties. During the first Olympic Games in Greece, garlic was consumed as a stimulant and in Roman times, soldiers chewed garlic before battle for strength.
Presently, these vegetables continue to hold their fascination for their unique flavor, chemistry, and biological properties. Epidemiological studies indicate some protective associations of Allium vegetable consumption against cancers, particularly cancers of the gastrointestinal tract. However, difficulties in assessing Allium consumption hamper efforts to further define these effects.
If garlic consumption does reduce the risk of cancer, the amount needed to lower risk remains unknown. Intervention studies have potential to overcome challenges present in epidemiological studies. Mechanistic studies indicate potential mechanisms of the anticancer activity of various Allium vegetable extracts and preparations, and highlight the activities of the sulfur-containing compounds. These compounds have effects at each stage of carcinogenesis and affect many physiological processes that modify cancer risk.
Several research gaps exist. Further research is warranted to improve methods for assessment of consumption of Allium vegetables for epidemiological studies. Randomized, controlled trials on the effects of garlic/onion consumption will help to address the issues with intake assessment and with possible confounding factors.
Furthermore, the effect of Allium vegetables on cancer processes cannot be considered in isolation; rather, they are likely dependent on several environmental and dietary variables. Dietary factors that have shown effect modification include total fat, selenium, methionine and vitamin A (69).
Other factors that may influence the relationship of Allium vegetables and cancer prevention are the interactions of these foods and their constituents with the oral and gut microbiota (93, 94). Additionally, the contribution of Allium vegetables to thiol signaling is an emerging area in redox biology that requires further study (95).
Finally, determining those who will respond most to increased consumption and determining the optimal amount(s) and preparation(s) of Allium vegetables for cancer prevention will aid the scientific community in making public health recommendations for garlic and onion consumption.
