Garlic and onions: their cancer prevention properties

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. Epidemiologic 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 biologic 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.

Figure 1

Global per capita production of garlic and onions, 1980–2012 (96). Onions refers to the sum of onions, shallots, green onions, and dried onions.

Figure 2

Bioactive sulfur compounds in Allium vegetables. S-alk(en)yl-L-cysteine sulfoxides (ASCOs) are the precursors to the biologically active compounds in Allium vegetables. ASCOs are formed form γ-glutamyl-S-alk(en)yl-L-cysteines in the vegetables cells. When the vegetables are crushed, chopped, or chewed, the enzyme alliinase is released from vacuoles and catalyzes the conversion of ASCOs to highly reactive sulfenic acid intermediates. Thiosulfinate compounds are formed from the condensation of these intermediates. Thiosulfinate compounds and their metabolites are responsible for the characteristic flavors and odors of Allium vegetables. Predominant compounds of ASCOs and thiosulfinate compounds/metabolites in garlic and onions are listed. See text for citations of references.