That garlic consumption can reduce the risk of cardiovascular disease in humans is well established in the nutritional literature, and mechanisms for cardio-protective effects are fairly well described. Various lines of evidence, mainly from animal and observational studies, also suggest a protective effect for garlic against some forms of cancer in humans. However, a clear understanding of the mechanisms of such an anti-cancer effect have not thus far been fully elucidated. A number of flavonol and organosulfur compounds have been identified from garlic and other vegetables in the Allium genus, and these may possess beneficial effects. Some animal studies have demonstrated an anti-carcinogenic activity for garlic, but have generally administered higher doses of garlic than might be expected from human consumption. Based on these animal experiments it is therefore difficult to draw any definitive conclusions regarding the benefits of garlic on cancer in humans.
The epidemiological evidence that garlic may inhibit cancer should be treated with caution because potential confounding variables exist. The strongest evidence for an anti-cancer effect of garlic relates to the protective effect shown for garlic against stomach and colon cancers. This may be due to the anti-bacterial effects of some garlic compounds on the growth of the bacteria Helicobacter pylori. The association between Helicobacter pylori infection and stomach ulcers and cancer is established, and the anti-bacterial effect of the garlic compound thiosulfinate, as established through experiments using 40 µg/mL standardised concentrations, might explain the inverse association between garlic consumption and stomach cancer. However, little data exists to suggest the minimum intake of garlic required to protect from cancer. Therefore, a greater number of intervention studies are required in order to confirm the data from observational studies, which cannot discount the possibility of confounding variables.
The problem of confounding variables in observational studies is particularly interesting with regard garlic. Evidence currently supports the theory that total vegetable intake is protective of cancer. Because garlic intake in associated with total vegetable intake, it is possible that the inverse association between garlic intake and cancer is attributable instead to the total vegetable intake. In addition, there exists a plethora of chemical composition studies that demonstrate quite different chemical properties for garlic depending on the method of preparation, and whether the garlic is in whole herb or supplemental form. Therefore the protective effects of garlic may vary depending on the method of preparation, cooking, storing and ageing, as well as the growing conditions. The protective effects of garlic on stomach cancer may for example, be based upon the requirement of the garlic to be in its raw whole herb form, where garlic displays the strongest bactericidal effects.