Reviewed: Song K, Milner JA. The influence of heating on the anticancer properties of garlic. American Society for Nutritional Sciences. 2001 [Supplement]:1054S-1057S.
The potential medicinal value of garlic (Allium sativum L., Alliaceae or Liliaceae) has been recognized for thousands of years. The authors cite studies that have shown that garlic has a variety of pharmacologic properties, including hypolipidemic (blood lipid-lowering), hypoglycemic (blood glucose-lowering), antibacterial, antifungal, antioxidant, and anticancer effects.
Support for the anticancer effect of garlic has come from epidemiologic, animal, and laboratory studies. Preclinical research has shown that garlic can protect a number of body tissues against different kinds of carcinogens and can interfere with cancer development at both the initiation and promotion stages. This article reviews research about the effects of heating garlic on its anticancer potential. "Although the minimum daily intake required to reduce cancer risk remains to be determined, garlic has been categorized as a dietary anticarcinogen," the authors write.
Laboratory research suggests that the anticancer substances in garlic include both lipid-soluble and water-soluble sulfur compounds. Garlic is distinguished from many other vegetables because of its high sulfur content, and it is thought that the many allyl sulfur compounds in garlic, including S-allylcysteine (SAC), a water-soluble sulfur-containing amino acid, probably explain many of its medicinal properties.
According to the authors' own research, microwave heating for as little as 30 seconds blocked 90 percent of the activity of alliinase, the enzyme that is activated when garlic is crushed or cut. Alliinase rapidly converts alliin to allicin. Allicin is responsible for garlic's odor, and is considered one of the most important biologically active compounds found in crushed garlic. Microwave heating of garlic for 60 seconds destroyed all alliinase activity. When alliinase is inactivated by heat, alliicin and its derivities cannot be formed. The authors cite studies showing that boiling garlic at 100 degrees C for 20 minutes inactivated its cardiovascular benefits, antifungal effects, antioxidant properties, and ability to inhibit cyclooxygenase (an enzyme that plays a role in the development of some types of cancer).
The authors also performed a study to measure the effects of heating garlic (dosage information was not included in the report) upon a rat anticancer assay. Raw garlic was effective in reducing formation of DNA adducts by 64 percent, and garlic that had been microwaved for 30 seconds provided a similar degree of protection. However, microwaving garlic for 60 seconds destroyed all of its anticancer benefits, whether the garlic had been crushed before heating or not. Letting crushed garlic sit at room temperature for 10 minutes before microwave heating preserved 70 percent of its anticarcinogenic effects, compared with the effects of raw garlic. Cutting the top off of whole, intact garlic and letting it sit for 10 minutes before oven heating also preserved some of its anticancer effects, whereas oven heating for 45 minutes without cutting the top of the garlic destroyed all of its anticarcinogenic potential.
Thus, this research suggests that many of the medicinal effects of garlic are reduced or destroyed by heating. Inactivation of alliinase and other heat-sensitive materials in garlic is probably the mechanism by which heating has this negative effect. "Although garlic is known for its many pharmaceutical effects, these abilities can be depressed by preparation or processing methods," the authors conclude.