Penn State researchers have shown that microwave heating or roasting garlic can diminish or destroy its anti-cancer activity – unless the herb is chopped or crushed, and allowed to “stand” for at least 10 minutes before cooking.
Kun Song, doctoral candidate in nutrition, and Dr. John A. Milner, professor and head of the Department of Nutrition, conducted the study.
The research was the first to show that as little as one minute of microwaving or 45 minutes of oven roasting can completely block garlic’s ability to retard the action of a known cancer-causing agent in rats. Garlic’s anti-cancer activity was retained, however, if the herb was first chopped or crushed and allowed to stand for 10 minutes before being heated. In the case of roasted whole garlic, anti-cancer activity was partially retained if the top of the bulb was sliced off prior to heating.
Song presented the results in a poster session titled, “Heating Blocks Garlic’s Protection Against 7,12 Dimethylbenz(a)anthrancene (DMBA) Induced Rat Mammary DNA Adducts,” at a conference on “Recent Advances on the Nutritional Benefits Accompanying the Use of Garlic as a Supplement” at the Marriott Newport Center, Newport Beach, Calif.
The conference is a continuing and distance education service of the Penn State College of Health and Human Development Department of Nutrition in cooperation with Wakunaga of America Co. Ltd. The conference is supported by Wakunaga, National Cancer Institute and Rexall-Sundown Inc.
In a recent interview, Song said that the 10-minute “standing period” after chopping or crushing the garlic enables an enzyme naturally present in certain garlic cells to come in contact with and act on chemicals in other cells. Chopping or crushing the garlic opens the cells and enables the enzyme to start a reaction that produces chemicals called allyl sulfur compounds that possess anti-cancer properties.
“The allyl sulfur compounds produced from the enzyme’s reaction are critical to garlic’s anti-cancer effects,” Song noted. “If garlic was heated or roasted immediately after crushing, the enzyme was de-activated by the heating process and garlic’s anti-cancer effects were blocked.”
Song and Milner conducted the study with rats given garlic by intubation six times over a two-week period. The rats received garlic equal to 2 percent of their daily food ration. After the feeding period was over, the rats were treated with a breast tumor inducer called DMBA. Genetic material (DNA) from the rats’ breast tissue then was examined in order to count the number of instances in which DMBA reaction products or metabolites had become attached to the DNA. The number of DMBA metabolites binding to DNA, called DNA adducts, was used as the measure of cancer incidence.
Rats that received no garlic had the highest number of adducts. Rats given raw garlic showed an average decrease of 64 percent in adduct formation compared with rats that had received no garlic. Rats given garlic that had been heated for one minute in the microwave oven or roasted in a convention oven for 45 minutes after being crushed and allowed to “stand” for 10 minutes showed 41 percent and 21 percent reductions in adduct formation, respectively. Rats given heated or roasted garlic that had not been allowed to stand showed no decrease in adducts compared with non-garlic fed rats.