Garlic is a perennial plant that is cultivated around the world and used widely as a culinary herb. A member of the lily family and closely related to onions and chives, garlic grows up to 2 feet in height with a smooth, round stem surrounded at the base by tubular sheathes from which grow long, flat, linear leaves. The stem is topped by a rounded cluster of small, white flowers. The garlic bulb (the usable portion of the plant) is made up of 4 to 15 cloves encased in a papery sheath that may be tan or pink in color.
Most of the health benefits of garlic are attributed to the sulfur-containing compound allicin and its derivatives. The primary source of garlic's pungent odor, allicin is produced when garlic is crushed or chewed and the enzyme alliinase reacts with the compound alliin. Allicin, in turn, may be converted into other sulfur compounds including ajoene, allyl disulfides, and vinyldithiins. Garlic is also naturally rich in selenium, an essential trace mineral with a variety of important roles in the body.
The desirable dosage of garlic may vary from person to person. Lipid-lowering effect can be expected with a daily dose of 600 to 900 mg of powdered garlic delivering 3.6 to 5.4 percent allicin. This amount is approximately equal to 1 clove (4 grams) of fresh garlic. For most people, garlic is nontoxic at this dosage. Large doses (20 grams or more per day) may cause heartburn or other gastrointestinal problems. Some people suffer allergic reactions as well. The alliin content (and allicin potential) of different commercial garlic supplements varies and should be stated on the label. Because the enzyme that converts alliin into allicin is inactivated by stomach acids, enteric coated tablets or other delivery systems that break down in the small intestines are the preferred supplemental forms. Due to garlic's anti-clotting activity, people on anticoagulant medications should only take garlic supplements under a physician's supervision.