Alliin and Allicin, two important sulfur compounds from Garlic (Allium sativum)
For further information, see Garlic Intermediate and Garlic Advanced pages

What's in a name?

Alliin (pronounced Al'-ee-in) and Allicin (Al'-liss-in) are two important sulfur-containing compounds found in Garlic and Onions and their relatives. Their names are derived from the Latin name of Garlic, Allium sativum. Both alliin and allicin are known as "organosulfur" compounds, which means they are "organic" - contain carbon - and also contain sulfur. Many medicinal organosulfur compounds are found in the Alliums, but Garlic contains the highest concentration of them (almost four times more than Onions).

Alliin is further classified as a "cysteine sulfoxide." Cysteine (pronounced Sis'-tuh-een) is a sulfur-containing amino acid; sulfoxide (sulf-ox'-eyed) means it contains a sulfur atom bonded to an oxygen atom. Garlic contains other cysteine sulfoxides, but alliin is the most abundant of these, as well as the most abundant sulfur compound in Garlic overall.

 Allicin, which is derived from alliin, is further classified as a "thiosulfinate." The prefix "thio" comes from the Greek word for sulfur. Thiosulfinates contain two sulfur atoms bonded together, one of which is also bonded to an oxygen atom. Allicin accounts for about 70 - 80% of total thiosulfinates in Garlic.

Odorless and stable, alliin is the most abundant sulfur compound in whole, unbruised Garlic. It is stored inside one kind of Garlic cell; in a separate type of cell, an enzyme called alliinase awaits. When the cells are broken open, alliin mixes with alliinase, and in about ten seconds all of the exposed alliin has been converted into a new group of compounds: allicin and its close relatives, which give off the aroma of fresh Garlic.

 Allicin is a fairly unstable compound that likes to react with itself. Much of it transforms into other, more stable sulfur compounds within a few hours to a few days, depending on storage conditions.

 Allicin is often thought of as the "active ingredient" in Garlic, and has been the subject of numerous scientific studies, but many of its transformation products have medicinal properties as well. Garlic chemistry is complicated, and much remains to be discovered. It could well be that there is a synergistic effect occurring between allicin and the herb's many other constituents and their derivatives.
 

Best sources of alliin and allicin

Both alliin and its enzyme, alliinase, are present in highest concentrations in fresh Garlic. As soon as the Garlic is cut or bruised, the alliin is converted to allicin; to get the maximum amount of allicin, the Garlic should be used as soon as possible. Refrigeration or placing the minced/crushed Garlic in water will extend the life of the allicin for a few days. Once formed, allicin begins reacting with itself, resulting in more stable organosulfur compounds. Cooking (including microwaving) destroys alliinase, preventing the conversion of alliin to allicin.

Significant amounts of alliin and alliinase can survive the drying and processing necessary to manufacture Garlic supplements. Freeze-drying preserves higher levels of these phytochemicals than other types of drying. Even if the supplements contain plenty of alliin and active alliinase, they must be enteric-coated if they are to deliver significant amounts of allicin to the body, because alliinase is inactivated by the low pH of stomach acid. The alliinase in the tablet must survive until it reaches the high-pH environment of the small intestine, where it can act on the alliin and convert it to allicin.

No Garlic supplement actually contains allicin itself. Some products are standardized to contain a certain amount of alliin and will list the allicin yield, or amount of allicin you can expect to be generated when the tablet reaches the small intestine. Some supplements are concentrated to yield as much or more allicin as a daily dose of fresh garlic; read the labels to be sure. The best will list "effective allicin yield," which relates to the amount of allicin actually formed during quality-control tests using simulated gastrointestinal conditions. Tests indicate a wide variation among commercially available supplements: in some brands the effective yield is less than 1%.

Garlic oil (steam distilled) is devoid of allicin. It does contain certain alliin-derived compounds, known as sulfides. The concentration of sulfides in this kind of Garlic oil should be similar to what you would derive from using whole Garlic; choose brands carefully, because high quality oils can have more than 50 times the sulfide content of inferior ones.

Macerate of Garlic is made by crushing Garlic in oil. In this preparation, the allicin is rapidly converted to other compounds, including sulfides, ajoene (Ah'-hoe-een, after the Spanish word for Garlic, ajo) and vinyldithiins (Vin-ul-die-thiy'-ins), which are medicinally active as well. You can make this at home, or purchase in supplement form. If you do buy capsules, make sure the label states the concentration of ajoene and vinyldithiins.

Aged Garlic extract is a somewhat different preparation. In order to remove the odor-forming chemicals, the Garlic is steeped in dilute alcohol for many months. It contains little alliin and no allicin; rather, its main ingredients are water-soluble sulfur compounds such as S-allylcysteine, which are also physiologically active.

Medicinal properties - Alliin and Allicin

Allicin and many of its derivatives have been reported in the scientific literature to be antimicrobial. Studies indicate that these compounds are toxic to many kinds of disease-causing bacteria (even drug-resistant ones) including Staphylococcus aureus (skin infections), Salmonella typhimurium (food poisoning), and Clostridium botulinum (botulism). They also inhibit the growth of Candida (the organisms involved in "yeast infections" and candidiasis) and certain kinds of amoebas which can cause dysentery. Fresh Garlic juice and freeze-dried Garlic juice powder both exhibit high, broad-spectrum antimicrobial activities, topically and in the intestine.

Garlic is known for its beneficial cardiovascular effects. It has been demonstrated to reduce serum cholesterol and triglycerides, lower blood pressure, reduce platelet aggregation (the "stickiness" of the blood), increase blood clotting time, and to discourage or even reverse the formation of plaque deposits in the arteries (atherosclerosis). Many studies have implicated allicin and its derivatives in these effects, although other constituents of Garlic appear to be responsible for lowering blood pressure and may be co-responsible for lowering cholesterol, plaque reduction, and blood-clotting effects.

A good deal of research also indicates that Garlic can have anti-cancer effects, detoxifying carcinogenic substances, inducing apoptosis ("programmed cell death") in cancer cells, and boosting the immune system. Allicin may be responsible for some of these effects, although other components of Garlic are important too.

Alliin and allicin are also involved (along with other compounds) in the proven antioxidant effects of Garlic. This effect is dose-dependent, as excessive concentrations of allicin act as pro-oxidants.

Toxicology

Garlic has been used for thousands of years and current has GRAS (generally recognized as safe) status in the U.S. Several studies and experts suggest that an upper limit of 10 grams of raw Garlic per day (2 to 3 cloves) have no side effects. Cooked Garlic, which contains no allicin, can be tolerated in much higher quantities.

Garlic is not without some toxicity, however. Allicin from crushed cloves will burn the skin if applied as a poultice and left on too long; and in high concentrations, it acts as an oxidizer rather than an antioxidant. Some people experience gastrointestinal discomfort when raw Garlic is ingested alone; when taken with food, this side effect is much less common.  Most sources attribute this to excess allicin, a phytochemical - like many - which is beneficial in small amounts and harmful in overdose. Those with irritated mucous membrane linings or ulcers in the esophagus, stomach, or intestines could experience serious irritation or "heartburn" from raw Garlic.

In very large doses (higher than one would reasonably encounter), both Garlic, Garlic oil and many of its purified sulfur compounds have demonstrated toxic or fatal effects in animal studies. Aged Garlic extract, being devoid of allicin, has not shown these effects at similar doses.

As with any herbal medicine, there may be persons who are individually allergic to Garlic or its extracts.

For further information, see Garlic Intermediate and Garlic Advanced pages