Pharmocokinetics

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MECHANISM OF ACTION

The intact cells of garlic contain an odorless, sulfur-containing amino acid derivative known as alliin. When the cells are crushed, alliin comes into contact with the enzyme alliinase located in neighboring cells and is converted to allicin. Allicin is a potent antibiotic, but it is highly odoriferous and unstable. The ajoenes are apparently responsible for the antithrombotic properties of garlic. Allicin is described as possessing antiplatelet, antibiotic, and antihyperlipidemic activity while allyl sulfides have anticancer activity. Most authorities agree that the best measure of the total activity of garlic is its ability to produce allicin, which, in turn, results in the formation of other active principles. In hyperlipidemia patients, garlic might lower cholesterol levels by acting as an HMG-CoA reductase inhibitor. For atherosclerosis, garlic is believed to reduce oxidative stress and low-density lipoprotein oxidation and have antithrombotic effects. For hypertension, it is thought to reduce blood pressure by causing smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxation factor. Garlic might stimulate both humoral and cellular immunity, causing T-cell proliferation, restoring suppressed antibody responses, and stimulating macrophage cytotoxicity on tumor cells. Garlic might increase selenium absorption with possible protection against tumororigenesis.

Preparation of garlic, such as heating, microwaving, or drying, can substantially reduce the allyl sulfur compounds (allicin and alliin). Crushed raw garlic is highest in these components. S-allyl cysteine (SAC) is well absorbed after oral administration and can be detected in the plasma, liver, and kidney. It is metabolized to N-acetyl-SAC and excreted in the urine. Allicin has not been detected in the bloodstream and is thought to undergo extensive first-pass hepatic metabolism. Allinase, required to convert alliin to allicin, is acid-labile.

Garlic Preparations

Fresh Garlic and Garlic Condiments

Home-prepared garlic and garlic condiments are among the most imprecise methods of garlic preparation. The amount of chemical constituents in fresh garlic from different parts of the world varies and depends upon growing conditions. Crushing or chewing fresh garlic is necessary to activate the release of alliinase, the enzyme essential to the conversion of alliin to allicin. Because allicin's stability depends on temperature and environment, the constituents in preparations will vary depending on whether water, oil, or vinegar is used and whether it is cooked and how long it is cooked. Garlic also is available in a variety of commercially processed condiments such as garlic powder and garlic salt.

Commercial Preparations

There are many commercial preparations available today. Many variables can influence the constituents in each formulation, the amounts of each constituent, and the bioavailability of the product after ingestion. Factors that have been shown to affect these parameters include the manufacturing processes, origins of the garlic used, and standardization methods. Furthermore, standardized formulations do not necessarily ensure standardized bioavailability of constituents, which can be affected by multiple factors such as gastric acidity.

"Aged Garlic Extract^TM"

The manufacturer of Kyolic exclusively prepares its garlic products using "aged garlic extracts^TM." "Aged garlic extracts^TM" purportedly allow volatile compounds that are found in whole garlic to slowly decompose into more stable allylcysteines. Garlic bulbs are aged up to 20 months. The extract is used to prepare tablets, capsules, and liquids that are measured for consistency by determining the amount of S- allylcysteine (SAC). This is the only product available on the market that is based on SAC content, rather than on alliin- or allicin-releasing potential.

Dehydrated Preparations

One of the most widely used forms of commercial garlic is garlic powder, which may or may not be enterically coated. Enteric coating reduces the tendency of the tablet to dissolve in the gastrointestinal tract and helps to preserve the activity of alliinase, which converts alliin to allicin. The preferred method of dehydration is to remove water from garlic at a low temperature to prevent inactivation of alliinase. The dried garlic is then pulverized and formed into tablets. This process allows the garlic preparation to remain odor-free until the tablet reaches the gastrointestinal tract after ingestion. Many garlic powder preparations are standardized according to varying amounts of alliin- and allicin-releasing potential; some products now coming onto the market are being standardized according to allicin content. Certain products also list standardized amounts of other constituents, with sulfur and g-glutamylcysteines being the most commonly reported. Two examples of standardized dehydrated garlic preparations are Kwai and Pure-Gar .

Distilled Garlic Oil

Distilled garlic oil involves heating crushed garlic in boiling water and collecting the steam as it vaporizes. The process produces diallyl trisulfide and diallyl disulfide as the main constituents.

Oil Macerates

A few commercial preparations contain macerated or chopped garlic mixed with oil (e.g., salad oil, rapeseed oil, or canola oil ) . Macerating garlic in the presence of food oils produces ajoene and dithiins. Oil macerates are usually packaged in soft gel capsules. Of note, Bordia's "garlic ether extract," which is used in some human trials, is similar in composition to commercially available garlic macerate.

Other Commercial Preparations

Numerous garlic preparations are combination or enriched products with other herbs, minerals, and vitamins. Some combination products include ginkgo biloba, hawthorn, selenium, vitamins C and E, b-carotenes, calcium, and enzymes.