Allium sativum, commonly called garlic, is a bulb-forming herb of the Liliaceae (lily family). Its medical use traces back to 5,000 years ago in Asia where it was used by nomadic tribes to ward off evil spirits and improve health (Aaron, 1996). The ancient Egyptians, Greeks, and Romans praised and used garlic. Hippocrates recommended its use to combat constipation and as a diuretic. Aristotle suggested its use for a cure against rabies (Anon., 1997a). It was believed to give strength to the men who built the pyramids, courage to the Roman armies, and fighting spirit to the English gamecocks (Dobelis, 1990). During the early 1900s and the outset of World War I, British army surgeons used garlic as a bactericide (Anon, 1997a).
Like most plants, garlic contains more than 100 biologically useful secondary metabolites including alliin, alliinase, allicin, S-allycystein, diallylsulfide, and allymethyltrisulfide (Challem, 1995). The oil of garlic contains the amino acid alliin which, once the bulbs are crushed, is converted to allicin (Dreidger, 1996). The enzyme alline lyase catalyses the formation of allicin, which is in turn the precursor to several sulfur-containing compounds responsible for the flavor, odor, and pharmacological properties of garlic (Ellmore and Fekldberg, 1994).
Once exposed to air, allicin is further converted to diallyldisulfide which has antibacterial effects (Mabey, et al., 1988). Reduction by cysteine will disrupt the disulfide bond in microbial proteins. Prescriptions containing extracts of Allium sativum, either used alone or with amphotericin B, have effects against human systemic fungal infections and cryptococcal meningitis (Howe, 1997). Ajoene, another sulfer-containing compound found in garlic oil, also decreases bacterial growth in gram negative and positive bacteria and yeast (Naganawa, et al., 1996). Ajoene is not found in commercial garlic preparations, it is only found in small quantities in the natural oil (Ishikawa et al., 1996).
The Ames test revealed that ajoene inhibits mutagenesis induced by both benzo[a]pyreded (B[a]P) and 4-nitro-1,2-phenylenediamine (Ishikawa et. al., 1996). The inhibitions of mutagenesis by ajoene is especially effective for transition-type mutations (Agarwal, 1996).
Garlic has been shown to reduce blood clotting and to reduce blood pressure, therefore making it an important part of the treatment for cardiovascular disease (Mabey, et. al., 1988). Allicin and adrenosine are the most potent antiplatelet constituents of garlic because of their in vitro effects (Agarwal, 1996). Garlic oil administered to healthy students and patients with coronary artery disease (CAD) inhibited platelet aggregation in vivo. Low doses of garlic also appear to be effective over a long term administration (Bordia et. al., 1996). Dithiins and ajoenes possess antithrombic properties (Passwater, 1997). Ajoene is currently being developed as a drug for the treatment of thromboembolic disorders (Agarwal, 1996). Dithiin and ajoenes decrease clotting time because they are anticoagulants and thin blood. This activity indirectly reduces the risk of stroke and cardiovascular disease.
Garlic improves cardiovascular function because it provides protection against hypercholesterolemic, artherosclerosis, ischemia-reperfusion-induced, arrhythmias and infarctions. Oxygen-free radical have been implicated as causative factors in these diseases and antioxidants have been shown to effectively treat these conditions because it scavenges free radicals (Prasad et. al., 1996).
Potent enzymes that inhibit the activities of adenosine deaminase and cyclic AMP phosphodiesterase have been identified from garlic extracts. The presence of such enzyme inhibitors in garlic may perhaps explain its effect as an antithrombotic, vasodilatory, and anticancer combatant (Agarwal, 1996). Many of the therapeutic actions of garlic parallel the physiological effects of nitric oxide and may be explained by its ability to increase nitric oxide synthase activity intracellularly (Das et. al., 1995).
Allium sativum has shown significant effects on cancers that affect the stomach and intestine. Persons who regularly ingest garlic have lower incidence of stomach cancer (Anon., 1994b). The Chinese Academy of Medical Sciences reports that epidemiological studies show that dietary intake of garlic is inversely related to gastric cancers (Howe, 1997).
Diallytrisulfide (DATS) is a compound in garlic that lowers the spread of human lung cancer cells. DATS is extremely effective in reducing growth of human lung carcinoma cells in culture. Also, two other compounds in garlic have anticarcinogenic properties: S-allycysteine (SAC) and diallyldisulfide (DADS) (Anon., 1997c).
Garlic stimulates the immune system. The garlic stimulates the activity of macrophages and bulbs increase the activity of helper T cells. It is also effective in treating upper respiratory viral infections and protects cell membranes from DNA damage (Holladay, 1997).
Although there are some negative effects noted in the literature, most are case studies, and some epidemiological surveys. For one, 38 cases of botulism appeared from a restaurant which pointed to a commercially chopped garlic in soybean oil (St. Louis et al., 1988). Another anecdotal occurrence involved over-usage of garlic. A case of spontaneous spinal epidural hematoma associated with platelet dysfunction from excessive garlic ingestion (Rose et. al., 1990). Other ailments pinned to garlic include a contact dermatitis allergy (Delaney & Donnely, 1996; Burden et al., 1994), and possibly occupational asthma caused by aromatic herbs or garlic powder (Lemiere, 1996). There are more severe links that have been implied such as cancer (which is against intuition because it has been an anti-cancer agent). But, the connection is very indirect: the mouthwash used to cover-up the garlic smell seems to be the cause of the increase (Kabat et. al., 1989). There has been recent concern has been raised stemming from new analysis of cancer concern that ancillary compounds that have in effect been ignored may be affiliated with overall cancer rates (Davis, 1989). The most indicting data from a scientific viewpoint comes from a true experiment that was conducted on primary rat hepatocytes. The finding show that while garlic was beneficial for detoxification and antioxidation at 1mM of extract, treatment at 5mM was associated with a significant decrease in cell viability, significant morphology changes, and lowered activity (Sheen et al., 1996). There have been a number of negative aspects that have been correlated to garlic, but most come as anecdotal evidence or as preliminary data.
Compared to the minute evidence against garlic, the positive effects are overwhelming in number and strength. As more research is conducted, it may soon be classified next to ginseng as a panacea at the present rate of positive discoveries being made.
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Effect of the Active Principle of Garlic--diallyl sulfide--on cell
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