Background

Garlic is the common name for Allium sativum, a member of the lily family. It is predominantly consumed for its aromatic qualities in food but has a history of medicinal use dating back to Egyptian times.

Uses

Garlic has been studied for its potential to modify the risk of atherosclerosis by reducing blood pressure, thrombus formation, and serum lipid and cholesterol levels. It may even promote the regression of atherosclerotic plaque. Garlic has also been promoted for treatment and prevention of cancer and infectious diseases.

Evidence of the therapeutic efficacy of garlic in lowering serum cholesterol is compelling, although the effect appears to be modest. A meta-analysis determined that the consumption of one half to one clove of garlic per day decreases total serum cholesterol levels by an average of 9%. In a German multi-center randomized controlled trial, standardized garlic power tablets reduced serum cholesterol levels by 12% and triglyceride levels by 17% in patients with hyperlipidemia. Yet, not all studies have found that garlic reduced serum cholesterol. It did not have a significant effect on children with familial hyperlipidemia. Differences may be explained by variations in treatment regimens, patient populations, study methodology, and publication bias.

Little is known about the mechanism of the cholesterol-lowering effect of garlic. In isolated rat hepatocytes, garlic inhibited acetate uptake and interfered with cholesterol biosynthesis. This mechanism has yet to be demonstrated in humans. Garlic may also decrease the susceptibility of lipoproteins to oxidation. The cholesterol-lowering effect of garlic may be mediated by a reduction in food intake.

Although garlic lowers blood pressure in animals, there is insufficient evidence to support the antihypertensive effect of garlic. Garlic may be useful in cases of mild hypertension but should not replace lifestyle modification and drug therapy. In nulliparous parturients, garlic therapy during the third trimester reduced the incidence of hypertension but not the incidence of preeclampsia.

Phytochemistry and pharmacology

Garlic contains organosulfur compounds, adenosine, trace minerals, and amino acids. The pharmacological effects are attributed to the sulfur-containing compounds, particularly allicin and its transformation products. When garlic is cut or crushed, alliin, the first compound found in nature to display optical isomerism, is exposed to the enzyme alliinase and converted to allicin.

Garlic’s constituents and their transformation products inhibit platelet aggregation dose-dependently. This activity is predominantly attributed to allicin, ajoene (4,5,9-trithiadodeca-1,6,11-triene 9-oxide), and methyl allyl trisulphide. The inhibition of platelet aggregation by ajoene appears to be irreversible and may potentiate the effect of other compounds such as prostacyclin, forskolin, indomethacin and dypiridamole. The mechanism behind these effects is unclear, although some investigators have implicated the cyclo-oxygenase pathway. Others have found a direct interaction with the platelet fibrinogen receptor. Still other possibilities surround the exogenous adenosine in garlic and inhibition of endogenous adenosine deamination and cyclic AMP phosphodiesterase. The extent of garlic’s antiplatelet activity in vivo is uncertain. In volunteers, inhibition of platelet aggregation to 5-HT was transient but potent. Another study showed no such activity. Garlic may also act as an anticoagulant by promoting fibrinolysis, which has been demonstrated in volunteers.

Little is known about the mechanism by which garlic may lower blood pressure. This effect may be mediated by nitric oxide, or by an as yet unknown mechanism. Allicin decreased pulmonary vascular resistance in isolated rat lungs independent of nitric oxide, ATP-sensitive potassium channels, activation of cyclooxygenase, and changes in bronchomotor tone.

The pharmacokinetics of garlic’s constituents are poorly understood. Allicin is not found in the blood after garlic consumption. It is unstable and converts readily into mono-, di-, tri-, and polysulfides; sulfur oxide; and other compounds such as ajoene. These organosulfur compounds readily react with cysteine in the intestinal tract or circulation. The sulfur-containing compounds found in the body after consumption of garlic are not known. Pharmacokinetic studies in animals have provided little insight.

Safety

The anticoagulant effect of garlic has raised concerns about bleeding in garlic users. One elderly patient developed a spontaneous epidural hematoma that was attributed to frequent garlic ingestion. Although unreliable, bleeding times were significantly elevated when the patient was hospitalized and returned to normal three days after the discontinuation of garlic.

Preparations and dosage

The usual dosage is 4 g (~2 cloves) of fresh bulb or its equivalent as an extract or tincture per day. Much larger doses (up to 28 cloves/day) have been advocated, and the development of concentrated garlic preparations has made these doses achievable. Commercial garlic preparations may be standardized to a fixed alliin and allicin content.