Facts & Comparisons > Garlic

Garlic

Scientific Name(s): Allium sativum L. Family: Liliaceae (lilies)

Common Name(s): Garlic , allium , stinking rose , rustic treacle , nectar of the gods , camphor of the poor , poor man's treacle 1

Clinical Overview

Uses of Garlic

Evidence suggests that garlic may beneficially affect cholesterol and lipids. Among its traditional uses, it has been employed for its antiseptic and antibacterial properties. Other potential areas of use include GI disorders and oncology.

Garlic Dosing

Garlic dosage is complicated by the volatility and instability of important constituents and by such products as “deodorized garlic,” “aged” extracts, and distilled oils. Doses of fresh bulbs studied in clinical trials for hyperlipidemia or atherosclerosis range from 2 to 4 g/day and a daily intake of 2 to 12 mg allicin has been proposed. Because garlic is a widely consumed foodstuff, dosage will remain a matter of personal tolerance.

Contraindications

Contraindications have not yet been identified.

Pregnancy/Lactation

Documented emmenagogue effects. Avoid use.

Garlic Interactions

Garlic may reduce saquinavir plasma concentrations. Patients taking saquinavir should be cautioned to limit their ingestion of garlic and to avoid garlic supplements without consulting their health care provider. Based on available reports, no special precautions are necessary in patients eating garlic and taking warfarin. However, since warfarin has a narrow therapeutic index, patients should be cautioned against use of herbal products without consulting their health care provider and to report any signs of bleeding. Based on an initial study, garlic does not appear to interact with alprazolam, dextromethorphan, or ritonavir.

Garlic Adverse Reactions

Ingestion of a single 25 mL dose of fresh garlic extract has caused burning of the mouth, esophagus, and stomach, nausea, sweating, and lightheadedness; safety of repeated doses of this amount has not been defined. Rarely, ingestion may also cause anaphylaxis. Topical exposure to crushed, uncooked garlic cloves for 3 to 5 minutes has resulted in toxic contact dermatitis. Repeated exposure to garlic dust can induce asthmatic reactions. Garlic may also cause allergic reactions.

Toxicology

Research reveals little or no information regarding the toxicology of this product.

Botany

A perennial bulb with a tall, erect flowering stem that grows to 2 to 3 feet. The plant produces pink to purple flowers that bloom from July to September. The bulb is odiferous.

History

The name Allium comes from the Celtic word all meaning burning or smarting. Garlic was valued as an exchange medium in ancient Egypt; its virtues were described in inscriptions on the Great Pyramid of Cheops. The folk uses of garlic have ranged from the treatment of leprosy in humans to managing clotting disorders in horses. Physicians prescribed the herb during the Middle Ages to cure deafness and the American Indians used garlic as a remedy for earaches, flatulence, and scurvy.

Chemistry

Fresh garlic is a source of numerous vitamins, minerals, and trace elements, although most are only found in minute quantities. Garlic contains the highest sulfur content of any member of the genus Allium. Two trace elements, germanium and selenium, are found in detectable quantities and have been postulated to play a role in the herb's antitumor effect.

Garlic contains about 0.5% of a volatile oil composed of sulfur-containing compounds (diallyldisulfide, diallyltrisulfide, methylallyltrisulfide). 2 The bulbs contain an odorless, colorless, sulfur-containing amino acid called alliin (S-allyl-L-cysteine sulfoxide), which has no pharmacologic activity. 3 When the bulb is ground, the enzyme allinase is released, resulting in the conversion of alliin to 2-propenesulfenic acid, which dimerizes to form allicin. Allicin gives the pungent characteristic odor to crushed garlic and is believed to be responsible for some of the pharmacologic activity of the plant.

Garlic Uses and Pharmacology

Hypercholesterolemia

A number of trials have examined the effects of garlic on lipoproteins and hypercholesterolemia. The exact mechanism for this action is uncertain, but it is thought that the organic disulfides present in garlic oil can reduce the activity of the thiol group found in many enzymes and can oxidize nicotinamide adenine dinucleotide phosphate (NADPH). These compounds can inactivate thiol enzymes such as coenzyme A and HMG-CoA reductase, and can oxidize NADPH, all of which are factors normally required for lipid synthesis.

Animal data

Research reveals no animal data regarding the use of garlic for hypercholesterolemia.

Clinical data

Individual randomized controlled trials comparing garlic to placebo have provided disparate results. Some studies have suggested that garlic has no effect in adults with mild to moderate hypercholesterolemia. 4 , 5 , 6 , 7 , 8 Evidence has shown that garlic has no significant effect on cardiovascular risk factors in pediatric patients with familial hyperlipidemia. 9 However, other studies looking specifically at moderate hypercholesterolemia in males have demonstrated that garlic has beneficial effects on lipid profiles (reduction in total cholesterol and LDL cholesterol). 10 , 11 Other data have shown that there may be a role for garlic as add-on therapy to traditional medicines (eg, reducing the dose of HMG-CoA reductase inhibitors). 12 Additional trials have demonstrated that allicin (the presumed active ingredient of garlic) may reduce total cholesterol and LDL cholesterol in adults with moderate hypercholesterolemia. 13

One meta-analysis report on the use of garlic for hypercholesterolemia specifically examined randomized, controlled trials comparing garlic with placebo. 14 The inclusion criteria were patients with a mean total cholesterol level of 5.17 mmol/L (200 mg/dL). Pooling data from 13 trials (including 796 patients) suggested that garlic is superior to placebo in reducing cholesterol levels. However the effect is modest (6% reduction in total cholesterol).

Another meta-analysis of 16 randomized, controlled trials (including 1365 patients) also showed a modest reduction in serum lipids. 15 Overall, a 12% greater reduction was observed with garlic therapy compared with placebo. This meta-analysis, however, consisted of small randomized studies of poor quality and not all patients recruited had hyperlipidemia. 15

Overall, these effects are generally short-term and whether they are sustainable beyond 3 months is unclear. The evidence for lowering LDL and total cholesterol is still questionable and may not be clinically meaningful.

Antithrombotic effects

Studies on the effects of platelet aggregation have produced inconsistent results, possibly related to variations in study design and in the garlic preparation used. The proposed mechanism for garlic oil inhibition of platelet function is by interfering with thromboxane synthesis. 16 Researchers isolated a component of garlic oil that inhibits platelet aggregation and identified it as methylallyltrisulphide (MATS). MATS is present in natural oil in a concentration of 4% to 10%. The purified compound inhibits ADP-induced platelet aggregation at a concentration of < 10 mcmol/L in plasma. 2

Further studies indicated that the most potent antithrombotic compound in garlic is 4,5,9, trithiadodeca-1,6,11-triene 9-oxide, also known as ajoene. This compound is formed by an acid-catalyzed reaction of 2 allicin molecules followed by rearrangement; the compound can be synthesized commercially. Unlike other antithrombotics now under investigation, ajoene appears to inhibit platelet aggregation regardless of the mechanism of induction. 17

Animal data

Scientists demonstrated the effect of ajoene in preventing clot formation caused by vascular damage. The experiment was designed to mimic the conditions of blood flow in small- and medium-sized arteries by varying the velocity of the blood; the compound proved to be effective in both conditions. The authors suggested that the compound may be useful in situations where emergency treatment is needed to prevent clot formation produced by vascular damage. 18

Clinical data

Clinical studies have demonstrated that inhibition of platelet aggregation is also observed in vivo after ingestion of fresh garlic. In 1 study, the platelets from healthy subjects who had eaten garlic cloves (100 to 150 mg/kg) showed complete inhibition to aggregation induced by 5-hydroxytryptamine. 19 Other studies have shown that ingestion of “aged” garlic extract can produce an inhibition of some of the platelet functions important for initiating thromboembolic events in the arterial circulation. 20 The effects of garlic on platelet aggregation may be dependent on the garlic preparation used. Differences appear to be mostly dependent on their content of organo-sulfur compounds, many of which are unstable or capable of interconversion during processing. 21

Blood pressure
Animal data

Research reveals no animal data regarding the use of garlic for blood pressure.

Clinical data

Strong evidence for the effect of garlic on blood pressure is lacking. The results of a meta-analysis suggest that garlic supplements of 600 to 900 mg/day for 1 to 3 months are associated with a clinically important reduction in blood pressure. 22 Their meta-analysis included 8 trials consisting of 415 patients. However, the trials were of generally moderate to poor quality and not all patients were hypertensive. A review of the literature suggested that the effects of garlic on blood pressure were insignificant. 23 No firm conclusions should be drawn from these trials.

Gastrointestinal effects

The effect of garlic on the GI system has been the topic of some debate.

Animal data

Oral administration of the oil (0.1 mL/10 g) in mice reduced the gastric transit time of a charcoal meal by 75% and prevented castor oil-induced diarrhea for up to 3 hours. 24 The investigators concluded that garlic oil can be investigated for its effectiveness in the management of hypermotile intestinal disorders. An additional role proposed in the literature is that garlic may be used in the treatment of Helicobacter pylori infection; however, the evidence does not support this. 25

Clinical data

Research reveals no clinical data regarding the use of garlic for GI effects.

Blood glucose reduction

Garlic has been suggested to reduce blood glucose levels, 3 increase serum insulin, and improve liver glycogen storage. 26

Animal data

Research reveals no animal data regarding the use of garlic for blood glucose reduction.

Clinical data

A review of the literature 27 demonstrated that glucose levels decreased from 89 to 9 mg/dL in healthy volunteers given garlic (800 mg dried powder for 35 days) as compared with placebo group. However, other reviews have shown that, in fact, garlic has no effect on glucose levels. 23 Garlic administration should not be recommended for this indication because of the lack of randomized controlled trials.

Antiseptic/Antibacterial properties

The antiseptic and antibacterial properties of garlic have been known for centuries. As recently as World War II, garlic extracts were used to disinfect wounds. During the 1800s, physicians routinely prescribed garlic inhalation for the treatment of tuberculosis.

Animal data

Research reveals no animal data regarding the use of garlic for antiseptic/antibacterial properties.

Clinical data

Garlic extracts inhibit the growth of numerous strains of Mycobacterium , but at concentrations that may be difficult to achieve in human tissues. 26 Preparations containing garlic extracts are used widely in Russia and Japan. Both gram-positive and gram-negative organisms are inhibited in vitro by garlic extracts. The potency of garlic is such that 1 mg is equivalent to 15 Oxford units of penicillin, making garlic about 1% as active as penicillin. 26

Garlic extracts have shown antifungal activity when tested in vitro 26 and their use has been suggested in the treatment of oral and vaginal candidiasis. In an attempt to quantitate the in vivo activity of garlic extracts, researchers administered 25 mL of fresh garlic extract orally to volunteers. 28 Serum and urine samples were tested for antifungal activity against 15 species of fungal pathogens. While serum exhibited anticandidal and anticryptococcal activity within 30 minutes after ingestion, no biological activity was found in urine. The findings suggest that while garlic extracts may exhibit some antifungal activity in vivo, they are probably of limited use in the treatment of systemic infections.

Oncology
Animal data

The antineoplastic activity of garlic has been studied in mice injected with cancer cells that had been pretreated with a garlic extract. No deaths occurred in this treatment group for up to 6 months, while mice injected with untreated cancer cells died within 16 days. 26 It is believed that the reaction of allicin with sulfhydryl groups (the concentration of which increases rapidly in dividing cells) may contribute to this inhibitory effect.

Clinical data

Scant data, primarily from case-control studies, suggest that dietary garlic consumption is associated with decreased odds of laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps. 29 The protective effect of garlic against colorectal and stomach cancers was addressed in meta-analyses of 18 studies. 30 It was concluded that high intake of garlic may offer protection. These results should be interpreted with caution because of the heterogeneity of the trials included in the meta-analyses.

Immunology

Garlic contains the trace elements germanium and selenium, which have been thought to play a role in improving host immunity.

Animal data

One study found that 2 oil-soluble compounds from garlic, diallyl sulfide and diallyl disulfide, when applied topically succeeded in protecting mice against carcinogen-induced skin tumors and increased survival rate. 18

Clinical data

Research reveals no clinical data regarding the use of garlic in immunology.

Other uses

Researchers demonstrated that allicin increased the levels of 2 important antioxidant enzymes in the blood: catalase and glutathione peroxidase. This discovery confirmed the antioxidant and free-radical scavenging potential of allicin. The clinical utility of antioxidant activity is not clear to researchers. Other researchers studied the sulfur compounds in aged garlic extract (a popular deodorized form of garlic) and found 5 sulfur compounds that inhibited lipid peroxidation in the liver, preventing a reaction that is considered to be one of the main features of aging in liver cells. According to the findings, the sulfur compounds “appear to be approximately 1000 times more potent in antioxidant activity than the crude, aged garlic extract.” 18

Dosage

Garlic dosage is complicated by the volatility and instability of important constituents and by such products as “deodorized garlic,” “aged” extracts, and distilled oils. Doses of fresh bulbs studied in clinical trials for hyperlipidemia or atherosclerosis range from 2 to 4 g/day and a daily intake of 2 to 12 mg allicin has been proposed. Because garlic is a widely consumed foodstuff, dosage will remain a matter of personal tolerance. 9 , 14 , 31 , 32

Pregnancy/Lactation

Documented emmenagogue effects. 33 Avoid use.

Interactions

Alprazolam

Based on available data, garlic does not appear to interact with alprazolam. In 14 healthy subjects, pretreatment with garlic extract (1800 mg twice daily for 14 days) did not alter the pharmacokinetics of a single oral 2 mg dose of alprazolam. 34

Dextromethorphan

Based on available data, garlic does not appear to interact with dextromethorphan. In 14 healthy subjects, pretreatment with garlic extract (1800 mg twice daily for 14 days) did not alter the pharmacokinetics of a single doe of dextromethorphan. 34

Ritonavir

Based on available data, garlic does not appear to interact with ritonavir. In 10 healthy subjects, garlic (7 does of 10 mg) did not alter the pharmacokinetics of ritonavir administered as a single 10 mg oral dose. 35

Saquinavir

In 10 healthy subjects, ingestion of garlic supplements have been reported to reduce saquinavir plasma concentrations. 36 Compared with administration of saquinavir alone, coadministration of garlic supplements (dose not specified) decreased the saquinavir area under the plasma concentration-time curve 51%, decreased the trough levels (8 hours after saquinavir administration) 49%, and decreased the peak concentration 54%. The mechanism of this interaction is unknown.

Warfarin

There is no report of an interaction occurring with administration of warfarin and ingestion of garlic. However, there are reports of inhibition of platelet aggregation in individuals ingesting garlic, 37 , 38 , 39 which may increase the risk of bleeding. Case reports of individuals ingesting garlic have been associated with spontaneous spinal epidural hematoma, 39 prolonged clotting time with increased bleeding, 40 and hemorrhaging. 41 In contrast, in a double-blind, placebo-controlled study in 14 men, garlic did not affect platelet aggregation. 42

There are no studies that evaluate the effect of garlic and its extracts in people who require stringent blood glucose control, but the potential for serious interactions should be kept in mind.

Adverse Reactions

Although garlic is used extensively for culinary purposes with essentially no ill effects, the safety of the long-term use of concentrated extracts is unclear. Ingestion of a single 25 mL dose of fresh garlic extract has caused burning of the mouth, esophagus, and stomach, nausea, sweating, and lightheadedness; safety of repeated doses of this amount has not been defined. Rarely, ingestion may also cause anaphylaxis. 43

Topical exposure to crushed, uncooked garlic cloves for 3 to 5 minutes has resulted in toxic contact dermatitis. 44 Additionally, repeated exposure to garlic dust can induce asthmatic reactions. 16 Garlic dust allergy, presenting as coughing, wheezing, chest tightness, difficulty breathing, blocked or runny nose, sneezing, and running or itching eyes is relatively rare. However, an IgE-mediated hypersensitivity reaction has been reported to affect mainly young atopic subjects. Cross-sensitivity to other members of the Liliaceae family may be observed. 45 The degree of cross-reactivity appears to vary among individuals. 46

Toxicology

Research reveals little or no information regarding the toxicology of this product.

Bibliography

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