Garlic: Background. Actions

2010

Historical Note

Garlic has been used as both a food and a medicine since antiquity. Legend has it that garlic was used in ancient Egypt to increase workers’ resistance to infection and later used externallyto prevent wound infection. Other ancient civilizations have also used it medicinally. Sanskrit records document the use of garlic approximately 5000 years ago and the Chinese have been using it for over 3000 years. One of the uses of garlic was as a treatment for tumours, a use which extends back to the Egyptian Codex Ebers of 1550 BC. Louis Pasteur was one of the first scientists to confirm that garlic had antimicrobial properties. Garlic was used to prevent gangrene and treat infection in both world wars. Traditionally, garlic has been used as a warming and blood cleansing herb to prevent and treat colds and flu, coughs, menstrual pain and expel worms and other parasites.

Common Name

Garlic

Other Names

Ail, ajo, allium, camphor of the poor, da-suan, knoblauch, la-juan, poor man’s treacle, rustic treacle, stinking rose

Botanical Name / Family

Allium sativum (family Liliaceae)

Plant Part Used

Bulb, and oil from the bulb

Chemical Components

Garlic bulbs contain organosulfur compounds, protein (mainly alliinase), amino acids (such as arginine, lysine, threonine and tryptophan), fibre, lipids, phytic acid, saponins, beta-sitosterol and small quantities of vitamins and minerals such as vitamin C, vitamin E, beta-carotene, chromium, iron and selenium.

Of the numerous constituents present, it is the alliin component and resultant degradation products, such as allicin and ajoene, that produce much of the herb’s pharmacological activity. These are formed when garlic is crushed or chewed and alliin is exposed to the enzyme alliinase.

According to Commission E, 1 mg of alliin produces 0.458 mg of allicin

Main Actions

ANTIOXIDANT

Garlic has strong antioxidant activity and is capable of directly scavenging free radicals, and indirectly by enhancing endogenous antioxidant systems such as glutathione, superoxide dismutase, catalase and glutathione peroxidase.

INHIBITS PLATELET AGGREGATION AND ANTITHROMBOTIC EFFECTS

It appears that the method of garlic preparation has a great influence over its antiplatelet activity in humans. Additionally, antithrombotic activity has been identified under clinical conditions.

STIMULATES FIBRINOLYSIS

A significant increase in fibrinolysis has been observed in several clinical tests for both raw and fried garlic.

REDUCES SERUM CHOLESTEROL LEVELS

A 2000 meta-analysis of 13 clinical trials concluded that garlic is superior to placebo in reducing total cholesterol levels, exerting a modest effect. The mechanism of action involves inhibition of cholesterol synthesis by deactivating HMG-CoA reductasevia enhanced phosphorylation, but not changing theamount of the enzyme, according to in vitro research. The compounds containing an allyl-disulfide or allyl-sulfhydryl group are most likely responsible for the inhibition of cholesterol synthesis by garlic and that this inhibition is likely to be mediated at sterol 4-alpha-methyl oxidase. Clinical evidence also suggests it raises HDL levels and reduces triglyceride levels.

ANTIHYPERTENSIVE ACTIVITY

Numerous clinical studies have identified antihypertensive activity with garlic. Although the mechanism of action has not been fully elucidated, evidence from in vivo research suggests that both the renin-angiotensin system and the NO system are responsible for this activity.

ANTIMICROBIAL AND IMMUNE-ENHANCING ACTIVITY

Garlic appears to exert both direct and indirect effects against various pathogens, as it enhances macrophageand T-lymphocyte function and has direct antibacterial, antifungal, antiviral and antiparasitic activity.

Allicin is believed to be chiefly responsible for garlic’s antimicrobial activity. More specifically, it has been found to exert antibacterial activity against a wide range of Gram-negative and Gram-positive bacteria, including multidrug-resistant enterotoxicogenic strains of Escherichia coli, Staphylococcus aureus, Mycobacterium tuberculosis, Proteus spp., Streptococcus faecalis and Pseudomonas aeruginosa; antifungal activity particularly against Candida albicans; antiparasitic activity against some of the major human intestinal protozoan parasites such as Entamoeba histoiytica and Giardia lamblia; and antiviral activity. Ajoene is another important antimicrobial constituent, with greater antiviral activity than allicin, according to one in vitro test.

Helicobacter pylori infection

Several in vitro and in vivo tests have shown that garlic has activity against Helicobacter pylori; however, results from clinical studies are equivocal. Two studies found that a combination of garlic and omeprazole produced synergistic effects against Helicobacter pylori.

ANTINEOPLASTIC AND CHEMOPREVENTATIVE EFFECTS

Many studies have reported antineoplastic effects of both oil and water soluble allyl sulfur compounds from garlic, but the effect is generally greater for the lipid-soluble compounds. Diallyl disulfide, one of the most studied oil-soluble organosulfur compounds in garlic, has demonstrated antineoplastic activity against both hormone-dependent and hormone-independent breast cancer cell lines. It also inhibits the proliferation of human tumour cell lines for colon, lung and skin cancer. Garlic derivatives inhibit proliferation of human prostate cancer cell lines and human breast cancer cell lines. In vitro results also show ajoene induces apoptosis in human leukaemic cells, whereas allicin, but not its precursor alliin, inhibits proliferation of human mammary, endometrial, and colon cancer cells.

Several mechanisms appear to be involved. Notable among these is a depression in nitrosamine formation and a reduction in carcinogen bioactivation. This is thought to involve an inhibitory effect on CYP2E1, one of the isoenzymes responsible for the activation of a number of carcinogens. Some of the organosulfur compounds may also aid detoxification by enhancing glutathione S-transferases, thereby reducing carcinogen loads. Suppression of proliferation associated with a depression in cell cycle progression and the induction of apoptosis has also been observed.

ANTIATHEROSCLEROTIC ACTIVITY

Evidence from in vitro, animal and human research has shown that garlic supplementation significantly reduces the atherosclerotic process. More specifically, garlic significantly decreased aortic tissue cholesterol as determined biochemically, fatty streak formation and the size of atherosclerotic plaque, compared with controls in an animal model.

Similar results have been obtained using ultrasound techniques in a randomised, double-blind, placebo-controlled clinical trial, involving 152 people. Not only did high-dose garlic powder significantly reduce arteriosclerotic plaquevolume, it also induced a slight regression in plaque spread within the 4-year test period. Later, these results were found to be significant only in females.

Results from several recently published animal studies further confirm anti-atherogenic effects and have investigated the mechanisms responsible. One in-vivo study found that garlic activated antioxidant systems and decreased peroxidation in aortic tissue whereas ajoene inhibited smooth muscle cell proliferation in another.

HYPOGLYCAEMIC ACTIVITY

Over a decade ago, one double-blind study reported that 800 mg garlic powder taken daily for a period of 4 weeks reduced blood glucose concentrations by 11.6%. However, a more recent study using a higher dose of 3 g/day over 26 weeks found no effects.

ANTI-INFLAMMATORY ACTIVITY

Fresh garlic extracts and garlic oil have been shown to inhibit COX activity in test tube and animal studies.

HEPATOPROTECTIVE EFFECTS

Aged garlic extract has a glutathione-sparing effect in the liver and specifically elevates reduced glutathione content, thereby enhancing endogenous protective mechanisms, according to in vitro tests.

ENHANCES MICROCIRCULATION

Jung et al (1991) found that 5 hours after the administration of garlic powder (Kwai: total dose 900 mg garlic powder) a significant increase in capillary skin perfusion (55%) occurred in the healthy volunteers, whereas Kiesewetter et al (1993) showed a 48% increase with a dose of 800 mg garlic.

MODULATES CYTOCHROME P450

Garlic may have the potential to alter CYP450 enzymes; however, results from in vitro, animal and human studies are confusing. Garlic oil induced CYP450 according to one animal study, whereas in vitro tests failed to identify an inhibitory effect on cytochrome P450 (2C9*1, 2C19, 3A4, 3A5 and 3A7). Other researchers using human volunteers found that garlic oil reduced CYP2E1 activity by 39%, but had no effect on CYP1A2, CYP2D6 or CYP3A4 activity.

Recently, an interaction between saquinivir and garlic has been reported, suggestive of a CYP induction effect, but human tests do not support this theory.