Garlic has been used for centuries for its reported benefits in promoting heart health and preventing infection. For more than 5,000 years, humans have been cultivating garlic for use as a spice and a medicine, and records document its medicinal use by Egyptian pharaohs, Chinese emperors, and soldiers from the middle Ages to World War II; among the latter, garlic juice was known as "Russian penicillin" for its antibiotic effects in wound healing.
Modern-day use of garlic as a dietary supplement generally centers on promotion of heart health by reducing scrum lipid levels (total cholesterol, LDL, triglycerides), lowering blood pressure, and inhibiting blood clotting. The cardio protective benefits associated with garlic are generally attributed to the various sulfur compounds that can be isolated from the raw clove. These compounds, which include allicin, allicin, S-allyl-cysteine, and S-methyl-cysleine, are found in varying concentrations.
In garlic, chives, leeks, shallots, and onions, the chemical composition may vary considerably depending on processing methods and are generally highest in garlic compared with, other plants in the allium family. The chemical responsible for the pungent smell of garlic, allicin, is produced from allicin (an odorless amino acid derivative) through the action of allicin and is thought to contribute to many of the health effects associated with garlic supplements.
A major concern with all garlic supplements is the total level of sulfur-containing compounds or the total allicin potential of commercial products. Raw garlic is more potent than cooked garlic, and fresh garlic is more potent old garlic. Deodorized and aged garlic supplements typically contain only a fraction of the allicin found in fresh garlic.
Because allicin is converted to the active allicin form (the source of garlic's unique odor) in the body, and because the precise mechanism by which garlic helps lower cholesterol is unknown, it is prudent to select a product with high allicin potential. General considerations for dosing are that each milligram of allicin yields approximately 50% of that amount as allicin; thus, 500 mg of a garlic extract standardized to 1% allicin would yield approximately 2,500 jig allicin (compared with a clove of fresh garlic, approximately 4 g, with 1% allicin yielding about 20,000 u.g allicin). Owing to differences in strength and preparations of various commercial garlic supplements, consumers should pay attention to die allicin potential of a particular product.
Garlic is mostly used for its antihyperlipidemic and antihypertensive effects. It also has been reported to possess antibacterial, antiviral, and antifungal effects, but these are generally confined to topical applications. In patients with hyperlipidemia, garlic might lower cholesterol levels by acting as a mild HMG-CoA reeducates inhibitor. Garlic is thought to protect vascular endothelial cells from injury by reducing oxidative stress and inhibiting LDL oxidation. Garlic has also been shown to have antithrombotic activity by increasing fibrinolytic activity and decreasing platelet aggregation. For hypertension, garlic is thought to reduce blood pressure by causing cell relaxation and vasodilation by activating production of nitric oxide.
The benefits of garlic supplements are controversial. Although quite a large number of studies appear to indicate a beneficial cardiovascular effect of garlic supplements, die most well-controlled studies generally suggest a lack of any beneficial effects or suggest benefits only at high doses. For example, in a study of children with elevated blood cholesterol and triglycerid.es, 8 weeks of garlic supplementation (900 mg/day) produced no significant effect on total cholesterol triglycerides, LDL, or HDL (Jepson et al., 2000). It is possible that these children, who had severe cases of familial hyperlipidemia, did not respond to the garlic supplements because their medical conditions were too advanced for treatment with a mild approach such as dietary supplementation. In support of this "non effect," however, a multicenter study carried out over 12 weeks (also using 900 mg/day) showed no significant lipid or lipoprotein changes following garlic supplementation (Jepson et al., 2000). The FDA has gone so far as to issue a ruling to prohibit claims on dietary supplements promoting a relationship between garlic, decreased serum cholesterol, and the risk in adults of cardiovascular disease.
The lack of effects in the above-mentioned studies may have been the results of the dose used, with 900 mg/day being too low. Larger doses of garlic (4-10 g/day) have been more consistently associated with beneficial effects. For example, in a study of 30 patients with coronary artery disease (Simons et al., 1995), garlic supplements (4 capsules per day equivalent to 4 g of raw garlic) showed a significant reduction in serum cholesterol and triglyceride levels as well as an inhibition of platelet aggregation (reduced blood clotting). Further supporting the cardiovascular benefits in humans is a well-controlled study that compared the effect of aged garlic extract on blood lipids in a group of 41 men with moderately elevated cholesterol levels. Each subject received about 7 g of garlic extract per day over the course of 6 months. The major findings were a reduction in total serum cholesterol of approximately 7%, a drop in LDL of 4-5%, a 5.5% decrease in systolic blood pressure, and a modest reduction of diastolic blood pressure. The study concluded that "dietary supplementation with aged garlic extract has beneficial effects on the lipid profile and blood pressure of moderately hypercholesterolemic subjects, "but this dose of garlic would certainly pose numerous practical issues such as compliance.
Adverse side effects associated with garlic supplements are rare. Occasionally, mild gastrointestinal symptoms such as heartburn and nausea may occur with high intakes. In some cases, high doses of garlic may potentiate the antithrombotic (blood-thinning) effects of anti-inflammatory medications such as aspirin and dietary supplements such as vitamin E and fish oil.