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 from Egyptian pharaohs to Chinese emperors to Middle Age and World War II soldiers where garlic juice was known as “Russian penicillin” for its antibiotic effects in wound healing.
Modern-day usage of garlic as a dietary supplement generally centers around promotion of heart health by reducing serum lipid levels (total cholesterol, LDL, triglycerides), lowering blood pressure, and inhibiting blood clotting. The cardioprotective benefits associated with garlic are generally attributed to the various sulfur compounds that can be isolated from the raw clove. These compounds, which include alliin, allicin, S-allyl-cysteine, and S-methyl-cysteine are found in varying concentrations in garlic, chives, leeks, shallots and onions, but the chemical composition may vary considerably depending on processing methods and are generally highest in garlic compared to other plants in the allium family. The chemical responsible for the pungent smell of garlic, allicin, is produced from alliin (an odorless amino acid derivative) via the action of alliinase 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 total allicin potential of commercial products. Raw garlic is more potent than cooked garlic, and fresh garlic is more potent than old garlic. De-odorized and aged garlic supplements typically contain only a fraction of the alliin found in fresh garlic. Because alliin 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 alliin yields approximately 50% of that amount as allicin – so 500mg of a garlic extract standardized to 1% alliin would yield approximately 2,500mcg allicin (compare this to a clove of fresh garlic, approximately 4 grams with 1% alliin, yielding about 20,000mcg allicin). Due to differences in strength and preparations of various commercial garlic supplements, consumers should pay attention to the “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, antifungal effects, but these are generally confined to topical applications. In patients with hyperlipidemia, garlic might lower cholesterol level by acting as a mild HMG-CoA reductase inhibitor. Garlic is thought to protect vascular endothelial cells from injury by reducing oxidative stress and inhibiting low density lipoprotein (LDL) oxidation. Garlic has also shown to have antithrombotic activity by increasing fibrinolytic activity and decreasing platelet aggregation. For hypertension, garlic is thought to reduce blood pressure by causing smooth cell relaxation and vasodilation by activating production of nitric oxide.
The health benefits of garlic supplements are a controversial area. Although there would appear to be quite a large number of studies indicating a beneficial cardiovascular effect of garlic supplements, the most well-controlled studies generally suggest a lack of any beneficial effects (Arora et al. 1981, Buck et al. 1979) – or suggest benefits only at high doses (Breithaupt-Grogler et al. 1997, Fogarty 1993). For example, a study of children with elevated blood cholesterol and triglycerides, 8 weeks of garlic supplementation (900mg/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 condition was too advanced for treatment with a mild approach such as dietary supplementation. In support of this “non-effect,” however, is a multi-center study carried out over 12 weeks (also using 900mg/d) also found no significant lipid or lipoprotein changes following garlic supplementation (Jepson et al. 2000). The Food and Drug Administration (FDA) has gone so far as to issue a ruling to prohibit the use a claim on dietary supplements relating to the relationship between garlic, decreased serum cholesterol, and the risk in adults of cardiovascular disease.
The lack of effect in the above studies may have been due to the dose used – with 900mg per day being too low. Larger doses of garlic (4-10 grams per day) have been more consistently associated with beneficial effects. For example, a study of 30 patients with coronary artery disease (Simons et al. 1995), garlic supplements (4 capsules per day equivalent to 4 grams 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 which compared the effect of aged garlic extract on blood lipids in a group of 41 men with moderately elevated cholesterol levels (Chutani and Bordia 1981). Each subject received about 7 grams of garlic extract per day over the course of 6 months. The major findings were a reduction in total serum cholesterol of approximately 7% and a drop in LDL of 4-5%. In addition, there was a 5.5% decrease in systolic blood pressure and a modest reduction of diastolic blood pressure in response to aged garlic extract. The overall conclusion of the study was 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 heart burn and nausea may occur with high intakes. In some cases, high doses of garlic may potentiate the anti-thrombotic (blood-thinning) effects of anti-inflammatory medications such as aspirin and dietary supplements such as vitamin E and fish oil. The German Commission E monographs recommend a dose of 4 grams of fresh garlic per day to lower blood lipids. This amount of garlic would be equivalent to approximately 18,000 mcg (18 mg) of alliin (9 mg of allicin) and 500 mcg of S-allyl cysteine.
1.Arora RC, Arora S, Gupta RK. The long-term use of garlic in ischemic heart disease--an appraisal. Atherosclerosis. 1981 Oct;40(2):175-9.
2.Breithaupt-Grogler K, Ling M, Boudoulas H, Belz GG. Protective effect of chronic garlic intake on elastic properties of aorta in the elderly. Circulation. 1997 Oct 21;96(8):2649-55.
3.Buck C, Simpson H, Willan A. Ischaemic heart-disease and garlic. Lancet. 1979 Jul 14;2(8133):104-5.
4.Chutani SK, Bordia A. The effect of fried versus raw garlic on fibrinolytic activity in man. Atherosclerosis. 1981 Feb-Mar;38(3-4):417-21.
5.Fogarty M. Garlic's potential role in reducing heart disease. Br J Clin Pract. 1993 Mar-Apr;47(2):64-5.
6.Jepson RG, Kleijnen J, Leng GC. Garlic for peripheral arterial occlusive disease. Cochrane Database Syst Rev. 2000;(2):CD000095.
7.Randerson K. Cardiology update. Garlic and the healthy heart. Nurs Stand. 1993 Apr 14-20;7(30):51.
8.Simons LA, Balasubramaniam S, von Konigsmark M, Parfitt A, Simons J, Peters W. On the effect of garlic on plasma lipids and lipoproteins in mild hypercholesterolaemia. Atherosclerosis. 1995 Mar;113(2):219-25.
EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.