Garlic is one of the top-selling herbs in Europe and the United States. It has been used since antiquity for the treatment of cardiovascular and infectious diseases. The German Federal Health Agency Commission E and the European Scientific Cooperative on Phytotherapy have approved its use for the treatment of hyperlipidemia and atherosclerosis. Garlic is available in multiple over-the-counter formulations (dried, powdered, oils). Many products are standardized to yield 0.6% allicin, the ingredient believed responsible for its odor and therapeutic benefit. Allicin is formed by the action of allinase enzymes when garlic is crushed. Both heat and acid destroy these enzymes, so some experts recommend that garlic is best ingested raw. The best-studied form is an enteric-coated capsule of dehydrated garlic. Freeze-drying helps retain most of the active ingredients found in raw garlic. Enteric coating permits allicin to be released in the small intestine, thereby enhancing absorption and reducing the breath odor. Allicin yield among powdered preparations varies as much as 230-fold in brands used in trials. This lack of standardization may contribute to inconsistent results in dozens of clinical trials.
Several mechanisms for garlic's beneficial effects on cardiovascular disease have been proposed: decreased low-density lipoprotein cholesterol (LDL-C) synthesis and oxidation, decreased platelet aggregation, decreased inflammation, lowered blood pressure, and antibacterial activity.
Doses of 600–900 mg of freeze-dried garlic (equivalent to one-half to one clove) taken for 4–6 weeks lower total cholesterol and LDL-C levels by 4–12%, have minimal effect on blood pressure (< 10 mm Hg), and no effect on glucose levels. In a well-designed 2007 trial, 192 adults with moderate hypercholesterolemia were randomized to one of four treatment arms for 6 months: raw garlic, two popular garlic supplements, or placebo. None of the three forms of garlic given at approximate doses of a 4-g clove garlic/d, 6 days/week for 6 months had statistically or clinically significant effects on plasma lipids in adults with initial LDL-C levels of 130–160 mg/dL. In addition, a double-blind randomized study in which 90 overweight smokers were randomized to 2.1 g/d garlic powder, 40 mg/d atorvastatin, or placebo for 3 months, showed no significant effect of garlic on inflammatory biomarkers, endothelial function, or lipid profile compared with placebo. Numerous observational studies have suggested that regular consumption of garlic might reduce the risk of developing certain malignancies, but no prospective controlled trials have been performed.
Garlic is well tolerated and apparently safe for long-term use. In addition to the well-known breath and body odor, common side effects include gastrointestinal upset, nausea, and flatulence.
A more than 50% reduction in blood levels of saquinavir after garlic supplementation has been reported. Although the induction of the cytochrome P450 system was hypothesized as the mechanism of action of this significant herb-drug interaction, a recent study of healthy volunteers did not reveal an effect of garlic on isozymes CYPP2D6 or CYP3A4. Garlic has been shown to have some antiplatelet activation activity, so there is a theoretical risk of increased bleeding, especially if taken with aspirin, anticoagulants, or NSAIDs. However, a 2006 study of 48 patients taking warfarin randomized to aged garlic extract or placebo for 12 weeks did not report any increased bleeding in those taking garlic. No changes in platelet function were noted in healthy volunteers after ingesting raw garlic and garlic oil, but some clinicians recommend stopping garlic 1–2 weeks prior to undergoing elective surgery.
References
Gardner CD et al. Effect of raw garlic vs. commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia. A randomized, clinical trial. Arch Intern Med. 2007 Feb 26;167(4):346–53. [PMID: 17325296]
Macan H. Aged garlic extract may be safe for patients on warfarin therapy. J Nutr. 2006 Mar;136(3 Suppl):793S–795S. [PMID: 16484565]
Rahman K et al. Garlic and cardiovascular disease. J Nutr. 2006 Mar;136(3 Suppl):736S–740S. [PMID: 16484553]
Scharbert G et al. Garlic at dietary doses does not impair platelet function. Anesth Analg. 2007 Nov;105(5):1214–8. [PMID: 17959943]
Van Doorn MB et al. Effect of garlic powder on C-reactive protein and plasma lipids in overweight and smoking subjects. Am J Clin Nutr. 2006 Dec;84(6):1324–9. [PMID: 17158412]
Wojcikowski K et al. Effects of garlic oil on platelet activity. Platelets. 2007 Feb;18(1):29–34. [PMID: 17365851]