(See also Overview of Dietary Supplements.)
Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can also be eaten raw or cooked. Because the active ingredients are volatile and destroyed by the act of crushing, the amount of active ingredient in the various forms of garlic varies greatly. Supplements are best standardized by the amount of active compound. Aged garlic extract (AGE), made from garlic allowed to age for at least 20 mo, has more stable active compounds than most forms. Consuming garlic supplements in this form appears to confer the greatest health benefits and freedom from adverse effects.
Garlic is said to have favorable effects on several cardiac risk factors, including reduction of BP and serum lipid and glucose levels; garlic inhibits platelets in vitro. Garlic is also said to protect against laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps.
The strongest evidence available for garlic supplementation, specifically AGE, is lowering blood pressure. A double-blind, randomized, placebo-controlled, dose-response trial of 79 general practice patients with uncontrolled systolic hypertension evaluated the effect of AGE supplementation for 12 wk. The study indicated that daily supplementation of 240, 480, and 960 mg AGE containing 0.6, 1.2, and 2.4 mg of S-allylcysteine, respectively, significantly reduced mean systolic blood pressure compared to placebo (1).
Results of the lipid-lowering effects of garlic supplementation have been quite inconsistent. A 2012 meta-analysis of 26 randomized, double-blind, placebo-controlled trials indicated that garlic supplementation was superior to placebo in reducing serum total cholesterol (TC) and triglyceride (TG) levels. The authors suggested that garlic supplementation could reduce serum TC and TG levels and that garlic therapy could benefit patients with risk of cardiovascular diseases (2).
Scientific evidence of either garlic intake or garlic supplement use shows limited or no protection against cancer or regulation of glucose. A prospective cohort study evaluating garlic intake in relation to colorectal cancer incidence did not find any protective effect (3). Evaluation of garlic supplement and glucose regulation is limited with little to no human placebo-controlled trials.
Garlic consumed in high doses has general antimicrobial effects in vitro (4).
Most of these studies lack the specific details with regard to the supplement and/or concentration of active ingredients in the supplement, which may account for the variable results.
Ried K, Frank OR, Stocks NP. Aged garlic extract reduces blood pressure in hypertensives: a dose-response trial. Eur J Clin Nutr 67(1):64-70, 2013.
Zeng T, Guo FF, Zhang CL, et al. A meta-analysis of randomized, double-blind, placebo-controlled trials for the effects of garlic on serum lipid profiles. J Sci Food Agric92(9):1892-1902, 2012.
Meng S, Zhang X, Giovannucci EL, et al. No association between garlic intake and risk of colorectal cancer. Cancer Epidemiol 37(2): 152-155, 2013.
Filocamo A, Nueno-Palop C, Bisignano C, et al. Effect of garlic powder on the growth of commensal bacteria from the gastrointestinal tract. Phytomedicine. 19(8-9):707-711, 2012.