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 months, 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.
(See also Overview of Dietary Supplements.)
Garlic is said to have favorable effects on several cardiac risk factors, including reduction of blood pressure 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 2016 meta-analysis evaluated 20 trials (970 subjects) in which study duration ranged from 2 to 24 weeks. A variety of garlic preparations included garlic powder and AGE. Mean systolic and diastolic pressure decreases were 5.1 mm Hg and 2.5 mm Hg, respectively (1).
Results of the lipid-lowering effects of garlic supplementation have been quite inconsistent. A 2013 meta-analysis of 39 randomized controlled studies (2298 participants) found that garlic lowered total cholesterol by 17 mg/dL (0.4 mmol/L) and LDL (low-density lipoprotein) cholesterol by 9 mg/dL (0.2 mmol/L) (2).
A 2015 meta-analysis of 7 studies (513 subjects) has verified efficacy of garlic in lowering fasting glucose (3). Other studies are needed to assess impact on hemoglobin A1C. Scientific evidence of either garlic intake or garlic supplement use shows limited or no protection against cancer. A 2016 systematic review and meta-analysis evaluating garlic intake in relation to colorectal cancer incidence did not find any protective effect (4).
Garlic consumed in high doses has general antimicrobial effects in vitro (5).
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: Garlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and review. J Nutr 146(2):389S-396S, 2016. doi: 10.3945/jn.114.202192.
Ried K, Toben C, Fakler P: Effect of garlic on serum lipids: an updated meta-analysis. Nutr Rev 71(5):282-299, 2013. doi: 10.1111/nure.12012.
Hou LQ, Liu YH, Zhang YY: Garlic intake lowers fasting blood glucose: meta-analysis of randomized controlled trials. Asia Pac J Clin Nutr 24(4):575-582, 2015. doi: 10.6133/apjcn.2015.24.4.15.
Chiavarini M, Minelli L, Fabiani R: Garlic consumption and colorectal cancer risk in man: a systematic review and meta-analysis. Public Health Nutr 19(2):308-317, 2016. doi: 10.1017/S1368980015001263.
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. doi: 10.1016/j.phymed.2012.02.018.
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
National Institutes of Health (NIH), National Center for Complementary and Integrative Health: General information on the use of garlic as a dietary supplement