The pharmacologic activity of garlic involves a variety of organosulfur compounds. Dried and powdered formulations contain many of the compounds found in raw garlic and will usually be standardized to allicin or alliin content. Allicin is responsible for the characteristic odor of garlic, and alliin is its chemical precursor. Dried powdered formulations are often enteric-coated to protect the enzyme allinase (the enzyme that converts alliin to allicin) from degradation by stomach acid. Aged garlic extract (AGE) has also been studied in clinical trials but to a lesser degree than dried, powdered garlic (GP). AGE contains no alliin or allicin and is odor-free. Its primary constituents are water-soluble organosulfur compounds, and packages may carry standardization to the compound S-allylcysteine.
1. Cardiovascular effects—In vitro, allicin and related compounds inhibit HMG-CoA reductase, which is involved in cholesterol biosynthesis (see Chapter 35), and exhibit antioxidant properties. Several clinical trials have investigated the lipid-lowering potential of garlic. The most recent meta-analysis (Ried et al, 2013) involved 39 randomized, double-blind, placebo-controlled trials with approximately 2300 patients. The investigators studied the effect of garlic mono-preparations on lipid parameters and found a moderate and significant reduction in both total serum cholesterol (–17 ±6 mg/dL) and LDL cholesterol (–9 ±6 mg/dL) when garlic was taken for 2 or more months by patients with elevated baseline cholesterol (>200 mg/dL). Subgroup analysis showed a greater effect on cholesterol reduction when AGE preparations were used than when GP preparations were used. While the benefit of garlic in lowering total cholesterol and LDL cholesterol is clinically relevant, optimal prescription drug therapy is far more efficacious (see Chapter 35).
Clinical trials report antiplatelet effects (possibly through inhibition of thromboxane synthesis or stimulation of nitric oxide synthesis) following garlic ingestion. A majority of human studies also suggest enhancement of fibrinolytic activity. These effects in combination with antioxidant effects (eg, increased resistance to LDL oxidation) and reductions in total cholesterol might be beneficial in patients with atherosclerosis. A randomized, controlled trial among persons with advanced coronary artery disease who consumed GP for 4 years showed significant reductions in secondary markers (plaque accumulation in the carotid and femoral arteries) as compared with patients on placebo, but primary end points (death, stroke, myocardial infarction) were not assessed. AGE preparations have similarly shown favorable effects in three small (<100 patients) randomized, double-blind, placebo-controlled trials in reducing coronary artery calcification (CAC) progression over 1 year. All trials involved patients with known coronary artery disease (CAD) or who were considered medium to high risk for CAD at baseline.