Garlic (Allium sativum)

Active Compounds

Garlic’s bioactive chemistry is distinctive in that the most pharmacologically active compounds do not exist preformed in the intact clove; they are generated enzymatically during cellular disruption.

The alliin-alliinase cascade: Intact garlic cells contain alliin (S-allyl-L-cysteine sulfoxide) stored in the cytoplasm, and the enzyme alliinase (alliin lyase) sequestered separately in vacuoles. When the cell is disrupted by crushing, chopping, chewing, or other mechanical action, alliin and alliinase come into contact. Alliinase converts alliin to allicin (diallyl thiosulfinate) within seconds. Allicin is thermolabile and decomposes rapidly to a range of secondary organosulfur compounds:

• Diallyl disulfide (DADS) and diallyl trisulfide (DATS) — formed when allicin breaks down in an oil or at elevated temperatures.
• Ajoene — formed when crushed garlic is macerated in edible oil (oil macerates); steam-distilled garlic essential oil does not contain it (MSK). Has antiplatelet activity.
• S-allylcysteine (SAC) and S-allylmercaptocysteine (SAMC) — water-soluble organosulfur compounds that accumulate in aged garlic extract (AGE); more stable than allicin and the primary bioactives in AGE-based supplements.

Practical implication: Cooking whole garlic cloves without crushing or chopping them first inactivates alliinase before the alliin-allicin conversion can occur, substantially reducing the formation of allicin and most downstream sulfur compounds. Letting crushed garlic rest before heating allows the conversion to proceed before alliinase is inactivated; in rat experiments, crushed garlic left to stand for 10 minutes before 60 seconds of microwaving kept part of its activity, while garlic microwaved immediately lost it (Song & Milner, 2001).

Epigenetic activity — HDAC inhibition: Diallyl disulfide (DADS) and other garlic organosulfur compounds have been shown in cell culture studies to inhibit histone deacetylase (HDAC) enzymes, producing histone hyperacetylation and altering gene expression in cancer cell lines; in colon tumor cells, the DADS metabolite allyl mercaptan was a stronger HDAC inhibitor than DADS itself (Druesne et al., 2004). This activity is being actively studied in the context of cancer chemoprevention but remains at a preclinical stage.

Other constituents: Garlic contains fructooligosaccharides (prebiotic fiber), flavonoids (quercetin, kaempferol), saponins, and amino acids in amounts that contribute to overall nutritional value but are not unique to garlic.

Evidence for Purported Benefits

Typical Use and Dosing

Photo: Ivar Leidus, CC BY-SA 4.0, via Wikimedia Commons

Culinary use: Raw garlic cloves have an average allicin potential of about 3.6 mg per gram fresh weight, so one medium clove (~3 g), crushed raw, can yield roughly 11 mg of allicin (Lawson & Hunsaker, 2018). Dose is highly variable based on preparation method — as noted above, cooking before crushing substantially reduces allicin yield.

Supplement forms and dosing:

• Garlic powder (dehydrated): Must be produced at temperatures low enough to preserve alliinase activity. Standardized to alliin content and “allicin potential” (the amount of allicin that would be released given optimal enzyme activity). The Kwai tablets used in most trials were standardized to 1.3% alliin and 0.6% allicin yield (1.8 mg allicin per 300 mg tablet), and three tablets (900 mg/day) was the most common trial dose — roughly the alliin of 1 g of raw garlic (Lawson & Hunsaker, 2018).
• Aged garlic extract (AGE / Kyolic): Produced by long-term extraction in ethanol at room temperature, converting allicin to more stable water-soluble sulfur compounds (primarily SAC). Contains little or no allicin. The most extensively studied AGE formulation (Kyolic) has been tested at a range of doses in cardiovascular trials; the AGE at Heart trial used 1.2 g/day containing 1.2 mg S-allylcysteine (Ried et al., 2016).
• Garlic oil: Steam-distilled garlic oil contains mainly diallyl sulfides (DADS, DATS) and no allicin or ajoene (MSK); ajoene is found in oil macerates of crushed garlic. Typically sold in softgel form.

The European Medicines Agency’s herbal committee has published an EU herbal monograph on garlic bulb covering traditional use for cough and cold and for circulatory disorders, but no clinical body has issued evidence-based dosing guidance. Most garlic powder trials used a daily dose equivalent to about 1–2 g of raw garlic, most commonly 900 mg/day of standardized powder (Lawson & Hunsaker, 2018).

Garlic is one of the staples covered in the anti-inflammatory pantry guide.

Product Selection Criteria

• Standardization: For garlic powder supplements, select products standardized to a defined alliin content and allicin potential (the Kwai preparation used in many trials was 1.3% alliin and 0.6% allicin yield) rather than to total garlic extract weight alone.
• Aged garlic extract pedigree: If selecting AGE-type products, Kyolic (Wakunaga) has the broadest clinical evidence base and has been used in most cardiovascular trials involving AGE.
• Enteric coating: Enteric coating is often marketed as protecting alliinase from stomach acid, but in a breath-metabolite bioassay enteric tablets gave variable and often lower allicin bioavailability (36–104%, falling to 22–57% with a high-protein meal), while non-enteric tablets gave 80–111% regardless of meal (Lawson & Hunsaker, 2018). Coating alone is not a marker of better delivery.
• Third-party testing: NSF or USP verification checks that a product contains the ingredients and amounts on its label and screens for contaminants such as heavy metals, which, as with any crop, can come from the soil it was grown in.
• Odor-modified claims: “Odorless” garlic products achieve reduced odor by modifying the allicin content or using preparations (like AGE) that minimize allicin. These products may not have equivalent bioactivity to standardized allicin-yielding preparations for all applications.

Frequently Asked Questions