Garlic bulb

Garlic Bulb (Allium sativum L.): A Comprehensive Reference

1. Identity and Botanical Description

Garlic (Allium sativum L.) is an aromatic herbaceous plant that is consumed worldwide as food and traditional remedy for various diseases. A multifunctional crop, garlic belongs to the Amaryllidaceae family. The plant originates from areas now known as Central Asia and Georgia on the Caucasus, from where the crop spread to the Mediterranean area.

Garlic has compact bulbs, and each bulb consists of several bulblets or cloves that are covered with a white skin. The shape and color of the garlic bulb is extremely diverse; its color ranges from white to brown to light brown to violet to dark violet, and its shapes may be rounded, elliptical, transverse broad elliptical, or transverse narrow elliptical. The perennial plant forms a bulb to store its reserves for winter; this bulb sits on top of a hard and flat ring covered in roots, and the main bulb consists of a varying number of closely packed and curved cloves.

Raw and intact garlic lacks substantial odor. The processing method significantly influences the bioavailability and efficacy of garlic-derived sulfur compounds through the generation of allicin; raw garlic contains alliin and alliinase, which react upon crushing to form allicin, a bioactive but highly unstable compound.

Common Preparations and Forms

Different forms of garlic—including fresh garlic, dried powder, aged garlic extract, black garlic, and garlic oil—contain fundamentally different active compounds in different concentrations, and the clinical evidence for each form varies significantly. Many garlic supplements are currently available on the market such as garlic powders, oil macerates, steam-distilled oils, and aged garlic extracts; each dose form varies in phytochemical content because of the different methods of preparation.

• Fresh/raw garlic: Fresh garlic and dried powders are typically used in food preparation and as spices but may also be presented as tablets.
• Garlic powder: Produced by dehydrating and grinding fresh garlic cloves. Most clinical trials have used garlic powder in dosages ranging from 300 mg/d to 900 mg/d, providing 1.8 mg to 5.4 mg of allicin.
• Aged garlic extract (AGE): The consumption of aged garlic extract, a commercial garlic preparation that contains S-allyl-L-cysteine (SAC), has been found to increase plasma SAC concentrations in humans.
• Steam-distilled garlic oil: Steam-distilled oils and aged extracts are used in tablets, soft gelatin capsules, or liquids. Garlic oil is an allicin derivative produced by the steam-distillation process.
• Black garlic: Black garlic is fresh garlic that has been fermented at a high temperature and under high humidity; it has less flavor due to a smaller content of allicin.
• Oil macerates: Produced by soaking crushed garlic in vegetable oils, yielding primarily ajoene and vinyldithiin compounds.
• Enteric-coated tablets: For those who prefer less odor, enteric-coated tablets or capsules with approximately 1.3% alliin are available.

2. Traditional and Historical Use

Interest in the potential benefits of garlic was noted in records found in Egyptian pyramids and ancient Greek temples, as well as in ancient texts—including medical literature—from China, Egypt, Greece, India, Israel, and Rome, in which its use was prescribed.

Ancient Egypt

Ancient Egyptian texts document the dual role of garlic as a food and sacred offering, consumed by pyramid builders to enhance strength and endurance, and used in rituals to symbolize protection and vitality. In ancient Egypt, the Ebers Papyrus prescribed garlic for heart-related issues; it was also implemented in treating tumors, abscesses, malaise, and parasitic or insect infestations. In ancient Egypt, garlic was sometimes termed the "plant of immortality," a testament to its sacred status and its use in rituals, mummification, and as nourishment for laborers building the pyramids.

Indian Medicine (Ayurveda)

The traditional medical literature of India first incorporated garlic as a Rasayana around 2000 BCE to boost immunity and support digestion. Throughout history, garlic has been employed in traditional medicine systems such as Ayurveda and Unani, symbolizing its significance in holistic health practices.

Traditional Chinese Medicine

Traditional Chinese medicine utilized the dynamic antibiotic properties of garlic to treat gastrointestinal and respiratory conditions, thus garnering the status of "the divine herb."

Greco-Roman and Medieval European Traditions

The influential Medical School at Salerno classified garlic as a "hot food" to be consumed in winter to protect against pulmonary or breathing disorders. The Abbess of Rupertsberg, St. Hildegard von Bingen (1098–1179 AD), who was a prominent medical writer, recommended raw garlic for many disorders.

From Pharmacographia Indica (1890), garlic was used to treat many ailments including coughs, mucus, gonorrhea, colic, fevers, swellings, rheumatism, worm infestation, hysteria, flatulence, sciatica, and heart disease.

Traditional Preparations

Garlic's promising therapeutic advantages in ethnomedicine include its application against hypertension, pneumonia, hair loss, snakebite, diabetes, wounds, cough, paralysis, scabies, malaria, hemorrhoids, carbuncles, heart diseases, asthma, pain, respiratory disorders, influenza, and female infertility, which are mainly attributed to its antidiabetic, anti-atherosclerotic, antimicrobial, antihypertensive, anticancer, cardioprotective, diuretic, aphrodisiac, sedative, carminative, and antipyretic properties evidenced by various studies.

The hypotensive effect of garlic was exploited in traditional Italian folk practice by swallowing crushed cloves wrapped in bread crumbs or in water, or by introducing large quantities of this plant. In the 19th century, Louis Pasteur observed that garlic exhibited antibacterial properties, laying the groundwork for modern scientific investigation.

3. Key Constituents and Active Compounds

Organosulfur Compounds

Allium sativum is rich in several sulfur-containing phytoconstituents such as alliin, allicin, ajoenes, vinyldithiins, and flavonoids such as quercetin. Bulbs of A. sativum are reported to contain hundreds of phytochemicals including sulfur-containing compounds such as ajoenes (E-ajoene, Z-ajoene), thiosulfinates (allicin), vinyldithiins (2-vinyl-(4H)-1,3-dithiin, 3-vinyl-(4H)-1,2-dithiin), sulfides (diallyl disulfide [DADS], diallyl trisulfide [DATS]), and others that account for 82% of the overall garlic sulfur content.

Alliin and the alliinase reaction: Allicin is produced upon tissue damage from the non-proteinogenic amino acid alliin (S-allylcysteine sulfoxide) in a reaction catalyzed by the enzyme alliinase. Crushing or cutting garlic cloves releases alliinase enzyme sequestered in the vacuoles, which encounters cytosolic alliin to convert it into an array of thiosulfinates, of which the most prominent is allicin. Alliinase is activated at a pH of 4.5 to 9 and can be irreversibly inactivated at higher temperatures; once released from the vacuoles, it encounters alliin, initiating a cleavage in the carbon–sulfur bond within amino acids, leading to the formation of an unstable complex that undergoes dehydration in the presence of pyridoxal phosphate, resulting in three intermediates: sulfenic acid, pyruvic acid, and ammonia.

Allicin: Allicin [S-(2-propenyl)-2-propene-1-sulfinothioate] is the most biologically active sulfur-containing compound of garlic and is responsible for its smell and taste. Alliin (S-allyl-L-cysteine sulfoxide) is the main precursor of allicin, which represents about 70% of total thiosulfinates in the crushed cloves. Being a thiosulfinate, allicin is a reactive sulfur species and undergoes a redox reaction with thiol groups in glutathione and proteins, which is thought to be essential for its biological activity. Allicin is a membrane-permeable compound that readily enters cells and interacts with sulfhydryl-containing molecules such as glutathione and cysteine residues in proteins and enzymes; this interaction can lead to cytotoxic effects due to the oxidative modification of thiol groups.

Secondary decomposition products: The highly reactive, unstable, and volatile allicin decomposes to yield a large number of sulfides such as diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), methyl allyl disulfide (MADS), methyl allyl sulfide, ajoene, and vinyl dithiins. The sulfur compounds enter a cascade of further reactions to produce alkyl disulfides and polysulfides, like vinyl dithiins and ajoene, depending on concentration, temperature, and pH.

S-allylcysteine (SAC): γ-Glutamyl-S-allyl-L-cysteine is thought to be absorbed intact and hydrolyzed to S-allyl-L-cysteine (SAC), since metabolites of these compounds have been measured in human urine after garlic consumption. Other allicin-derived compounds, including diallyl sulfides, ajoenes, and vinyldithiins, have not been detected in human blood, urine, or stool, even after the consumption of up to 25 g of fresh garlic or 60 mg of pure allicin.

Ajoene: Ajoene is produced through allicin breakdown and non-enzymatic repair. The ajoenes have been shown to be powerful inhibitors of platelet aggregation.

Non-Sulfur Bioactive Compounds

Beside its major constituents of water, proteins, and carbohydrates, garlic is enriched with trace nutrients and an impressive array of bioactive compounds; saponins, organic acids, flavonoids, and organosulfur compounds (OSCs) are the primary bioactive compounds present in garlic bulb. As a rich natural source of bioactive compounds, including polysaccharides, saponins, tannins, linalool, geraniol, phellandrene, β-phellandrene, ajoene, alliin, S-allyl-mercapto cysteine, and β-phellandrene, garlic has many therapeutic applications and may play a role in drug development against various human diseases.

Effect of Processing on Compound Composition

Heating non-crushed garlic inactivates alliinase, preventing allicin formation during cooking and facilitating alliin formation instead. Aged garlic extract is notable for its water-soluble compounds. The AGE product contains primarily the water-soluble compounds S-allylcysteine (SAC) and SAMC, but also contains the lipid-soluble compounds diallyl sulfide, triallyl sulfide, and DADS.

4. Established Mechanisms of Action

Cardiovascular and Antihypertensive Mechanisms

Potential mechanisms of action for blood pressure effects include increased nitric oxide production, improved endothelial function, and antioxidant properties; some of the proposed pathophysiological mechanisms include vasodilation via nitric oxide release, enhanced endothelial function, antioxidant properties, inhibition of angiotensin-converting enzyme (ACE) activity, sodium and water excretion, and anti-inflammatory effects. Blood pressure-reducing properties of garlic are also related to hydrogen sulfide production and allicin content liberated from alliin and the enzyme alliinase, which is assumed to possess angiotensin II–inhibiting and vasodilating effects.

Allicin is also thought to be responsible for inhibiting β-hydroxy β-methylglutaryl-CoA (HMG-CoA), and thus lowering total cholesterol and LDL while increasing HDL. Alliin reduces blood triglycerides, boosts HDL cholesterol, and inhibits myocardial lipid accumulation; allicin prevents vascular calcification, promotes endothelial cell migration, and lowers blood pressure and plasma lipids.

Antioxidant Mechanisms

Garlic organosulfur compounds can activate Nrf2, a transcription factor that translocates to the nucleus and binds to the antioxidant response element (ARE), driving expression of genes coding for antioxidant and detoxifying enzymes including GCL, glutathione S-transferases, thioredoxin, NAD(P)H quinone oxidoreductase 1 (NQO-1), and heme oxygenase 1 (HO-1).

Antimicrobial Mechanisms

Studies on inhibition of Salmonella typhimurium using allicin suggest that inhibition of RNA synthesis is a primary target of allicin action; allicin and other thiosulfinates are also known to react with cysteine to abolish antimicrobial activity and to inhibit acetyl-CoA synthases from plants, yeasts, and mammals.

Anticancer Mechanisms (Preclinical)

Anticancer effects of garlic may affect cancer cells by modulation of many pathways, including alteration in carcinogen-metabolizing enzymes, cell cycle arrest, induction of apoptotic cell death, and suppression of oncogenic signal transduction pathways. In cancer cells, ajoene has been shown to trigger apoptosis, thus exerting an anti-cancer impact.

Immunomodulatory Mechanisms

Garlic and its derived bioactive constituents, including organosulfur compounds, may enhance immune system performance via pathways such as oxidative stress, interleukins, immunological and inflammatory response, and others; upon exposure to polysaccharides derived from garlic, RAW 264.7 macrophages exhibit distinct variations in tumor necrosis factor-α, IL-10, IL-6, and interferon-γ expression, and these polysaccharides possess notable immunomodulatory properties.

5. Scientific Evidence by Health Area

5.1 Cardiovascular Health — Blood Pressure

Evidence level: Moderate, consistent signal in individuals with elevated baseline blood pressure; insufficient evidence for outcomes such as mortality reduction.

A meta-analysis of 17 randomized controlled trials found that garlic intake caused a 3.75 mm Hg reduction (95% CI: −5.04 to −2.45; I² = 30.7%; P < .001) in systolic blood pressure. Clinical studies that investigated the effects of garlic and garlic-based supplements on blood pressure suggest that garlic consumption may modestly reduce blood pressure, particularly in individuals with mild hypertension.

In controlled clinical trials, daily intakes of aged garlic extract at doses between 1.2 g and 2.4 g (containing 1.2 to 2.4 mg of S-allyl-L-cysteine) consistently resulted in reductions in systolic blood pressure by 9–10 mm Hg and reductions in diastolic blood pressure by 4–8 mm Hg in a majority of patients with uncontrolled hypertension.

A 2012 Cochrane review of two randomized controlled trials found insufficient evidence to determine if garlic provides a therapeutic advantage compared to placebo in terms of reducing the risk of mortality and cardiovascular morbidity in patients diagnosed with hypertension; although garlic appeared to reduce mean supine systolic and diastolic blood pressure compared with placebo, the reviewers noted that the difference in blood pressure reduction falls within the known variability in blood pressure measurements, making it difficult to determine the impact, if any, of garlic on lowering blood pressure.

5.2 Cardiovascular Health — Lipid Profile and Cholesterol

Evidence level: Modest, with consistent signal for total cholesterol and variable effects on LDL and HDL; evidence is mixed across individual studies.

A 2024 meta-analysis incorporating 108 trials involving 7,137 participants found that consumption of garlic resulted in significant improvement in serum levels of triglycerides (WMD: −5.82 mg/dL), total cholesterol (WMD: −10.21 mg/dL), LDL cholesterol (WMD: −5.90 mg/dL), and HDL cholesterol (WMD: +2.18 mg/dL).

A 2023 review found evidence demonstrating mostly consistent total cholesterol reduction with garlic supplementation; however, its effect on LDL-C and HDL-C is variable. A 2016 meta-analysis and review of 39 randomized controlled trials involving 2,300 participants treated for a minimum of 2 weeks found garlic to be effective in reducing total cholesterol and LDL-C by 10 percent if taken for more than 2 months by individuals with slightly elevated concentrations.

The NCCIH states there is no evidence that garlic supplements prevent heart disease; evidence is mixed about whether garlic supplements lower cholesterol levels or change other known cardiovascular disease risk factors, and the available evidence on efficacy of garlic supplementation for disease prevention consists of mostly small, preliminary, or low-quality studies.

Formulation matters significantly: recent evidence suggests that variation in trial results may be due to variability arising from dose formulation causing inconsistent allicin release, and these results could be interpreted to mean that the therapeutic efficacy of garlic is dose-dependent, relying upon the quantity of allicin released.

5.3 Blood Glucose and Glycemic Control

Evidence level: Preliminary to moderate in specific populations; results inconsistent across studies, with high heterogeneity.

Among the studies focusing on fasting blood glucose (FBG), 8 eligible studies with 12 effect sizes were examined in one systematic review and meta-analysis; the garlic intervention demonstrated a significant influence on FBG levels (mean deviation = −7.01; 95% CI: −8.53, −5.49; P < 0.001; I² = 91.8%). In cases where heterogeneity surpasses the 50% threshold, the reliability of the combined outcomes may be compromised, necessitating a critical evaluation of the appropriateness of these results.

Current data confirm that garlic supplement plays positive and sustained roles in blood glucose, total cholesterol, and high/low density lipoprotein regulation in the management of type 2 diabetes mellitus (T2DM). However, due to the limited sample size and verified outcomes, there is not yet a comprehensive and quantitative analysis with high reliability.

Despite extensive animal research on garlic's anti-diabetic properties, there is limited evidence of its effectiveness in human investigations; garlic has been shown to affect blood glucose levels in the majority of clinical investigations, but not consistently in diabetic patients, proving that further research is still needed to determine its role in the treatment of diabetic patients.

5.4 Cancer — Epidemiological and Clinical Evidence

Evidence level: Weak and insufficient for clinical conclusions from human studies; preclinical evidence is extensive but does not translate directly.

Some studies indicate that certain groups of people who eat more garlic may be less likely to develop certain cancers, such as stomach and colon cancers; however, garlic in dietary supplement form has not been shown to help reduce the risk of these cancers. The National Cancer Institute recognizes garlic as one of several vegetables with potential anticancer properties but does not recommend using garlic dietary supplements for cancer prevention.

Although the beneficial effects of garlic intake against cancer have been reported in animal studies, benefits in human studies still remain controversial. Few clinical trials have been conducted to examine the potential anticarcinogenic effects of garlic. A systematic review of randomized clinical trials and cohort studies identified insufficient evidence to draw conclusions for the causal relationship between garlic intake and cancer.

5.5 Antimicrobial and Antiviral Activity

Evidence level: Strong preclinical evidence; limited and inconclusive human clinical evidence.

Extracts and isolated compounds of A. sativum have been evaluated for various biological activities including antibacterial, antiviral, antifungal, antiprotozoal, antioxidant, anti-inflammatory, and anticancer activities among others. Most previous studies of the antibacterial activity of garlic extracts have focused on Escherichia coli and Staphylococcus aureus.

Regarding the common cold, a 2014 Cochrane review of a single randomized controlled trial involving 146 participants who received either a garlic supplement or a placebo for 12 weeks concluded that there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold; the single trial suggested that garlic may prevent occurrences of the common cold but more studies are needed to validate this finding.

Regarding Helicobacter pylori, although garlic preparations and organosulfur compounds could inhibit the growth of H. pylori in the laboratory, there is little evidence to suggest that high garlic intakes or garlic supplementation may help prevent or eradicate H. pylori infection in humans; higher intakes of garlic were not associated with a significantly lower prevalence of H. pylori infection in China or Turkey, and clinical trials using garlic cloves, aged garlic extract, steam-distilled garlic oil, garlic oil macerate, or garlic powder have not found garlic supplementation to be effective in eradicating H. pylori infection in humans.

5.6 Antioxidant Activity

Evidence level: Some human biomarker evidence; clinical significance not well established.

Garlic supplementation at 600 mg/daily markedly reduced the susceptibility of apolipoprotein B-containing lipoproteins for oxidation in clinical studies. A 2024 comprehensive meta-analysis also demonstrated that garlic intake significantly improves oxidative stress and inflammatory biomarkers, particularly in adults with unfavorable baseline risk factors.

6. Body Systems and Health Areas of Association

Substantial studies have shown that garlic and its bioactive constituents exhibit antioxidant, anti-inflammatory, antibacterial, antifungal, immunomodulatory, cardiovascular protective, anticancer, hepatoprotective, digestive system protective, anti-diabetic, anti-obesity, neuroprotective, and renal protective properties. The primary body systems with which garlic research has been associated include:

• Cardiovascular system: Blood pressure, lipid metabolism, platelet aggregation, atherosclerosis, endothelial function.
• Endocrine and metabolic system: Blood glucose regulation, insulin sensitivity, lipid profiles in metabolic syndrome.
• Immune system: Garlic and its derived bioactive constituents, including organosulfur compounds, may enhance immune system performance via pathways such as oxidative stress, interleukins, and immunological and inflammatory response.
• Microbial defense: Antibacterial, antifungal, and antiviral activity at the cellular level.
• Digestive system: Historical use for gastrointestinal complaints; limited modern human evidence for H. pylori eradication.
• Oncological associations: Epidemiological associations with lower gastrointestinal cancer risk in dietary (not supplement) contexts.

7. Dosage Forms and Reported Dosages from Studies

A number of controlled clinical trials have examined the effect of powdered or dehydrated garlic supplements on cardiovascular risk factors; the most commonly used doses ranged from 600 to 900 mg/day and provided 3.6 to 5.4 mg/day of potential allicin.

Clinical trials have used 600–900 mg (delivering approximately 5,000–6,000 mcg of allicin potential) per day in two or three divided amounts; aged garlic extracts have been studied in amounts ranging from 2.4 to 7.2 grams per day.

In controlled clinical trials, daily intakes of aged garlic extract at doses between 1.2 g and 2.4 g (containing 1.2 to 2.4 mg of S-allyl-L-cysteine) consistently resulted in clinically meaningful blood pressure reductions in patients with uncontrolled hypertension.

Supplements included garlic powder, aged garlic extract, and garlic oil, which may have different effects on blood pressure; in one major meta-analysis, most trials used garlic powder in dosages ranging from 300 mg/d to 900 mg/d, providing 1.8 mg to 5.4 mg of allicin.

It is important to note that many manufacturers provide information on the "allicin potential" of their powdered garlic supplements, but few provide information on the "allicin release," a distinction with real clinical significance since most medicinal garlic supplements sold as garlic powder tablets or capsules show poor allicin release.

8. Safety Considerations and Drug Interactions

General Safety Profile

Garlic is probably safe for most people in the amounts usually eaten in foods; taking garlic supplements may increase the risk of bleeding. Side effects include breath and body odor, heartburn, and upset stomach; these side effects can be more noticeable with raw garlic. Allicin, the active substance of the garlic, can induce gastric agitation especially if administered in high doses.

Bleeding Risk

There is convincing evidence associating garlic supplementation with surgical bleeding; garlic has been associated with surgical bleeding in many case reports, and multiple RCTs show that garlic decreases platelet aggregation; a recent systematic review concluded that, based on the aggregated evidence, garlic does predispose to surgical bleeding. Taking garlic supplements may increase the risk of bleeding; this is especially important if surgery is planned or if a person takes medicines such as anticoagulants or aspirin that may also affect bleeding.

Interactions with Anticoagulants

Garlic possesses antiplatelet properties and may increase bleeding risk, particularly when combined with anticoagulants (warfarin, direct oral anticoagulants such as apixaban, rivaroxaban, edoxaban, or dabigatran) or antiplatelet medications (aspirin, clopidogrel, prasugrel, ticagrelor). Caution is advised when direct oral anticoagulants (DOACs) are used with garlic.

Interactions with HIV Protease Inhibitors

Garlic has been found to interfere with the effectiveness of some drugs, including saquinavir. In a study in healthy volunteers, a 3-week administration of a garlic supplement decreased plasma concentrations of the protease inhibitor saquinavir by about 50%. Chronic (21-day) administration of dehydrated garlic significantly reduced the AUC and Cmax of the HIV protease inhibitor saquinavir (a CYP3A4 and P-glycoprotein substrate) in nine healthy volunteers; it was proposed that garlic constituents lowered the saquinavir plasma concentrations by inducing the drug-metabolizing enzyme CYP3A4 in the intestine and the liver, and/or the membrane efflux transporter P-glycoprotein at the intestinal mucosa.

In two cases of HIV-infected patients, significant interactions between garlic and darunavir resulted in sub-therapeutic darunavir plasma concentrations and viral rebound; in one case the patient ingested 15 garlic cloves per week; other studies found garlic reduced blood levels of saquinavir and may reduce its effectiveness.

CYP Enzyme Interactions

Animal and in vitro studies with known organosulfur constituents of garlic, as well as garlic extracts, are known to simultaneously inhibit and induce several CYP enzymes (1A, 2B, 2C, 2E, and 3A subfamilies) in the liver, as well as the efflux transporter P-glycoprotein in the intestinal mucosa. The effects of garlic on CYP3A4 are controversial, as some in vitro studies reported that garlic had an inhibitory effect on CYP3A4, while other studies showed no significant effect of garlic on CYP3A4.

Interactions with Antidiabetic Agents

Dose adjustments may be required from potential blood-sugar-lowering effects with garlic supplements when taken concomitantly with insulin.

Pregnancy and Lactation

Garlic may not be safe for use during pregnancy or while breastfeeding when taken orally in amounts greater than those found in foods; little is known about the safety of using garlic topically during pregnancy or while breastfeeding.

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