Alliin: A Comprehensive Reference
1. Identity and Chemical Characterization
Alliin is a naturally occurring, non-proteinogenic sulfur-containing amino acid. Its chemical name is S-Allyl-L-cysteine sulfoxide, and it is a white crystalline powder that is almost odorless, with a melting point of 163–165 °C (at which point it decomposes and carbonizes), and a specific rotation of [α]D = +60°. Its molecular weight is 177.22.
Alliin has the molecular formula C6H11NO3S and is registered in PubChem (NIH) as CID 9576089. Alliin is an organic sulfur-containing product derived from the amino acid cysteine, making it a member of the cysteine sulfoxide family of organosulfur compounds found across the plant genus Allium.
The precursor alliin is found in four stereoisomers in nature; however, only one form — (+)-S-allyl-l-cysteine-sulfoxide — is found in garlic.
1.1 Botanical Source
The major focus of attention in the chemistry of Allium species has been on garlic compounds, where the starting point for the complex chemistry is alliin (S-allylcysteine sulfoxide). The Allium species are widely consumed in the regular diet worldwide, and among them, garlic (Allium sativum) is the most commonly studied because of its outstanding health benefit claims. However, other vegetables like onion (Allium cepa), leeks (Allium ampeloprasum var. porrum), chives (Allium schoenoprasum), and shallots (Allium ascalonicum) are also representative species as sources of organosulfur compounds (OSC), specifically the S-alk(en)yl-l-cysteine sulfoxides (ACSO).
The ACSOs present in Allium vegetables include alliin, methiin, propiin, and isoalliinin, and their presence differentiates the species: propiin is characteristic of shallots, and isoalliinin is typical of both onions and shallots.
Garlic (Allium sativum) is a perennial flowering plant that is native to Central Asia. Numerous cuneiform records show that garlic has been cultivated in Mesopotamia for at least 4,000 years.
1.2 Biosynthesis Within the Plant
Within the garlic plant, S-allyl-cysteine is oxidized to alliin, which is the "inactive" precursor of allicin. Alliin is enzymatically hydrolyzed to produce allyl sulfenic acid, which condenses spontaneously to form allicin. Alliin itself is stored in an essentially odorless state in intact garlic cloves. Allicin is not found in the intact garlic bulb; both the enzyme alliinase and alliin are found in different compartments of the garlic bulb.
2. Common Forms and Supplement Preparations
Several different types of garlic preparations are available commercially, and each type provides a different profile of organosulfur compounds depending on how it was processed. Not all garlic preparations are standardized, and even standardized brands may vary with respect to the amount and the bioavailability of the organosulfur compounds they provide.
- Dried/Powdered Garlic Tablets and Capsules: Powdered or dehydrated garlic is made from garlic cloves that are usually sliced and dried at a low temperature to prevent alliinase inactivation; the dried garlic is then pulverized and often made into tablets. To meet United States Pharmacopeial Convention (USP) standards, powdered garlic supplements must contain no less than 0.1% γ-glutamyl-S-allyl-L-cysteine and no less than 0.3% alliin (dry weight). Although powdered garlic supplements do not actually contain allicin, the manufacturer may provide a value for the "allicin potential" or "allicin yield" of a supplement on the label, representing the maximum achievable allicin yield.
- Enteric-Coated Tablets: Because alliinase is inactivated by the acidic pH of the stomach, most powdered garlic tablets are enteric-coated to keep them from dissolving before they reach the neutral pH of the small intestine.
- Steam-Distilled Oil and Aged Garlic Extract: Steam distilled oils and aged extracts are used in tablets, soft gelatin capsules, or liquids. Aged garlic extract is processed under conditions that convert most alliin and allicin into more stable compounds such as S-allylcysteine (SAC), and consequently contains little to no alliin or allicin.
- Raw Garlic (Food Form): Crushing or chopping garlic and letting it sit for 5–10 minutes allows alliinase to convert alliin to allicin before heat inactivates the enzyme; raw garlic delivers the most allicin but causes the most gastrointestinal irritation and odor. Roasting whole cloves at high temperatures largely destroys alliinase and significantly reduces allicin production.
2.1 Standardization and Allicin Potential
The United States Pharmacopeial Convention (USP) requires that powdered garlic supplements contain no less than 0.2% γ-glutamyl-(S)-allyl-L-cysteine and 0.5% alliin on a dried basis. Most clinical studies have used standardized garlic powder extract at 1.3% alliin (13 mcg alliin per milligram of powder).
The formation of allicin in the body after consumption of garlic powder supplements is questionable because alliinase is inactive at pH 3.5 or below, a pH range commonly found in the stomach, although a moderate to high-protein meal can briefly raise the pH to 4.4 or higher, a range in which alliinase is active. Hence, many brands of garlic supplements have been enteric-coated to prevent disintegration in the stomach, and the U.S. Pharmacopeia (USP) has established a monograph for estimating allicin formation and release from such products under simulated gastrointestinal dissolution conditions.
For enteric tablets, allicin bioavailability varied from 36–104%, but it was reduced to 22–57% when consumed with a high-protein meal, due to slower gastric emptying. Independent of meal type, non-enteric tablets gave high allicin bioavailability (80–111%), while garlic powder capsules gave 26–109%.
3. Traditional and Historical Use
Alliin, as the principal bioactive precursor compound in garlic, has been central to garlic's medicinal reputation across millennia, even though its precise chemical identity was not characterized until the twentieth century. The historical record pertains to preparations of garlic as a whole, within which alliin constitutes the primary sulfur compound.
3.1 Ancient Near East and Egypt
Interest in the potential benefits of garlic has origins in antiquity and is one of the earliest documented examples of plants employed for treatment of disease and maintenance of health; garlic was in use at the beginning of recorded history and was found in Egyptian pyramids and ancient Greek temples. Well-preserved garlic was found in the tomb of Tutankhamun (c. 1325 BC).
Ancient Egyptian and Roman sources describe garlic as a natural remedy for strength and endurance; pyramid workers consumed it to boost stamina, while Roman soldiers used it to recover from physical exertion and wounds.
In the past, garlic has been utilized as a remedy during various epidemics such as typhus, dysentery, cholera, and influenza, and whenever an epidemic emerged, garlic was among the first preventive and curative remedies employed.
3.2 Ancient Greece and Rome
Hippocrates, the revered physician, prescribed garlic for a variety of conditions. Garlic was given to the original Olympic athletes in Greece, as perhaps one of the earliest "performance enhancing" agents. Hippocrates prescribed garlic for respiratory issues, poor digestion, and fatigue.
Garlic was consumed by ancient Greek and Roman soldiers, sailors, and rural classes, and according to Pliny the Elder (Natural History xix. 32), by the African peasantry.
3.3 Ayurvedic Medicine (India)
In ancient Indian medicine, garlic was a valuable remedy used as a tonic, to cure a lack of appetite, common weakness, cough, skin disease, rheumatism, and haemorrhoids. In the Vedas — the Indian holy book — garlic was mentioned among other medicinal plants. Garlic appeared in Sanskrit texts dating back about 5,000 years. In Asia, garlic is a vital component of Ayurvedic medicine, used to treat infections, high blood pressure, and digestive disorders, among other health conditions.
3.4 Traditional Chinese Medicine (TCM)
Garlic first appeared in traditional Chinese medicine (TCM) at least 3,000 years ago. In TCM, garlic was considered to have a special influence on the spleen, kidney, and stomach — organs tied to metabolism, energy production, and digestion. It was also thought to remove poisons from the body, correct the unwholesomeness of water, and to eliminate the noxious effects of putrid meat and fish, and to keep plagues away.
3.5 Cross-Cultural Traditions
Ayurveda and traditional Chinese medicine have emphasized the digestive and detoxifying effects of garlic, using it to balance bodily humors and eliminate intestinal parasites. Garlic is one of the few herbs that was and still is used in all three great healing systems of the world — Ayurveda, Traditional Chinese Medicine, and Traditional European Medicine.
4. Key Constituents, Biochemistry, and Mechanisms of Action
4.1 The Alliin–Alliinase–Allicin Cascade
Allicin is produced upon tissue damage from the non-proteinogenic amino acid alliin (S-allylcysteine sulfoxide) in a reaction that is catalyzed by the enzyme alliinase. Alliin is acted upon by the cysteine-S-lyase enzyme alliinase when cell damage mixes substrate and enzyme; the first major volatile product is allicin (diallylthiosulfinate), giving crushed garlic its characteristic odor.
The reaction proceeds rapidly; alliin conversion to allicin is approximately 97% complete after 30 seconds at 23 °C. A single clove of approximately 10 g fresh weight can liberate up to 5 mg of allicin.
The dehydration reaction is supported by pyridoxyl phosphate (PLP), and results in the production of pyruvate, allyl sulfenic acid, and ammonia. Allicin is an intermediate compound, since it is rapidly and spontaneously decomposed and condensed into different substances, which are equally active.
Alliinase (EC 4.4.1.4), an enzyme found in garlic and other Allium plants in exceptionally high concentrations, is responsible for the formation of bioactive compound allicin from its stable precursor alliin. This reaction serves as a self-defence mechanism of Allium plants initiated upon tissue damage.
4.2 Chemical Identity and Reactivity of Alliin's Primary Metabolite (Allicin)
Being a thiosulfinate, allicin is a reactive sulfur species (RSS) and undergoes a redox-reaction with thiol groups in glutathione and proteins that is thought to be essential for its biological activity. Allicin is physiologically active in microbial, plant, and mammalian cells.
Allicin is hydrophobic in nature, can efficiently cross cellular membranes, and behaves as a reactive sulfur species (RSS) inside cells. It is a physiologically active molecule with the ability to oxidize the thiol groups of glutathione and between cysteine residues in proteins.
4.3 Downstream Organosulfur Compounds
Allicin produced from alliin is itself unstable and rapidly degrades into a cascade of additional bioactive compounds. Allicin is highly unstable and quickly degrades into other organosulfur-containing compounds such as diallyl disulfide and diallyl trisulfide, which also contribute to garlic's bioactivity. The thiosulfinates further degrade to vinyldithiins and ajoenes within 24 hours; the thiosulfinate allicin accounts for approximately 70% of the total thiosulfinates produced and is thought to be the principal bioactive compound responsible for the health-promoting benefits of garlic.
4.4 Direct Biological Activities of Alliin Itself
While most downstream biological effects proceed via allicin and its metabolites, alliin itself has been identified as a direct bioactive agent in certain contexts. Application of alliin helps to promote glucose metabolism and insulin sensitivity, and its applications have shown positive effects on the blood lipid profile and have been associated with prevention of heart attack. Alliin, the biosynthetic precursor of allicin, has been shown to suppress viral replication in infected cells while exhibiting minimal cytotoxicity toward uninfected host cells, suggesting a degree of selectivity in its antiviral action.
4.5 Antioxidant Mechanisms
As an antioxidant, allicin (derived from alliin) fights reactive oxygen species (ROS) by downregulation of NOX (NADPH oxidizing) enzymes, and can directly interact to reduce the cellular levels of different types of ROS produced by a variety of peroxidases.
4.6 Antimicrobial Mechanism
Allicin is physiologically active in microbial, plant, and mammalian cells. In a dose-dependent manner, allicin can inhibit the proliferation of both bacteria and fungi or kill cells outright, including antibiotic-resistant strains like methicillin-resistant Staphylococcus aureus (MRSA). A combination of the short half-life, high reactivity, and non-specificity to particular proteins are reasons most bacteria cannot deal with allicin's mode of action and develop effective defence mechanisms.
4.7 Anticancer Mechanisms (Preclinical)
Allicin and its organosulfur derivatives induce cell cycle arrest in the G2/M phase, through the inactivation of Cdk1 and the induction of its hyperphosphorylation, reducing its binding with cyclin B1. Allicin also reduces the cellular levels of HIF-1α, thereby inhibiting the expression of VEGF and consequent stimulation of angiogenesis.
5. Scientific Evidence by Area of Use
The following sections present the available evidence for health applications associated with alliin-containing garlic preparations. Because alliin is the direct precursor to allicin and other active garlic organosulfur compounds, the majority of relevant human clinical research has been conducted using standardized garlic products characterized by their alliin content or "allicin potential." Evidence strength is characterized based on the nature of the available studies.
5.1 Cardiovascular Health: Lipid Profile
Garlic supplements, mainly dried and pulverized whole-clove supplements, have been used in a large number of controlled clinical trials since the mid-1980s, focusing primarily on serum cholesterol and blood pressure.
Study specifics: One 12-week randomized, double-blind, placebo-controlled trial tested the effect of dried garlic powder tablets (providing 10.8 mg alliin per day, corresponding to about three garlic cloves) on blood lipids, blood pressure, and arterial stiffness in 75 healthy, normo-lipidaemic volunteers (men and women aged 40–60 years).
For meta-analytic evidence on aged garlic extract (which differs from alliin-standardized preparations), a systematic review and meta-analysis of 19 randomized controlled trials found that aged garlic consumption significantly reduced systolic blood pressure (WMD: −2.49 mmHg; 95% CI: −4.02 to −0.95) and LDL cholesterol (WMD: −4.41 mg/dL; 95% CI: −8.28 to −0.54). However, it did not significantly affect diastolic blood pressure or HDL cholesterol, and the effect on total cholesterol only approached significance (WMD: −4.74 mg/dL; 95% CI: −9.49 to 0.01; I² = 74.84%).
An 8-week controlled, randomized, double-blind, parallel-group study of a dietary supplement containing onion and garlic extract revealed significant reductions in both LDL cholesterol and total cholesterol levels among participants who received the extract. Additionally, improvements in blood pressure and in oxidative and inflammatory markers were observed, suggesting its potential as a therapeutic intervention for managing mild hypercholesterolemia.
Evidence strength: The effects on blood pressure have been moderately consistent for hypertensive subjects, while the effects on serum lipids have been inconsistent; of the 23 qualifying trials on serum cholesterol with a garlic powder product, 43% found no effect. Authors of meta-analyses most frequently cite the high heterogeneity among the trials — due to high variation in dose, variable product types, identification of active compounds, standardization concerns, and unknown bioavailability — as the reason for caution in recommending garlic products for the treatment of hypercholesterolemia and hypertension. Overall, evidence in this area is mixed and preliminary, with modest effects where found.
5.2 Cardiovascular Health: Blood Pressure
Subgroup analyses in the aged garlic meta-analysis indicated that aged garlic significantly affected systolic blood pressure and triglycerides in participants with cardiovascular diseases, while it also considerably impacted diastolic blood pressure in patients with hypercholesterolemia. However, as above, this body of work has high heterogeneity and results vary substantially by preparation type. Evidence is preliminary and inconsistent for alliin-standardized products specifically.
5.3 Antimicrobial Activity
Allicin has been extensively studied and reported for its therapeutic potential as an antioxidant with antimicrobial, anticancer, and anti-inflammatory activities. In vitro, in a dose-dependent manner, allicin can inhibit the proliferation of both bacteria and fungi or kill cells outright, including antibiotic-resistant strains like methicillin-resistant Staphylococcus aureus (MRSA).
Evidence strength: The antimicrobial evidence for alliin's primary metabolite, allicin, is substantial in vitro, but robust human clinical trials specifically testing alliin or allicin formulations against defined infections remain limited. A review of the mostly in vitro antimicrobial effects of allicin concluded that determination of allicin bioavailability from various products is necessary before proper clinical studies can be conducted. This area is currently supported by in vitro and some animal evidence, with insufficient high-quality human clinical trial data.
5.4 Antioxidant Activity
Several bioactive compounds from garlic, including allicin, allyl sulphides, alliin, ajoenes, and 1,2-vinyldithiin have therapeutic effects as antioxidants, anti-inflammatory, cardioprotective, antimicrobial, anticancer, and immunomodulatory agents.
Hydrolyzing allicin and alliin has been reported to produce water-soluble compounds with enhanced antioxidant, bioavailability, and stability properties.
Evidence strength: Antioxidant effects of garlic organosulfur compounds including alliin and allicin are well-documented in laboratory settings. Human clinical evidence specifically attributing antioxidant outcomes to alliin itself is limited; most human data involve composite garlic preparations. Evidence is promising but largely preclinical.
5.5 Anticancer Properties
Preclinical research has investigated allicin (derived from alliin) for potential anticancer activity. In one study, alliinase was chemically conjugated to a monoclonal antibody directed against a specific pancreatic cancer marker (CA19-9); after the conjugate bound to targeted pancreatic cancer cells, addition of alliin caused the cell-localized alliinase to produce allicin, effectively inducing apoptosis in MIA PaCa-2 cells. Allicin-induced caspase-3 expression, DNA fragmentation, cell cycle arrest, p21(Waf1/Cip1) cyclin-dependent kinase inhibitor expression, ROS generation, GSH depletion, and led to various epigenetic modifications resulting in stimulation of apoptosis.
Evidence strength: There are numerous reports on neuroprotection and improvement of cognitive abilities by the application of allicin; however, almost all study reports on these subject areas are based on preclinical studies conducted on animal models or human cell lines, and only two or three specific clinical studies have been reported. Anticancer evidence is currently in vitro and animal-based only; no human clinical trial evidence directly supports anticancer efficacy for alliin or allicin.
5.6 Metabolic Effects: Glucose and Insulin Sensitivity
Animal research has demonstrated effects of allicin on glucose metabolism. Insulin resistance is one of the main causes of elevated blood glucose, and studies have shown that allicin can alleviate insulin resistance. In a rodent model, after treatment with allicin, the blood glucose of rats consistently fluctuated within a limited range and was more stable than that of the model group, with the allicin group showing a significant decrease in blood glucose in the sixth week.
Evidence strength: These findings come from animal models; human clinical data specifically examining alliin's independent contribution to glycemic control remain limited. Overall, this area shows preliminary animal evidence with insufficient human clinical trial data to draw firm conclusions.
5.7 Neuroprotective and Cognitive Effects
Allicin has shown anticancer, antimicrobial, and antioxidant properties and also serves as an efficient therapeutic agent against cardiovascular diseases; researchers have described allicin as an antioxidant and neuroprotective molecule that can ameliorate cognitive abilities in cases of neurodegenerative and neuropsychological disorders. Most of the neuroprotective actions of allicin are mediated via redox-dependent pathways.
Evidence strength: Neuroprotective effects attributed to alliin-derived compounds are largely preclinical. As noted above, clinical evidence specifically for neuroprotection is highly limited.
5.8 Antiviral Activity
Beyond allicin, other garlic-derived organosulfur compounds exhibit broad-spectrum antiviral activity through diverse mechanisms. Alliin itself has been shown to suppress viral replication in infected cells while exhibiting minimal cytotoxicity toward uninfected host cells, suggesting a degree of selectivity in its antiviral action.
Computational (in silico) research has also been applied to alliin: results from molecular docking studies suggested that alliin has potential antiviral activity against COVID-19 by binding to the main protease (Mpro) of SARS-CoV-2. However, the WHO declared that there is no indication that garlic consumption has safeguarded patients from the novel coronavirus in the recent outbreak, and no clinical studies have been reported to confirm the prevention of Allium sativum against COVID-19.
Evidence strength: Antiviral evidence for alliin and its metabolites is in vitro and computational at this stage, with no confirmed human clinical trial evidence.
6. Body Systems and Health Areas of Association
Based on the available scientific and traditional literature, alliin and its derived compounds are associated with the following body systems:
- Cardiovascular System: Blood pressure regulation, lipid profile modulation (cholesterol and triglycerides), antiplatelet activity, and anti-atherogenic effects.
- Immune System: Alliin and garlic-derived compounds have documented immunomodulatory activity alongside antioxidant and anti-inflammatory properties.
- Gastrointestinal System: Ayurveda and TCM have emphasized the digestive and detoxifying effects of garlic, including its use against intestinal parasites.
- Metabolic System: Associations with glucose metabolism, insulin sensitivity, and lipid homeostasis observed in preclinical models.
- Nervous System: Neuroprotective and antioxidant effects mediated via redox pathways, studied primarily in preclinical models.
- Antimicrobial Defense: Broad-spectrum in vitro activity against bacteria, fungi, and viruses, primarily via allicin and its derived compounds.
7. Dosage Forms and Doses Reported in Studies
A typical dosage of garlic reported in the literature is 900 mg daily of a garlic powder extract standardized to contain 1.3% alliin, providing about 12,000 micrograms of alliin daily, or 4 to 5 mg of "allicin potential." Alliin-free aged garlic is taken at a dose of 1 to 7.2 grams daily.
In one 12-week randomized trial, volunteers received dried garlic powder tablets providing 10.8 mg alliin per day, corresponding to approximately three garlic cloves.
In meta-analyzed trials, garlic powder used in each trial was standardized to contain 2.5–13 μg/mg allicin or 13 μg/mg alliin; trial durations ranged from 7 days to one year.
In a pilot double-blind, randomized, placebo-controlled study of aged garlic extract combined with warfarin therapy, the study medication was administered at a dose of 5 mL twice a day for 12 weeks.
Most clinical trials with non-aged garlic have used preparations yielding at least 3,600–5,400 mcg of allicin per day, roughly the amount from one small clove.
Alliin content in commercial garlic powder supplements varies from 6.5 to 49 mg/g of garlic powder, reflecting substantial inter-product variation.
8. Safety Considerations and Known Drug Interactions
8.1 General Tolerability
In a pilot study of aged garlic extract combined with warfarin, there was no evidence of increased hemorrhage in either the placebo or the AGE group. Adverse events included headache, fatigue, colds, and dizziness; however, no significant difference was found in the incidence of these minor adverse events between the groups, suggesting they are unlikely to be attributable to the garlic extract.
8.2 Anticoagulant and Antiplatelet Drug Interactions
The most extensively documented interaction involves anticoagulant and antiplatelet therapies. Allicin and other garlic organosulfur compounds exhibit antiplatelet activity, perhaps through inhibition of thromboxane synthesis and platelet aggregation, and may potentiate the effects of anticoagulant and antiplatelet drugs such as warfarin, aspirin, and clopidogrel, thereby increasing the risk of bleeding, particularly with high-dose or prolonged garlic supplementation.
Garlic is one of the most widely used herbal medicines and is commonly ingested by people receiving warfarin; in vitro studies with the garlic constituent allicin and its degradation products indicate that these compounds possess antiplatelet effects. It has been suggested that herbal medicines such as garlic, either as a dietary supplement or in cooking, may interact with warfarin, resulting in poor international normalized ratio (INR) control.
Electronic health record analyses have detected signals of interactions between warfarin and several dietary supplements including garlic. Despite increased awareness of the potential for herb–drug interactions, the lack of rigorous clinical evidence regarding the significance and possible mechanisms provides a challenge for clinicians and consumers making rational decisions about the safe combination of complementary and conventional medicines.
8.3 CYP Enzyme Effects
An experiment evaluating eight water-soluble garlic compounds (including alliin) on CYP enzyme isoforms 1A2, 2B6, 2C9, 2C19, 2D6, and 3A in human liver microsomes revealed that only SMC and SAC caused an inhibition greater than 50% on CYP3A4; other compounds, including alliin, presented a ratio lower than 50%.
8.4 Gastrointestinal Effects and Odor
Raw garlic, which contains alliin and delivers the highest conversion to allicin, causes the most gastrointestestinal irritation and odor. Enteric-coated and deodorized preparations have been developed to mitigate these effects while preserving bioavailability.
8.5 Perioperative Considerations
The NCCIH emphasizes that individuals taking anticoagulants, aspirin, or any other medication should discuss garlic supplement use with their healthcare provider before use. Taking garlic supplements may increase the risk of bleeding, which is especially important if surgery is planned.
8.6 Standardization and Product Variability
The range of allicin bioavailability is due to variation in tablet/capsule size (0.1–1.0 g garlic powder), variation in alliin content (6.5–49 mg/g garlic powder), and variation in allicin bioavailability (22–111%), highlighting the need to carefully evaluate garlic supplements being considered for use in clinical trials.
References
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