Alliin

Alliin at a Glance

Alliin — S-allyl-L-cysteine sulfoxide — is the molecule that explains why an intact garlic clove smells of almost nothing. It is a stable, water-soluble, odourless sulfur amino acid that garlic stores in the cytosol of its storage cells, physically separated from the enzyme that destroys it ^{1Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →}. Break the cell and the two meet: alliinase converts alliin to allicin within seconds, and the whole familiar cascade of garlic chemistry — allicin, ajoene, the diallyl sulfides, the vinyldithiins — unfolds from there ^{1,3Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →Reference 3Rose P et al. · 2005Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agentsView study →}.

That compartmentalisation is the spine of the whole account. It makes alliin the stable, measurable thing about garlic and allicin the moving target, which is why analytical chemists, breeders and supplement manufacturers all deal in alliin while the pharmacology literature deals in allicin.

Its own pharmacology is a genuinely open question. There is a surprisingly large body of isolate-level preclinical work — rodent metabolic, anti-inflammatory and cytoprotective studies using pure alliin — but no human trial has ever tested isolated alliin against a clinical endpoint, and the one human tissue study that looked for it found no significant accumulation from a garlic supplement ^{29Reference 29Livingstone TL et al. · 2022Accumulation of sulforaphane and alliin in human prostate tissueView study →}. Treat what follows as an honest precursor page with a preclinical appendix, not as a second copy of allicin’s evidence.

The Compartmentalisation Mechanism

Garlic runs a two-part chemical weapon that is only armed by damage.

Part one — the substrate. S-alk(en)yl cysteine sulfoxides, of which alliin is garlic’s dominant form, accumulate in the cytosol of storage mesophyll cells ^{1Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →}. They are stable, water-soluble, non-volatile and biologically inert to the plant itself. A clove can hold roughly 1% of its fresh weight as alliin — about 27.7 ± 5.8 mg/g dry weight — without any metabolic cost of keeping a reactive toxin around ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

Part two — the enzyme. Alliinase (alliin lyase, EC 4.4.1.4) is a pyridoxal-phosphate-dependent glycoprotein dimer of ~51.5 kDa subunits, abundantly expressed in the bulb, with a K_m of about 1.1 mM for synthetic S-allylcysteine sulfoxide and a pH optimum near 6.5 ^{4Reference 4Rabinkov A et al. · 1994Alliin lyase (alliinase) from garlic (Allium sativum)View study →}. It is a vacuolar enzyme, and in Allium shoots it is concentrated in specific cells — immunohistochemistry in Allium tuberosum localised it predominantly to the bundle sheath ^{1,5Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →Reference 5Manabe T et al. · 1998Alliinase [S-alk(en)yl-L-cysteine sulfoxide lyase] from Allium tuberosum (Chinese chive) — purification, localization, cDNA cloning and heterologous functional expressionView study →}.

The trigger. Tissue damage — a knife, a tooth, an insect’s mandible — ruptures the vacuole and brings enzyme and substrate together. Alliinase cleaves alliin to allyl sulfenic acid plus pyruvate and ammonia; two sulfenic acids then condense spontaneously to allicin, and allicin rearranges further into ajoene, vinyldithiins and the diallyl polysulfides ^{1Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →}. The speed is the point: when water is added to garlic powder, allicin formation is essentially complete in about 30 seconds ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

The strict “two sealed compartments” picture may be a slight simplification. Genome and transcriptome work in onion found that of 64 alliinase genes, only 14 carried the N-terminal signal peptide needed for vacuolar targeting — leading the authors to propose that cytosolic alliinases maintain a low constitutive level of the reactive product even in undamaged tissue, with the vacuolar enzymes providing the damage-triggered surge ^{6Reference 6Cho H et al. · 2024Understanding the defense mechanism of Allium plants through the onion isoallicin-omics studyView study →}. If that holds for garlic, the defence is a dual system rather than a pure trip-wire.

What “Allicin Potential” Actually Measures

This is the practical consequence of the mechanism, and the reason alliin matters commercially.

Because allicin only exists after damage and then degrades, you cannot meaningfully standardise a garlic product to its allicin content. What manufacturers standardise instead is “allicin potential” (the US Pharmacopeia calls it “potential allicin”): the amount of allicin a product would yield if you pulverised it and stirred it in water for 30 minutes ^{2,7Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 7Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →}. That figure is really a composite of two things — how much alliin is present, and how much alliinase activity survives processing, tabletting and the stomach.

Both halves fail routinely, and the failures have shaped the clinical literature:

• Applying the USP dissolution method to all 24 known brands of enteric-coated garlic tablets, Lawson and Wang found that nearly all met their allicin-potential claim when crushed in water — but 83% released under 15% of that potential under simulated gastrointestinal conditions, defeated by tablet excipients that impaired alliinase and by slow disintegration ^{7Reference 7Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →}.
• The same group traced a run of negative cholesterol trials to the tablets themselves: older lots of a single brand released 44% of their potential, newer lots only 15%, so subjects in the later, negative studies plausibly received a fraction of the allicin that subjects in the earlier, positive ones did ^{8Reference 8Lawson LD et al. · 2001Clinical trialAllicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterolView study →}.
• A later bioavailability study using breath allyl methyl sulfide as the readout found allicin bioequivalence ranging from 36–104% for enteric tablets (dropping to 22–57% with a high-protein meal, which slows gastric emptying), 80–111% for non-enteric tablets, and 26–109% for garlic-powder capsules. Alliin content across the same powders ranged 6.5–49 mg/g — a seven-fold spread ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

The honest reading: alliin content is the reliable number on a garlic label; allicin delivery is not. Notably, that study also found alliinase activity was not the limiting variable once a minimum was met — only that a minimum was required ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Alliinase is also inactivated below about pH 3.5, which is why enteric coating exists at all, and why stomach acid, not the tablet, is often the real destination of the alliin you swallow ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

What It's Used For

Alliin’s own pharmacology is almost entirely isolate-level preclinical work in rodents and cell lines. It is more voluminous than most precursor molecules manage — but it does not reach a human endpoint anywhere, and it should not be read as evidence for garlic.

Evidence by IndicationStrength of support · Beta

Support is an experimental score — weighted by study type (human > animal > in vitro) and volume. A beta, not a validated metric. Every score here is capped low by design: all of it is isolate preclinical work, with no human clinical endpoint anywhere in the set.

01Metabolic and glycaemic regulationShaky30%Three independent diet-induced-obesity mouse studies ^{20,21Reference 20Zhai B et al. · 2018AnimalHypoglycemic and hypolipidemic effect of S-allyl-cysteine sulfoxide (alliin) in DIO miceView study →Reference 21Sánchez-Sánchez MA et al. · 2020AnimalAlliin, an Allium sativum nutraceutical, reduces metaflammation markers in DIO miceView study →}

02Anti-inflammatoryShaky26%Rodent colitis + LPS macrophage/mouse models ^{22,23Reference 22Shi L et al. · 2017Alliin, a garlic organosulfur compound, ameliorates gut inflammation through MAPK-NF-κB/AP-1/STAT-1 inactivation and PPAR-γ activationView study →Reference 23Liu M et al. · 2022AnimalAlliin alleviates LPS-induced pyroptosis via promoting mitophagy in THP-1 macrophages and miceView study →}

03Antioxidant and radical scavengingShaky22%Direct radical-chemistry assays + Nox1 suppression in osteoclasts ^{19,26Reference 19Chung LY · 2006The antioxidant properties of garlic compounds: allyl cysteine, alliin, allicin, and allyl disulfideView study →Reference 26Chen Y et al. · 2016Alliin attenuated RANKL-induced osteoclastogenesis by scavenging reactive oxygen species through inhibiting Nox1View study →}

04Organ cytoprotection (heart, brain, kidney)Shaky18%Rodent ischaemia-reperfusion and ferroptosis models ^{24,25Reference 24Zhao R et al. · 2019Alliin alleviates myocardial ischemia-reperfusion injury by promoting autophagyView study →Reference 25Meng J et al. · 2025In vitroAlliin from garlic as a neuroprotective agent attenuates ferroptosis in vitro and in vivo via inhibiting ALOX15View study →}

05Anticancer and anti-angiogenicShaky12%One CAM/endothelial study; contradicted in a direct alliin-vs-allicin comparison ^{27,28Reference 27Mousa AS · 2005Anti-angiogenesis efficacy of the garlic ingredient alliin and antioxidants: role of nitric oxide and p53View study →Reference 28Rosas-González VC et al. · 2020Differential effects of alliin and allicin on apoptosis and senescence in luminal A and triple-negative breast cancer: caspase, ΔΨm, and pro-apoptotic gene involvementView study →}

Mechanisms

The first row is the only one with established, non-controversial standing. Everything below it is preclinical, and none of it has a demonstrated direct molecular target — alliin’s chemical inertness is what makes it a good storage form and a poor drug candidate.

Pharmacokinetics

Alliin is well absorbed and rapidly cleared, and — importantly — is not efficiently converted to allicin inside the body. When pure L(+)-alliin was consumed by humans, breath allyl methyl sulfide (the standard allicin metabolite) was detectable at 0.2 h and peaked at ~3.2 h, but only about 5% of the dose was metabolised down that route, which the authors interpret as the ceiling on what intestinal bacterial alliinase can contribute ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Synthetic L(±)-alliin behaved no differently from the natural L(+) form ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

Dog data summarised in the same work show rapid and essentially complete absorption, with <0.1% appearing as S-allylcysteine or its N-acetyl form, 22% of the dose excreted in urine as N-acetyl-alliin, 6% as unchanged alliin, and 72% unaccounted for ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Uptake has a plausible carrier: alliin is a substrate of LAT1 (SLC7A5), the essential-amino-acid transporter ^{30Reference 30Scanga R et al. · 2022The nutraceutical alliin from garlic is a novel substrate of the essential amino acid transporter LAT1 (SLC7A5)View study →} — unsurprising for a cysteine derivative, and a reasonable explanation for its ready absorption.

The human distribution data are thin and negative. In a randomised, double-blinded, four-week 2×2-factorial supplement study in 39 men scheduled for prostate biopsy, alliin and alliin-derived metabolites were detected in prostate tissue — but at no significantly higher concentration in the men taking the garlic supplement than in those who were not, in contrast to the same study’s clear positive result for sulforaphane from broccoli ^{29Reference 29Livingstone TL et al. · 2022Accumulation of sulforaphane and alliin in human prostate tissueView study →}. No half-life, volume of distribution or oral bioavailability figure has been established for alliin in humans.

Clinical Trials

There are no clinical trials of isolated alliin at a clinical endpoint. The closest human data are a four-week tissue-accumulation study using a garlic supplement, which was negative for alliin ^{29Reference 29Livingstone TL et al. · 2022Accumulation of sulforaphane and alliin in human prostate tissueView study →}, and the large body of garlic-powder cardiovascular trials in which alliin appears only as the standardisation figure, not the intervention ^{2,8Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 8Lawson LD et al. · 2001Clinical trialAllicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterolView study →}.

Last checked: July 2026.

Isolate vs. Plant Studies

This molecule sits at the sharpest version of the isolate-vs-plant problem, and it points the opposite way from most.

The preclinical work is almost all isolate. Pure alliin (usually synthetic, and interchangeable with the natural form in vivo ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}) is what was dosed in the DIO-mouse metabolic studies, the colitis model, the osteoclast assays and the angiogenesis work ^{20,21,22,26,27Reference 20Zhai B et al. · 2018AnimalHypoglycemic and hypolipidemic effect of S-allyl-cysteine sulfoxide (alliin) in DIO miceView study →Reference 21Sánchez-Sánchez MA et al. · 2020AnimalAlliin, an Allium sativum nutraceutical, reduces metaflammation markers in DIO miceView study →Reference 22Shi L et al. · 2017Alliin, a garlic organosulfur compound, ameliorates gut inflammation through MAPK-NF-κB/AP-1/STAT-1 inactivation and PPAR-γ activationView study →Reference 26Chen Y et al. · 2016Alliin attenuated RANKL-induced osteoclastogenesis by scavenging reactive oxygen species through inhibiting Nox1View study →Reference 27Mousa AS · 2005Anti-angiogenesis efficacy of the garlic ingredient alliin and antioxidants: role of nitric oxide and p53View study →}. That is normally a strength. Here it creates an odd inversion: the isolate has a preclinical file, and the plant has the human file — but they are not the same story. Garlic’s human cardiovascular evidence comes from garlic powder standardised to alliin, where the presumed active agent is the allicin generated after swallowing, not the alliin itself ^{2,8Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 8Lawson LD et al. · 2001Clinical trialAllicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterolView study →}.

The one head-to-head comparison went against the precursor. In matched breast-cancer lines, allicin did the pharmacology and alliin did not — indeed alliin increased clonogenicity ^{28Reference 28Rosas-González VC et al. · 2020Differential effects of alliin and allicin on apoptosis and senescence in luminal A and triple-negative breast cancer: caspase, ΔΨm, and pro-apoptotic gene involvementView study →}. That is the single most useful sanity check available: where the two have been tested in the same assay, the activity travels with the reactive product.

Practical consequence: it is not legitimate to read any garlic clinical result onto alliin. It is equally not legitimate to dismiss alliin’s preclinical file as contamination — those studies used the pure compound in systems with no alliinase. The defensible position is that alliin may have modest activity of its own, unproven in humans, and that this is a genuinely open question rather than a settled one either way.

Biosynthesis & Its Role in Plants

Alliin is built from glutathione, not assembled from free cysteine — the pathway proposed from radiotracer work involves alk(en)ylation of the cysteine residue within glutathione, followed by cleavage of the peptide and oxidation of the sulfur to the sulfoxide ^{1,9Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →Reference 9Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}. Two steps of the garlic route are now enzymatically defined:

• Deglutamylation. γ-Glutamyl transpeptidases remove the glutamyl group from the stored intermediate γ-glutamyl-S-allylcysteine, releasing S-allylcysteine ^{11Reference 11Yoshimoto N et al. · 2014Garlic γ-glutamyl transpeptidases that catalyze deglutamylation of biosynthetic intermediate of alliinView study →}.
• S-oxygenation. AsFMO1, a cytosolic NADPH/FAD-dependent flavin-containing monooxygenase, then stereoselectively oxidises S-allylcysteine to almost exclusively the (R_CS_S) sulfoxide — the same stereochemistry as natural alliin ^{10Reference 10Yoshimoto N et al. · 2015Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlicView study →}. AsFMO1 prefers free S-allylcysteine over the γ-glutamyl form, which fixes the order: deglutamylate first, oxidise second ^{10Reference 10Yoshimoto N et al. · 2015Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlicView study →}.

The route runs in two settings. In storage leaves of pre-emergent bulbs, stored γ-glutamyl-S-allylcysteine is converted to alliin during sprouting; in green foliage leaves, alliin is made de novo ^{10Reference 10Yoshimoto N et al. · 2015Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlicView study →}. Transcriptional control is beginning to be mapped — the WRKY transcription factor AsWRKY9 binds the W-box in the AsFMO1 promoter and raises both AsFMO1 expression and alliin accumulation in transgenic garlic callus ^{13Reference 13Wu J et al. · 2025Identification and functional characterization of AsWRKY9, a WRKY transcription factor modulating alliin biosynthesis in garlic (Allium sativum L.)View study →}.

Tissue distribution and wounding response follow the defence logic. Alliin is lowest in roots and highest in buds, and in mature leaves it falls after wounding — while genes for cysteine and serine biosynthesis are upregulated, consistent with the pool being consumed and then replenished ^{12Reference 12Yang X et al. · 2021Parallel analysis of global garlic gene expression and alliin content following leaf woundingView study →}. AsWRKY9 expression itself rises sharply after leaf injury ^{13Reference 13Wu J et al. · 2025Identification and functional characterization of AsWRKY9, a WRKY transcription factor modulating alliin biosynthesis in garlic (Allium sativum L.)View study →}.

The ecological reading is straightforward: garlic maintains an expensive, biologically inert stockpile precisely so that it can generate a broad-spectrum antimicrobial and anti-herbivore toxin instantly, on damage, at the site of damage — without ever having to store the toxin itself ^{1,6Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →Reference 6Cho H et al. · 2024Understanding the defense mechanism of Allium plants through the onion isoallicin-omics studyView study →}.

Alliin & S-Allylcysteine

These two are constantly confused, and the relationship is worth stating precisely, because it runs in both directions.

Biosynthetically, S-allylcysteine is alliin’s immediate precursor. AsFMO1 oxidises S-allylcysteine’s sulfur to give alliin ^{10Reference 10Yoshimoto N et al. · 2015Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlicView study →}. The pathway is glutathione → γ-glutamyl-S-allylcysteine → S-allylcysteine → alliin.

Chemically, the arrow can also reverse. Alliin is not thermally robust: heated in solution at 60–89 °C it degrades by first-order kinetics, and S-allylcysteine is one of the identified degradation products (alongside a set of allyl-alanine and dialanine di-, tri- and tetrasulfides) ^{18Reference 18Chen Z et al. · 2017Thermolysis kinetics and thermal degradation compounds of alliinView study →}.

Analytically, they mark opposite ends of processing. Alliin is the marker of fresh, intact garlic and the standardisation figure for garlic powder. S-allylcysteine — stable, water-soluble, odourless and well absorbed — is the marker of aged garlic extract, where it accumulates over months while alliin is consumed ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. The two are pharmacokinetically distinguishable in humans: breath allyl methyl sulfide peaks at ~3.2 h after alliin and ~8.4 h after S-allylcysteine, indicating genuinely different metabolic routes ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

Black garlic completes the picture — long low-temperature heating leaves alliin undetectable, and because alliinase was never activated, the alliin was not converted to allicin but to some other, still unidentified set of compounds ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

Prevalence in Nature

Alliin is essentially a garlic molecule. It is the characteristic cysteine sulfoxide of Allium sativum, and its distribution across the genus is what gives each species its own flavour signature: garlic accumulates alliin, onion accumulates isoalliin (S-trans-1-propenyl cysteine sulfoxide), methiin (S-methyl cysteine sulfoxide) is present in most Alliums and in some Brassicaceae, and propiin (S-propyl cysteine sulfoxide) occurs in onion and relatives ^{9Reference 9Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}.

Within garlic, the content is high but variable. Cloves average 27.7 ± 5.8 mg/g dry weight (9.7 ± 2.0 mg/g fresh weight) across 46 sampled varieties, and alliin is typically 65–85% of a product’s total S-allyl compounds ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Cultivar and growing conditions both matter: hardneck (var. ophioscorodon) garlic carries more alliin and methiin than softneck, with the rocambole cultivar ‘Music Pink’ the richest of eleven tested ^{16Reference 16Lee J · 2005Free amino acid and cysteine sulfoxide composition of 11 garlic (Allium sativum L.) cultivars by gas chromatography with flame ionization and mass selective detectionView study →}; across Andalusian trial sites, purple-type cultivars averaged the highest alliin and methiin but the lowest isoalliin, and planting date significantly affected alliin content while genotype contributed relatively little to the total variance ^{15Reference 15Montaño A et al. · 2011Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic (Allium sativum L.) grown in Andalusia, SpainView study →}.

Storage changes the mix rather than depleting alliin outright. Tracking cultivars over seven months — from a month pre-harvest through six months of storage — a LC-MS/MS study found high variability between cultivars and between sites for the same cultivar, and a striking up-to-54-fold increase in isoalliin over six months of storage ^{14Reference 14Rektorisova M et al. · 2020Variability in S-alk(en)yl-L-cysteine sulfoxides in garlic within a seven-month period determined by a liquid chromatography–tandem mass spectrometry methodView study →}.

Outside A. sativum, giant/elephant garlic (Allium ampeloprasum) carries alliin at a much lower ~2.7 mg/g dry weight, alongside comparable S-allylcysteine ^{17Reference 17Peterssen-Fonseca D et al. · 2021Chemometric optimisation of pressurised liquid extraction for the determination of alliin and S-allyl-cysteine in giant garlic (Allium ampeloprasum L.) by liquid chromatography tandem mass spectrometryView study →}.

Major Natural Sources

Source	Part	Alliin content	Note
Garlic (Allium sativum)	Fresh clove	9.7 ± 2.0 mg/g fresh wt	~1% of fresh weight; the reference source 2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →
Garlic powder	Dried bulb	6.5–49 mg/g	Seven-fold spread across commercial supplement powders 2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →
Giant/elephant garlic (Allium ampeloprasum)	Clove	~2.7 mg/g dry wt	Roughly a tenth of true garlic 17Reference 17Peterssen-Fonseca D et al. · 2021Chemometric optimisation of pressurised liquid extraction for the determination of alliin and S-allyl-cysteine in giant garlic (Allium ampeloprasum L.) by liquid chromatography tandem mass spectrometryView study →
Black garlic	Heat-aged clove	Undetectable	Converted during aging, but not to allicin 2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →
Onion, leek, chive (Allium spp.)	Bulb / leaf	Trace to absent	Dominated by isoalliin, methiin, propiin instead 9Reference 9Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →

Stability, Cooking & Processing

Alliin is stable in the intact clove and reasonably stable dry, but three things destroy it or divert it:

• Damage. Crushing, chopping, chewing or blending converts it to allicin within seconds ^{1,2Reference 1Yoshimoto N · 2019S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivitiesView study →Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Even gentle handling counts for something — dicing garlic into 3 mm cubes converted only about 3% of the alliin, because most cell walls survive intact ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.
• Heat. Above 60 °C alliin degrades by first-order kinetics into S-allylcysteine and a family of allyl-alanine polysulfides ^{18Reference 18Chen Z et al. · 2017Thermolysis kinetics and thermal degradation compounds of alliinView study →}. Separately, heat inactivates alliinase, so cooking uncrushed garlic locks the alliin in place — it neither becomes allicin nor survives as a reliable precursor.
• Acid. Alliinase is inactivated below about pH 3.5, so alliin that reaches an empty, acidic stomach in an intact matrix will largely never meet an active enzyme ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

The practical kitchen implication is the familiar one, now with a mechanism: crush first, wait, then heat. And the practical clinical implication is that “garlic” in a trial is not a single exposure — a boiled, roasted, pickled, black or raw preparation of the same clove delivers very different downstream chemistry from the same starting alliin ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}.

Analytical Chemistry & Quality Control

Alliin is the workhorse analyte of garlic quality control, for exactly the reasons above: it is stable, non-volatile, UV-detectable after derivatisation and present in quantities that are easy to measure.

The standard approach is HPLC or LC-MS/MS on an alliinase-inactivated extract — samples are typically blanched or extracted in the presence of an alliinase inhibitor so that the alliin is not destroyed during workup ^{2,17Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 17Peterssen-Fonseca D et al. · 2021Chemometric optimisation of pressurised liquid extraction for the determination of alliin and S-allyl-cysteine in giant garlic (Allium ampeloprasum L.) by liquid chromatography tandem mass spectrometryView study →}. Methods have been developed to quantify alliin and allicin simultaneously (including a digital-subtraction HPTLC method) ^{39Reference 39Bosca B et al. · 2023Novel simultaneous determination of alliin and allicin in Allium sp. using digital subtraction HPTLCView study →} and to assay allicin, alliin and alliinase activity together with a chromogenic thiol ^{40Reference 40Miron T et al. · 2002A spectrophotometric assay for allicin, alliin, and alliinase (alliin lyase) with a chromogenic thiol: reaction of 4-mercaptopyridine with thiosulfinatesView study →}.

Two subtleties matter for anyone reading a certificate of analysis:

• Stereochemistry. Natural alliin is a single diastereoisomer, (+)-S-allyl-L-cysteine sulfoxide; chemical synthesis produces a diastereoisomeric mixture, and separating them requires a dedicated method (porous graphitic carbon HPLC, for example) ^{38Reference 38Dethier B et al. · 2012Analysis of the diastereoisomers of alliin by HPLCView study →}. In commercial products the L(−)-isomer, which tends to increase with age, is usually under 6% of the total ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}. Absolute configurations of alliin and allo-alliin have been confirmed by X-ray diffraction ^{37Reference 37Hakamata W et al. · 2015A simple synthesis of alliin and allo-alliin: X-ray diffraction analysis and determination of their absolute configurationsView study →}.
• What the number does and doesn’t tell you. Alliin content sets the ceiling on allicin, nothing more. A tablet can hit its alliin claim exactly and still deliver almost nothing, because the enzyme, the excipients, the disintegration time and the meal all sit between the label and the bloodstream ^{2,7,8Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 7Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →Reference 8Lawson LD et al. · 2001Clinical trialAllicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterolView study →}.

Discovery & Synthesis

Alliin was isolated in 1948 by Arthur Stoll and Ewald Seebeck at Sandoz in Basel, who published it as “the genuine mother substance of garlic oil” — the odourless precursor that garlic actually contains, as distinct from the pungent allicin that Chester Cavallito had characterised a few years earlier and that only appears after damage ^{34Reference 34Stoll A · 1948Stoll A, Seebeck E (1948). [About alliin, the genuine mother substance of garlic oil]. Helvetica Chimica Acta. https://pubmed.ncbi.nlm.nih.gov/18912392/View study →}. The pair followed it in 1950 with a synthesis of natural alliin ^{35Reference 35Stoll A · 1950Stoll A, Seebeck E (1950). [Synthesis of natural alliin]. Experientia. https://pubmed.ncbi.nlm.nih.gov/14773552/View study →}, and consolidated the chemistry in a 1951 review ^{36Reference 36Stoll A · 1951Chemical investigations on alliin, the specific principle of garlicView study →}. The name follows the botanical genus Allium, and gave its name in turn to the enzyme (alliinase, or alliin lyase) that consumes it.

Modern synthesis is straightforward and cheap: S-allyl-L-cysteine is prepared by S-alkylation of L-cysteine and then oxidised at sulfur, a route reworked in 2015 into a simple procedure for both alliin and allo-alliin with the absolute configurations confirmed by X-ray diffraction ^{37Reference 37Hakamata W et al. · 2015A simple synthesis of alliin and allo-alliin: X-ray diffraction analysis and determination of their absolute configurationsView study →}. Because chemical oxidation is not fully stereoselective, synthetic alliin arrives as a diastereoisomeric mixture requiring separation or acceptance of the co-product ^{38Reference 38Dethier B et al. · 2012Analysis of the diastereoisomers of alliin by HPLCView study →} — in contrast to the plant, where AsFMO1 delivers essentially one isomer ^{10Reference 10Yoshimoto N et al. · 2015Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlicView study →}.

Manufacture today is a mixture of both. Research-grade alliin is synthetic (and behaves identically to the natural form in humans ^{2Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →}). Commercially, the alliin that people actually consume is not isolated at all — it is delivered as garlic powder, made by slicing and low-temperature-drying cloves fast enough to inactivate alliinase before it can act, then standardised on alliin content or allicin potential ^{2,7Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 7Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →}.

Patents: not yet researched.

Toxicity & Safety

Alliin carries a low flag. It is an odourless, chemically unreactive amino acid that humans eat in gram quantities as ordinary garlic, it has no reported acute toxicity, and it has no established interaction profile of its own. An alliin-rich garlic extract fed to Drosophila melanogaster raised total thiols and glutathione-S-transferase activity, reduced acetylcholinesterase activity, and produced no histological changes in brain, fat body or gut ^{32Reference 32Oyaluna ZE et al. · 2024Chemical analysis of alliin-rich Allium sativum (garlic) extract and its safety evaluation in Drosophila melanogasterView study →}.

The honest caveats are about scope rather than hazard.

The risk is what it becomes. Alliin’s practical meaning in the body is allicin and the downstream sulfides, and it is those that carry garlic’s real cautions — antiplatelet activity and bleeding risk (relevant with anticoagulants and before surgery), gastrointestinal irritation, and reflux. Those belong on the garlic monograph and the allicin page, where the exposure is actually decided.

The garlic allergen is the enzyme, not the precursor. IgE-mediated garlic allergy has been characterised, and the identified allergen is alliin lyase (alliinase) itself — a protein, not alliin ^{33Reference 33Kao SH et al. · 2004Identification and immunologic characterization of an allergen, alliin lyase, from garlic (Allium sativum)View study →}. People with Allium allergy should avoid garlic regardless; alliin is not the sensitising agent.

Interactions are unassessed for the isolate. The only relevant data are indirect: rats fed 5% garlic powders for two weeks showed increased UDP-glucuronosyltransferase and glutathione-S-transferase activity, slightly increased CYP1A2, and decreased CYP2E1 — and UGT activity correlated with the alliin content of the powder, though the other changes did not ^{31Reference 31Le Bon AM et al. · 2003AnimalEffects of garlic powders with varying alliin contents on hepatic drug metabolizing enzymes in ratsView study →}. That is a whole-powder effect with an alliin-dose correlation, not a demonstrated alliin interaction; the CYP2E1 and phase II changes are conventionally attributed to diallyl disulfide and its relatives ^{31Reference 31Le Bon AM et al. · 2003AnimalEffects of garlic powders with varying alliin contents on hepatic drug metabolizing enzymes in ratsView study →}. No isolate-level CYP, P-glycoprotein or transporter-inhibition study exists. Alliin is a substrate (not a demonstrated inhibitor) of LAT1/SLC7A5 ^{30Reference 30Scanga R et al. · 2022The nutraceutical alliin from garlic is a novel substrate of the essential amino acid transporter LAT1 (SLC7A5)View study →} — a theoretical competition point with other LAT1 substrates that has not been examined clinically.

Reproductive and genotoxicity data are absent for isolated alliin. No dedicated reproductive-toxicity, developmental or repeat-dose genotoxicity package has been characterised for the pure compound. Pregnancy and lactation decisions belong to the herb or product page where intake is decided, not to this molecule page.

Safety at a Glance

✅ Low / no toxicityAllergy risk

• Ordinary dietary exposure is well established. Alliin is roughly 1% of fresh garlic and is eaten in gram quantities worldwide with no toxicity signal attached to the molecule itself.
• Chemically inert as stored. Odourless and non-reactive until alliinase reaches it — the property that makes it a storage form rather than a toxin.
• The real caution is downstream. Bleeding risk, GI irritation and reflux belong to allicin and the garlic sulfides that alliin generates, not to alliin as swallowed.
• Allium allergy. Garlic’s characterised IgE allergen is the enzyme alliinase, not alliin; anyone allergic to Allium species should avoid garlic products regardless of alliin content.
• No isolate safety package. No human safety, interaction, reproductive or genotoxicity data exist for isolated alliin at supplemental doses — silence, not reassurance.

Dosage

There is no established human dose of isolated alliin, and no product is sold as pure alliin for a therapeutic purpose. What exists is a standardisation figure, which is a different kind of number.

Two caveats do most of the work here. First, an alliin figure is a ceiling, not a delivery: the same 18 mg of alliin can yield anywhere from near-zero to full allicin equivalence depending on the enzyme, the tablet and the meal ^{2,7,8Reference 2Lawson LD · 2018Allicin bioavailability and bioequivalence from garlic supplements and garlic foodsView study →Reference 7Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →Reference 8Lawson LD et al. · 2001Clinical trialAllicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterolView study →}. Second, the rodent doses in the pharmacology section above are not scaled-down-able to a garlic clove — a 500 mg/kg mouse dose has no dietary human equivalent.

These are research and standardisation figures, not a personal recommendation.

References

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