Methiin

Methiin at a Glance

Methiin is S-methyl-L-cysteine sulfoxide (SMCS or SMCSO) — a small, odourless, water-soluble sulfur amino acid, and the methyl analogue of alliin. The two molecules differ by a single substituent: alliin carries an allyl group on its sulfur, methiin carries a methyl. That one difference decides what each becomes when the plant is cut, and it is the whole story of this molecule.

In garlic, methiin is a minor player — roughly a tenth of the alliin content of a fresh clove ^{22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →}. Its real territory is the cabbage family, where it is the dominant cysteine sulfoxide and makes up around 1% of dry matter ^{4Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →}. That agricultural fact is why the molecule has a name in veterinary medicine that it does not have in human medicine: it is the “kale anaemia factor”, the compound behind a Heinz-body haemolytic anaemia in cattle and sheep grazing brassica forage ^{6,7Reference 6Smith RH · 1980Kale poisoning: the brassica anaemia factorView study →Reference 7Whittle PJ et al. · 1976Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root cropsView study →}.

Its human evidence is thin and honest about it. A 2025 scoping review found 21 studies, essentially all animal or in vitro, with about two-thirds reporting favourable anti-hyperglycaemic, cholesterol-lowering or antioxidant effects — and concluded that human studies are what is missing ^{3Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →}. What humans do have is measurement: methiin is absorbed and excreted largely intact, and is a validated urinary biomarker of cruciferous-vegetable intake ^{18,19,20,21Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →}.

Methiin vs. Alliin & Isoalliin

The most useful thing to know about methiin is where it sits in a family of four. Allium and Brassica plants store their pungency as stable, odourless S-alk(en)yl-L-cysteine sulfoxides (ACSOs), differing only in the group attached to sulfur ^{2Reference 2Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}:

Precursor	S-substituent	Cleavage product	Characteristic of
Methiin (MCSO)	methyl	methyl thiosulfinates (S-methyl methanethiosulfinate)	brassicas; present in every Allium surveyed
Alliin (ACSO)	allyl	allicin → diallyl disulfide, ajoene	garlic
Isoalliin (PECSO)	trans-1-propenyl	lachrymatory factor (the onion tear compound)	onion, chive, leek
Propiin (PCSO)	propyl	propyl thiosulfinates	onion and relatives

When tissue is crushed, the vacuolar enzyme alliinase (a C–S lyase, EC 4.4.1.4) cleaves whichever precursors are present into a sulfenic acid plus pyruvate and ammonia; the sulfenic acids condense into thiosulfinates ^{2,27Reference 2Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →Reference 27Keusgen M et al. · 2002Characterization of some Allium hybrids by aroma precursors, aroma profiles, and alliinase activityView study →}. Alliinase is not selective for alliin — it acts on methiin too. Crushing garlic therefore generates a small stream of methyl-containing thiosulfinates alongside the allyl chemistry that dominates. In brassicas, where the equivalent C–S lyase is a cystine lyase, the principal product from methiin is S-methyl methanethiosulfinate, which degrades on heating to dimethyl disulfide and dimethyl trisulfide — the cooked-cabbage smell ^{4Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →}.

This makes the cysteine-sulfoxide profile a chemotaxonomic fingerprint. A survey of the genus Allium found methiin present in every sample investigated, with the other three sulfoxides varying in a pattern that tracks both evolutionary lineage and culinary use ^{1Reference 1Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}. Two clear chemical types emerge: an “onion type” dominated by isoalliin (onion, chive, top onion) and a “garlic type” dominated by alliin (garlic, wild leek, sand leek). Pearl onion runs relatively high in methiin and leek relatively high in propiin; ramsons (Allium ursinum) carries a near-equal triple mix of methiin, alliin and isoalliin; and cultivated Chinese leek is the rare species where alliin and isoalliin co-dominate ^{1Reference 1Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}. Methiin-dominated Allium species are common in the genus — and, tellingly, are the ones people rarely eat ^{1Reference 1Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}. In crossbred Allium hybrids, methiin was the single most abundant sulfoxide across the set ^{27Reference 27Keusgen M et al. · 2002Characterization of some Allium hybrids by aroma precursors, aroma profiles, and alliinase activityView study →}.

Brassica Anaemia & the Kale-Poisoning Finding

This is the one place where methiin has a large, unambiguous literature, and it is veterinary, not human.

The syndrome. Severe haemolytic anaemia in cattle grazing kale was described in Europe in the 1930s; the causative link to a breakdown product of S-methylcysteine sulfoxide was only established some 35 years later ^{5Reference 5Stoewsand GS · 1995ReviewBioactive organosulfur phytochemicals in Brassica oleracea vegetables — a reviewView study →}. Affected animals develop Heinz bodies — clumps of oxidatively denatured haemoglobin — in their red cells, then anaemia, poor weight gain and, in bad cases, haemoglobinuria and death. It has been reported in cattle and sheep on kale, forage rape and cabbage ^{6,7,10Reference 6Smith RH · 1980Kale poisoning: the brassica anaemia factorView study →Reference 7Whittle PJ et al. · 1976Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root cropsView study →Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}.

The mechanism, and why it needs a rumen. Methiin itself is not the toxin. In the rumen, microbial fermentation converts it to dimethyl disulfide, an oxidant that attacks haemoglobin directly. Feeding dimethyl disulfide to birds reproduces the picture without any brassica at all: Heinz bodies appeared in most red cells by day 7, with lethargy and leg weakness by days 8–12 and significantly depressed haemoglobin and packed cell volume ^{9Reference 9Maxwell MH · 1981Production of Heinz body anaemia in the domestic fowl after ingestion of dimethyl disulphide: a haematological and ultrastructural studyView study →}. The rumen is what makes the conversion efficient. In sheep fed kale, the rumen degradation rate of SMCO was 1.2/h — twice the rate of the most rapidly fermented dietary carbohydrate and eight times the rate of water outflow from the rumen — so essentially all of it is fermented before it can leave ^{8Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →}. The kale in that study contained 11.4 g SMCO per kg dry matter, and lambs grazing it developed severe haemolytic anaemia with depressed live-weight gain over the first five weeks ^{8Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →}.

Two scoping boundaries matter here.

1. It is a forage-intake problem. The exposures are ruminants eating brassica as a sole or dominant diet, in kilogram quantities of dry matter, continuously for weeks. Field reports involve herds fed cabbage for five to six months a year over several years ^{10Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}. The syndrome in those herds was also multifactorial — the high sulphur load simultaneously raised methiin, depressed copper availability (plasma copper was severely reduced in every animal) and raised glucosinolate content, with goitre in the lambs alongside the anaemia ^{10Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}.
2. Human gut chemistry runs the other way. People have no rumen, and the human gut bacteria studied so far do not oxidise methiin toward disulfides — Escherichia isolates from human faeces reduce SMCSO to its deoxy analogue, S-methylcysteine ^{21Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →}. Consistent with that, the molecule shows up in human urine largely unchanged, which is precisely why it works as a dietary biomarker ^{18,19,20Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}.

The honest bottom line: brassica anaemia is a real, well-documented, mechanistically explained toxicity of this molecule — in grazing livestock. There is no evidence that eating cabbage, kale or garlic causes haemolytic anaemia in people, and no human case has been attributed to methiin. Plant breeders treat SMCO as a trait to select down in forage brassicas for exactly this reason ^{7Reference 7Whittle PJ et al. · 1976Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root cropsView study →}. Do not read the veterinary literature as a human dietary warning; equally, do not pretend the finding is not there.

What It's Used For

Everything below is preclinical. The applications cluster tightly — glucose, lipids, antioxidant defence — and largely trace to a small number of research groups.

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. No application here has human trial evidence; the ceiling is deliberately low across the whole table.

01Cholesterol and lipid loweringShaky32%Rat isolate studies from three independent groups, 1973–2007 ^{13,14,16Reference 13Kumari K · 2007AnimalLipid lowering effect of S-methyl cysteine sulfoxide from Allium cepa Linn in high cholesterol diet fed ratsView study →Reference 14Komatsu W et al. · 1998AnimalSuppression of hypercholesterolemia in hepatoma-bearing rats by cabbage extract and its component, S-methyl-L-cysteine sulfoxideView study →Reference 16Itokawa Y et al. · 1973AnimalEffect of S-methylcysteine sulfoxide, S-allylcysteine sulfoxide and related sulfur-containing amino acids on lipid metabolism of experimental hypercholesterolemic ratsView study →}

02Glycaemic controlShaky28%Alloxan-diabetic rat studies, largely one group ^{11,12,15Reference 11Kumari K et al. · 1995Antidiabetic and hypolipidemic effects of S-methyl cysteine sulfoxide isolated from Allium cepa LinnView study →Reference 12Kumari K · 2002AnimalAntidiabetic and antioxidant effects of S-methyl cysteine sulfoxide isolated from onions (Allium cepa Linn) as compared to standard drugs in alloxan diabetic ratsView study →Reference 15Sheela CG et al. · 1995AnimalAnti-diabetic effects of onion and garlic sulfoxide amino acids in ratsView study →}

03Antioxidant defenceShaky22%Rat lipid-peroxidation and antioxidant-enzyme markers ^{12Reference 12Kumari K · 2002AnimalAntidiabetic and antioxidant effects of S-methyl cysteine sulfoxide isolated from onions (Allium cepa Linn) as compared to standard drugs in alloxan diabetic ratsView study →}; review synthesis ^{3Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →}

04Antimutagenic activityShaky16%In vitro / mouse work on the cleavage product, not the parent ^{5,17Reference 5Stoewsand GS · 1995ReviewBioactive organosulfur phytochemicals in Brassica oleracea vegetables — a reviewView study →Reference 17Nakamura YK et al. · 1996S-methyl methanethiosulfonate, bio-antimutagen in homogenates of Cruciferae and Liliaceae vegetablesView study →}

Mechanisms

The final two rows are the load-bearing contrast. The same molecule is metabolised oxidatively in a rumen and reductively in a human colon, and that difference — not dose alone — is why kale poisoning is a livestock syndrome.

Pharmacokinetics

Methiin is a small, highly water-soluble, odourless amino acid, and the human data are unusually clear on one point: it survives to the urine largely intact. It was identified by NMR metabonomic profiling as one of the urinary markers distinguishing a high cruciferous diet (250 g/day each of broccoli and Brussels sprouts, 14 days) from a restricted one in 20 healthy men, along with three structurally related metabolites ^{18Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →}. Quantitatively, after an acute 20 g dose of black garlic, methiin was the single largest organosulfur compound recovered in 24-hour urine at 17,954 ± 6,040 nmol — more than a quarter of the 64,312 nmol total ^{19Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →}; after black onion it was again the largest, at 13.6 ± 3.9 µmol ^{20Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}. Both studies propose hepatic and renal N-acetylation as the main conjugation route for this compound class ^{19,20Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}.

Colonic handling is reductive: Escherichia isolated from human faecal communities converted SMCSO to its reduced analogue in vitro — the first demonstration of human gut bacteria acting on this compound ^{21Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →}. In-vitro digestion of fresh garlic found methiin stable through the gastric phase (concentrations were unchanged or rose while alliin fell sharply), but it was not detected after the intestinal phase, implying alkaline instability or absorption at that point ^{22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →}. No half-life, Cmax or absolute-bioavailability figure has been published for isolated methiin in any species — a real hole given the 200 mg/kg rat doses in the efficacy work.

Clinical Trials

There are none. No completed, ongoing or terminated human trial of isolated methiin appears in the literature; the 2025 scoping review searched five databases and found 21 original studies, none of them human interventions, and closed by stating that human studies are especially needed ^{3Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →}. The only human work is observational metabolite profiling ^{18,19,20Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}.

Last checked: July 2026.

Isolate vs. Plant Studies

Unusually for a phytochemical page, this molecule’s isolate-vs-plant balance is good in the animal literature and absent in the human one.

The rodent work is genuinely isolate work: SMCS purified from Allium cepa was dosed as a single compound at a defined mg/kg ^{11,12,13,15Reference 11Kumari K et al. · 1995Antidiabetic and hypolipidemic effects of S-methyl cysteine sulfoxide isolated from Allium cepa LinnView study →Reference 12Kumari K · 2002AnimalAntidiabetic and antioxidant effects of S-methyl cysteine sulfoxide isolated from onions (Allium cepa Linn) as compared to standard drugs in alloxan diabetic ratsView study →Reference 13Kumari K · 2007AnimalLipid lowering effect of S-methyl cysteine sulfoxide from Allium cepa Linn in high cholesterol diet fed ratsView study →Reference 15Sheela CG et al. · 1995AnimalAnti-diabetic effects of onion and garlic sulfoxide amino acids in ratsView study →}, and one study went further by testing cabbage extract and the isolated compound side by side and finding the isolate reproduced the extract’s effect on cholesterol metabolism ^{14Reference 14Komatsu W et al. · 1998AnimalSuppression of hypercholesterolemia in hepatoma-bearing rats by cabbage extract and its component, S-methyl-L-cysteine sulfoxideView study →}. That is a stronger attribution than most plant constituents can claim.

The human side inverts it. Every human observation is of whole food — broccoli and Brussels sprouts ^{18Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →}, black garlic ^{19Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →}, black onion ^{20Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}, broccoli leachate ^{21Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →} — and measures where the molecule goes, not what it does. So the transferability caveat here is not the usual “the plant was tested, not the molecule”; it is the sharper one that no human has ever been given this molecule to see what happens.

One further caution specific to garlic: because methiin is a minor constituent of garlic (about a tenth of alliin by concentration ^{22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →}), no garlic clinical outcome — cardiovascular, antimicrobial, lipid — should be credited to methiin. Garlic’s human evidence belongs to the allyl chemistry: alliin, allicin, diallyl disulfide, diallyl trisulfide, ajoene and S-allylcysteine.

Its Role in Plants

Methiin is a storage form of sulfur and a flavour precursor held in reserve. It sits inert and odourless in the cytoplasm while its cleaving enzyme is compartmentalised away in the vacuoles of vascular-bundle-sheath cells; only when tissue is damaged do the two meet, releasing reactive thiosulfinates in seconds ^{2Reference 2Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}. That is a classic two-component chemical defence: the plant stores an inactive precursor and a separate enzyme, and pays the metabolic cost of the toxin only when it is bitten.

Biosynthetically, methiin derives from methionine. Labelling work in garlic demonstrated the conversion of methionine to S-methyl-L-cysteine and then to its sulfoxide as early as 1963 ^{30Reference 30Sugii M et al. · 1963Biosynthesis of S-methyl-L-cysteine and S-methyl-L-cysteine sulfoxide from methionine in garlicView study →}. The broader picture developed since is that ACSOs form either by alkylation of the cysteine residue of glutathione, followed by cleavage of the glutamate and glycine residues and S-oxidation, or by direct alkylation of cysteine or a precursor such as O-acetylserine — with the route depending on tissue physiology, and with sulfur availability in the soil influencing which precursors accumulate ^{2Reference 2Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}. The final oxidation step is performed by a flavin-containing monooxygenase ^{2Reference 2Jones MG et al. · 2004Biosynthesis of the flavour precursors of onion and garlicView study →}.

In brassicas the enzymology differs slightly: two isoforms of cystine lyase correlate with the recovery of volatile sulfur compounds from crushed cabbage, and the primary product is S-methyl methanethiosulfinate rather than an allyl thiosulfinate ^{4Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →}.

Prevalence in Nature

Methiin is almost exclusive to two plant families — Amaryllidaceae (the alliums) and Brassicaceae (the crucifers) ^{3Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →}. Within them it is close to universal: a survey of the genus Allium detected methiin in every sample investigated, across all three evolutionary lineages ^{1Reference 1Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}, and cysteine sulfoxides with methiin most abundant were found in every one of a set of Allium interspecific hybrids ^{27Reference 27Keusgen M et al. · 2002Characterization of some Allium hybrids by aroma precursors, aroma profiles, and alliinase activityView study →}.

Source	Part	Methiin content	Basis
White cabbage (Brassica oleracea)	head	3.2–10.2 µmol/g fresh wt (~1% of dry matter)	LC quantification across 3 months of sampling 4Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →
Red cabbage (Brassica oleracea)	head	3.9–10.3 µmol/g fresh wt	as above 4Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →
Forage kale (Brassica oleracea)	whole crop	11.4 g/kg dry matter	the diet used in the sheep-anaemia study 8Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →
Garlic (Allium sativum)	fresh clove	4,425 nmol/g fresh wt (vs 41,706 for alliin)	UHPLC-HRMS profiling 22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →
Black garlic (aged A. sativum)	aged clove	243 nmol/g fresh wt	same study; ageing depletes it ~18-fold 22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →
Leek (Allium ampeloprasum)	white shaft	3–16 mg/g dry wt (isoalliin: 15–53)	31 cultivars by HPLC-MS/MS 23Reference 23Bernaert N et al. · 2012Influence of cultivar and harvest time on the amounts of isoalliin and methiin in leek (Allium ampeloprasum var. porrum)View study →
Leek (Allium ampeloprasum)	green leaves	1–10 mg/g dry wt (isoalliin: 9–45)	as above 23Reference 23Bernaert N et al. · 2012Influence of cultivar and harvest time on the amounts of isoalliin and methiin in leek (Allium ampeloprasum var. porrum)View study →

Two comparisons in that table carry the page. First, per gram of fresh food, cabbage carries as much methiin as garlic does or more — the molecule’s centre of gravity is the brassica aisle, not the allium one. Second, within garlic itself it is clearly subordinate to alliin, at roughly a tenth of the concentration ^{22Reference 22Moreno-Ortega A et al. · 2020In vitroBioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestionView study →}; the same ordering appears in the routine multi-analyte garlic assays, where methiin is quantified alongside alliin, isoalliin and cycloalliin as a standard minor component ^{24,25,26Reference 24Ichikawa M et al. · 2006Determination of seven organosulfur compounds in garlic by high-performance liquid chromatographyView study →Reference 25Montaño A et al. · 2011Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic (Allium sativum L.) grown in Andalusia, SpainView study →Reference 26Ferioli F et al. · 2022Non-volatile cysteine sulphoxides and volatile organosulphur compounds in cloves of garlic (Allium sativum L.) and elephant garlic (Allium ampeloprasum L.) local accessions from northern and central ItalyView study →}. Purple-type Spanish garlic cultivars carry the highest methiin (and the highest alliin) of the ecotypes surveyed, with the profile varying enough by location and cultivar to classify bulbs by origin at ~81–86% accuracy ^{25Reference 25Montaño A et al. · 2011Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic (Allium sativum L.) grown in Andalusia, SpainView study →}. Italian garlic and elephant garlic accessions likewise vary several-fold in total cysteine sulfoxide content, with elephant garlic (A. ampeloprasum) lower in total sulfoxides and proportionally lower in alliin than true garlic ^{26Reference 26Ferioli F et al. · 2022Non-volatile cysteine sulphoxides and volatile organosulphur compounds in cloves of garlic (Allium sativum L.) and elephant garlic (Allium ampeloprasum L.) local accessions from northern and central ItalyView study →}.

Across the wider genus, methiin-dominated species are common but are mostly the ones humans do not cultivate; the species that became vegetables and spices are the ones whose profiles shifted toward alliin or isoalliin ^{1Reference 1Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}. Beyond cabbage and kale, the compound is reported through the cultivated brassicas generally — broccoli, Brussels sprouts, cauliflower, turnip, swede and forage rape ^{4,5,7Reference 4Friedrich K et al. · 2022Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetablesView study →Reference 5Stoewsand GS · 1995ReviewBioactive organosulfur phytochemicals in Brassica oleracea vegetables — a reviewView study →Reference 7Whittle PJ et al. · 1976Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root cropsView study →}.

Discovery & Synthesis

First isolation, 1955–56. The compound was described as “a new free amino acid in cabbage” by R.L.M. Synge and J.C. Wood in a 1955 Biochemical Society communication, then characterised in full the following year as (+)-(S-methyl-L-cysteine S-oxide) in Biochemical Journal ^{28Reference 28Synge RLM · 1956Synge RLM, Wood JC (1956). (+)-(S-methyl-L-cysteine S-oxide) in cabbage. Biochemical Journal. https://pubmed.ncbi.nlm.nih.gov/13363835/View study →}. In the same year, C.J. Morris and J.F. Thompson independently reported the identification of (+)-S-methyl-L-cysteine sulfoxide in plants in the Journal of the American Chemical Society ^{29Reference 29Morris CJ · 1956The identification of (+)S-methyl-L-cysteine sulfoxide in plantsView study →}. The trivial name methiin came later, coined on the pattern set by alliin — Stoll and Seebeck’s name for the garlic precursor — with the “-iin” suffix marking the family of Allium cysteine sulfoxides.

Biosynthetic origin, 1963. Sugii, Nagasawa and Suzuki demonstrated the conversion of methionine to S-methyl-L-cysteine and to its sulfoxide in garlic, establishing methionine as the sulfur donor ^{30Reference 30Sugii M et al. · 1963Biosynthesis of S-methyl-L-cysteine and S-methyl-L-cysteine sulfoxide from methionine in garlicView study →}.

Synthesis. Methiin is prepared by S-methylation of L-cysteine to S-methyl-L-cysteine, followed by controlled oxidation of the sulfide to the sulfoxide; because oxidation creates a second stereocentre at sulfur, the naturally occurring (+)-(R_S) diastereomer must be separated from the mixture. There is no significant commercial market for isolated methiin — it is produced at analytical-standard scale for use as a chromatographic reference. Practical work with the molecule is analytical rather than preparative, and a mature toolkit exists: normal-phase HPLC for four sulfoxides and three γ-glutamyl peptides in a single garlic run ^{24Reference 24Ichikawa M et al. · 2006Determination of seven organosulfur compounds in garlic by high-performance liquid chromatographyView study →}, non-derivatised capillary electrophoresis for methiin and alliin (with simultaneous pyruvate detection as a blanching check) ^{31Reference 31Horie H · 2006Non-derivatized analysis of methiin and alliin in vegetables by capillary electrophoresisView study →}, and dansyl-chloride HPLC covering the whole class of S-substituted cysteine derivatives ^{32Reference 32Kubec R · 2009Chromatographic methods for determination of S-substituted cysteine derivatives — a comparative studyView study →}.

Patents: not yet researched.

Toxicity & Safety

Methiin carries a low flag on the human axis. It is a normal component of ordinary vegetables, it is absorbed and excreted largely unchanged, and no human toxicity has been attributed to it at any dietary intake ^{3,18Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →}. The rodent efficacy studies dosed 200 mg/kg/day for 45–60 days without reporting adverse effects ^{11,12,13Reference 11Kumari K et al. · 1995Antidiabetic and hypolipidemic effects of S-methyl cysteine sulfoxide isolated from Allium cepa LinnView study →Reference 12Kumari K · 2002AnimalAntidiabetic and antioxidant effects of S-methyl cysteine sulfoxide isolated from onions (Allium cepa Linn) as compared to standard drugs in alloxan diabetic ratsView study →Reference 13Kumari K · 2007AnimalLipid lowering effect of S-methyl cysteine sulfoxide from Allium cepa Linn in high cholesterol diet fed ratsView study →} — though none was designed as a toxicology study.

The badge is species-scoped on purpose. The molecule’s substantial toxicity literature is agricultural, and does not transfer to people (see Brassica Anaemia above): at forage-level intake, rumen fermentation converts methiin to dimethyl disulfide, producing Heinz-body haemolytic anaemia in cattle and sheep ^{6,8,9,10Reference 6Smith RH · 1980Kale poisoning: the brassica anaemia factorView study →Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →Reference 9Maxwell MH · 1981Production of Heinz body anaemia in the domestic fowl after ingestion of dimethyl disulphide: a haematological and ultrastructural studyView study →Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}. Humans have no rumen, and the human gut bacteria studied reduce rather than oxidise the molecule ^{21Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →}.

Safety at a Glance

✅ Low / no toxicity

• Ordinary dietary constituent. A normal component of cabbage, kale, broccoli, garlic, onion and leek, absorbed and excreted largely intact — it is used as a biomarker of vegetable intake, not a hazard.
• Ruminant forage toxicity (“kale poisoning”). In cattle and sheep grazing brassica forage, rumen fermentation to dimethyl disulfide causes Heinz-body haemolytic anaemia. A livestock management issue; not documented in humans.
• All efficacy evidence is preclinical. No human trial of isolated methiin exists; glycaemic, lipid and antioxidant claims come from alloxan-diabetic and hypercholesterolaemic rats at 200 mg/kg/day.
• No dedicated isolate toxicology. No genotoxicity, repeat-dose or reproductive-toxicity package has been published for pure methiin, and no drug-interaction or CYP data exist. Absence of findings is not evidence of safety.

Full Safety and Interactions Detail

The ruminant syndrome in full. Brassica (“kale”) anaemia was recognised in European cattle in the 1930s and traced to a breakdown product of S-methylcysteine sulfoxide roughly 35 years later ^{5,6Reference 5Stoewsand GS · 1995ReviewBioactive organosulfur phytochemicals in Brassica oleracea vegetables — a reviewView study →Reference 6Smith RH · 1980Kale poisoning: the brassica anaemia factorView study →}. SMCO content is high enough in forage brassicas to matter — 11.4 g/kg dry matter in the kale fed to lambs in the definitive New Zealand study — and rumen degradation is essentially complete because the fermentation rate (1.2/h) far outstrips rumen outflow ^{8Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →}. The resulting dimethyl disulfide is directly haemolytic: fed alone to fowl, it produced Heinz bodies in most erythrocytes within seven days, with falling haemoglobin and packed cell volume ^{9Reference 9Maxwell MH · 1981Production of Heinz body anaemia in the domestic fowl after ingestion of dimethyl disulphide: a haematological and ultrastructural studyView study →}. Field cases in cabbage-fed Jersey cattle and Dorper sheep showed Heinz-body haemolytic anaemia together with severely depressed plasma copper and goitre — a multifactorial sulfur-overload picture in which methiin is one contributor alongside impaired copper availability and glucosinolates ^{10Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}. SMCO content varies widely between brassica forage and root crops and is treated as a breeding target to be selected downward ^{7Reference 7Whittle PJ et al. · 1976Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root cropsView study →}.

Why this does not extend to humans at culinary intake. Three independent reasons. (i) Anatomy — the conversion to dimethyl disulfide is a rumen fermentation; the human gut bacteria examined convert SMCSO reductively to S-methylcysteine instead ^{21Reference 21Kellingray L et al. · 2021In vitroEffects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiomeView study →}. (ii) Disposition — in humans the intact molecule appears in urine, which is the basis for its use as a cruciferous-intake biomarker ^{18,19,20Reference 18Edmands WM et al. · 2011Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profilingView study →Reference 19Moreno-Ortega A et al. · 2023Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humansView study →Reference 20Moreno-Ortega A et al. · 2023Urinary excretion of organosulfur compounds after acute ingestion of black onionView study →}. (iii) Dose and pattern — the toxic exposures are ruminants eating brassica as the dominant diet, in kilograms of dry matter daily for weeks to months ^{8,10Reference 8Barry TN · 1985Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxideView study →Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}, not portions of cabbage in a mixed diet. No human case of methiin-attributed haemolysis has been reported.

What is genuinely unknown. There is no published genotoxicity, repeat-dose or developmental-toxicity study on isolated methiin, no CYP or transporter interaction data, and no human tolerability data at supraphysiological doses. The scoping review that assembled the whole evidence base found 21 studies and no human intervention among them ^{3Reference 3Hill CR et al. · 2025ReviewS-methyl cysteine sulfoxide and its potential role in human health: a scoping reviewView study →}. Anyone concentrating this molecule beyond food levels would be doing so without a safety dataset.

Reproductive note (prose only). No reproductive or developmental toxicity study exists for isolated methiin. The one veterinary report touching reproduction — reduced conception rates and abortions in a cabbage-fed dairy herd — attributed the picture to overall sulfur load, copper depletion and glucosinolates rather than to methiin specifically ^{10Reference 10Taljaard TL · 1993Cabbage poisoning in ruminantsView study →}. Pregnancy and lactation decisions belong to the herb or product page where intake is decided, not to this molecule page.

Dosage

There is no human dose for methiin. No clinical trial has administered the isolated compound, so nothing below is a dosing guide — it is the exposure record, given so the animal figures can be read against real-world intake.

The gap between the two halves of that table is the point: the rat studies used 200 mg per kilogram of body weight per day, which for a 70 kg adult would be roughly 14 g/day — two orders of magnitude beyond what any plausible vegetable intake supplies. No supplement is standardised to methiin, and garlic products are standardised to alliin or allicin yield, not to this molecule. These are doses studied in research and are not a personal recommendation.

References

1. Fritsch RM, Keusgen M (2006). Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae). Phytochemistry. https://pubmed.ncbi.nlm.nih.gov/16626766/
2. Jones MG, et al. (2004). Biosynthesis of the flavour precursors of onion and garlic. Journal of Experimental Botany. https://pubmed.ncbi.nlm.nih.gov/15234988/
3. Hill CR, et al. (2025). S-methyl cysteine sulfoxide and its potential role in human health: a scoping review. Critical Reviews in Food Science and Nutrition. https://pubmed.ncbi.nlm.nih.gov/37819533/
4. Friedrich K, et al. (2022). Formation of volatile sulfur compounds and S-methyl-L-cysteine sulfoxide in Brassica oleracea vegetables. Food Chemistry. https://pubmed.ncbi.nlm.nih.gov/35247727/
5. Stoewsand GS (1995). Bioactive organosulfur phytochemicals in Brassica oleracea vegetables — a review. Food and Chemical Toxicology. https://pubmed.ncbi.nlm.nih.gov/7797181/
6. Smith RH (1980). Kale poisoning: the brassica anaemia factor. Veterinary Record. https://pubmed.ncbi.nlm.nih.gov/7434535/
7. Whittle PJ, Smith RH, McIntosh A (1976). Estimation of S-methylcysteine sulphoxide (kale anaemia factor) and its distribution among brassica forage and root crops. Journal of the Science of Food and Agriculture. https://pubmed.ncbi.nlm.nih.gov/957611/
8. Barry TN, Manley TR (1985). Responses to oral methionine supplementation in sheep fed on kale (Brassica oleracea) diets containing S-methyl-L-cysteine sulphoxide. British Journal of Nutrition. https://pubmed.ncbi.nlm.nih.gov/3870695/
9. Maxwell MH (1981). Production of Heinz body anaemia in the domestic fowl after ingestion of dimethyl disulphide: a haematological and ultrastructural study. Research in Veterinary Science. https://pubmed.ncbi.nlm.nih.gov/7255913/
10. Taljaard TL (1993). Cabbage poisoning in ruminants. Journal of the South African Veterinary Association. https://pubmed.ncbi.nlm.nih.gov/8410951/
11. Kumari K, Mathew BC, Augusti KT (1995). Antidiabetic and hypolipidemic effects of S-methyl cysteine sulfoxide isolated from Allium cepa Linn. Indian Journal of Biochemistry & Biophysics. https://pubmed.ncbi.nlm.nih.gov/7665195/
12. Kumari K, Augusti KT (2002). Antidiabetic and antioxidant effects of S-methyl cysteine sulfoxide isolated from onions (Allium cepa Linn) as compared to standard drugs in alloxan diabetic rats. Indian Journal of Experimental Biology. https://pubmed.ncbi.nlm.nih.gov/12587728/
13. Kumari K, Augusti KT (2007). Lipid lowering effect of S-methyl cysteine sulfoxide from Allium cepa Linn in high cholesterol diet fed rats. Journal of Ethnopharmacology. https://pubmed.ncbi.nlm.nih.gov/16987625/
14. Komatsu W, Miura Y, Yagasaki K (1998). Suppression of hypercholesterolemia in hepatoma-bearing rats by cabbage extract and its component, S-methyl-L-cysteine sulfoxide. Lipids. https://pubmed.ncbi.nlm.nih.gov/9625597/
15. Sheela CG, Kumud K, Augusti KT (1995). Anti-diabetic effects of onion and garlic sulfoxide amino acids in rats. Planta Medica. https://pubmed.ncbi.nlm.nih.gov/7480182/
16. Itokawa Y, et al. (1973). Effect of S-methylcysteine sulfoxide, S-allylcysteine sulfoxide and related sulfur-containing amino acids on lipid metabolism of experimental hypercholesterolemic rats. Journal of Nutrition. https://pubmed.ncbi.nlm.nih.gov/4682454/
17. Nakamura YK, et al. (1996). S-methyl methanethiosulfonate, bio-antimutagen in homogenates of Cruciferae and Liliaceae vegetables. Bioscience, Biotechnology, and Biochemistry. https://pubmed.ncbi.nlm.nih.gov/8987591/
18. Edmands WM, et al. (2011). Identification of human urinary biomarkers of cruciferous vegetable consumption by metabonomic profiling. Journal of Proteome Research. https://pubmed.ncbi.nlm.nih.gov/21770373/
19. Moreno-Ortega A, et al. (2023). Bioavailability of organosulfur compounds after the ingestion of black garlic by healthy humans. Antioxidants. https://pubmed.ncbi.nlm.nih.gov/37107300/
20. Moreno-Ortega A, et al. (2023). Urinary excretion of organosulfur compounds after acute ingestion of black onion. Food & Function. https://pubmed.ncbi.nlm.nih.gov/37194921/
21. Kellingray L, et al. (2021). Effects of in vitro metabolism of a broccoli leachate, glucosinolates and S-methylcysteine sulphoxide on the human faecal microbiome. European Journal of Nutrition. https://pubmed.ncbi.nlm.nih.gov/33067661/
22. Moreno-Ortega A, et al. (2020). Bioaccessibility of bioactive compounds of “fresh garlic” and “black garlic” through in vitro gastrointestinal digestion. Foods. https://pubmed.ncbi.nlm.nih.gov/33142731/
23. Bernaert N, et al. (2012). Influence of cultivar and harvest time on the amounts of isoalliin and methiin in leek (Allium ampeloprasum var. porrum). Journal of Agricultural and Food Chemistry. https://pubmed.ncbi.nlm.nih.gov/23020262/
24. Ichikawa M, et al. (2006). Determination of seven organosulfur compounds in garlic by high-performance liquid chromatography. Journal of Agricultural and Food Chemistry. https://pubmed.ncbi.nlm.nih.gov/16506797/
25. Montaño A, et al. (2011). Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic (Allium sativum L.) grown in Andalusia, Spain. Journal of Agricultural and Food Chemistry. https://pubmed.ncbi.nlm.nih.gov/21247176/
26. Ferioli F, Giambanelli E, D’Antuono LF (2022). Non-volatile cysteine sulphoxides and volatile organosulphur compounds in cloves of garlic (Allium sativum L.) and elephant garlic (Allium ampeloprasum L.) local accessions from northern and central Italy. Journal of the Science of Food and Agriculture. https://pubmed.ncbi.nlm.nih.gov/35211974/
27. Keusgen M, et al. (2002). Characterization of some Allium hybrids by aroma precursors, aroma profiles, and alliinase activity. Journal of Agricultural and Food Chemistry. https://pubmed.ncbi.nlm.nih.gov/11982415/
28. Synge RLM, Wood JC (1956). (+)-(S-methyl-L-cysteine S-oxide) in cabbage. Biochemical Journal. https://pubmed.ncbi.nlm.nih.gov/13363835/
29. Morris CJ, Thompson JF (1956). The identification of (+)S-methyl-L-cysteine sulfoxide in plants. Journal of the American Chemical Society. https://doi.org/10.1021/ja01589a028
30. Sugii M, Nagasawa S, Suzuki T (1963). Biosynthesis of S-methyl-L-cysteine and S-methyl-L-cysteine sulfoxide from methionine in garlic. Chemical and Pharmaceutical Bulletin. https://doi.org/10.1248/cpb.11.135
31. Horie H, Yamashita K (2006). Non-derivatized analysis of methiin and alliin in vegetables by capillary electrophoresis. Journal of Chromatography A. https://pubmed.ncbi.nlm.nih.gov/16999975/
32. Kubec R, Dadáková E (2009). Chromatographic methods for determination of S-substituted cysteine derivatives — a comparative study. Journal of Chromatography A. https://pubmed.ncbi.nlm.nih.gov/19733357/

View all 32 references▾