ZHICHENG BIO
L-ergothioneine: the authorised novel food is an enantiomer, and your assay probably cannot see it
A regulatory name, a transporter, and an assay that is blind to the difference between them.
The short answer
The substance authorised for sale in the European Union is designated "L-ergothioneine" — the enantiomer is written into the law, not just the marketing. Ergothioneine does not enter cells passively at dietary concentrations; it is accumulated by a dedicated carrier, the ergothioneine transporter (ETT/OCTN1, gene SLC22A4), for which the measured affinity is a Km of about 21 µmol/L. A conventional reversed-phase HPLC assay reporting "ergothioneine ≥ 98%" measures the molecule and is blind to which enantiomer it is looking at, because enantiomers co-elute on achiral stationary phases. If enantiomeric purity is not a separate line on the specification, it has not been specified.
- The molecule
- L-ergothioneine, C9H15N3O2S, Mr 229.30, CAS 497-30-3
- The transporter
- ETT / OCTN1, gene SLC22A4. Saturation analysis gives Km ≈ 21 µmol/L; transport of the previously presumed substrates TEA and carnitine is negligible by comparison (PNAS 2005;102(14):5256–5261, PMID 15795384, doi 10.1073/pnas.0408624102)
- The legal name
- Commission Implementing Regulation (EU) 2017/2470, as amended by (EU) 2018/462: "The designation of the novel food on the labelling of the foodstuffs containing it shall be 'L-ergothioneine'"
- The authorised levels
- Food supplements 30 mg/day (general population) and 20 mg/day (children over 3); alcohol-free beverages and milk-based drinks 0.025 g/kg; fresh milk products 0.040 g/kg; cereal bars 0.2 g/kg; chocolate confectionery 0.25 g/kg
- The toxicology behind it
- NOAEL 800 mg/kg bw per day from two subchronic rat studies; margins of safety 470 for adults and 216 for children over 3 (EFSA Journal 2016;14(11):4629)
- What we could not verify
- We did not locate a published transport measurement for D-ergothioneine specifically. The argument on this page rests on the legal designation and on the assay gap, not on a claimed Km for the D-form
Why the transporter makes the enantiomer question a real one
Most dietary compounds get into cells by diffusing. Ergothioneine does not, at the concentrations food delivers — it is a zwitterion at physiological pH and it depends on a carrier. That carrier was identified in 2005: SLC22A4, previously catalogued as an organic cation transporter (OCTN1) on the assumption that its job was moving things like tetraethylammonium. The 2005 work showed that tetraethylammonium and carnitine transport are negligible next to ergothioneine, and that saturation analysis of the uptake rate gives a Km of about 21 µmol/L — high affinity, for a transport protein.
That matters commercially for a reason that has nothing to do with efficacy claims. A molecule whose distribution depends on a specific binding site is a molecule whose stereochemistry is a functional variable rather than a cosmetic one, because a binding site is a chiral environment and recognises one hand at a time. This is the general principle behind why pharmacopoeias specify enantiomers at all.
We should be precise about what has and has not been published. The affinity figure above is for the L-form, which is the naturally occurring one and the one the transporter was characterised with. We were not able to locate a peer-reviewed measurement of D-ergothioneine transport by ETT, and we are not going to state one. What we can state is the regulatory position, which is unambiguous, and the analytical position, which is where buyers actually get caught.
The regulation names the enantiomer
Ergothioneine reached the EU market as a novel food. EFSA's 2016 opinion assessed synthetic l-ergothioneine, deriving a NOAEL of 800 mg/kg body weight per day from two subchronic rat studies and concluding that the margins of safety — 470 for adults, 216 for children over three — were sufficient at the proposed intakes. The authorisation that followed, and the extension of use in Commission Implementing Regulation (EU) 2018/462 amending the Union list at (EU) 2017/2470, sets the conditions: 30 mg/day in food supplements for the general population, 20 mg/day for children over three, and per-category limits of 0.025 g/kg in alcohol-free beverages and milk-based drinks, 0.040 g/kg in fresh milk products, 0.2 g/kg in cereal bars and 0.25 g/kg in chocolate confectionery.
The labelling clause is the part worth quoting to a supplier: the designation on the label "shall be 'L-ergothioneine'". That is the name of the authorised substance. A racemate is not that substance — it is that substance diluted with something the Union list does not cover, and the burden of demonstrating otherwise sits with whoever wants to sell it.
This is a rare and useful situation for a buyer. Enantiomeric purity is normally a technical argument you have to win on chemistry. Here it is a compliance question with a regulation number attached, and compliance questions get answered faster.
Why your certificate of analysis will not tell you
Enantiomers have identical mass, identical UV spectra, identical solubility in achiral solvents and identical retention on an achiral stationary phase. A standard reversed-phase C18 HPLC method — which is what almost every commercial ergothioneine COA is based on — separates ergothioneine from everything that is not ergothioneine, and then reports a single number. The number is total ergothioneine. It is the correct answer to a question you did not mean to ask.
The same blindness applies to the other routine tests. Assay by titration, if used, counts protons. Loss on drying counts water. Heavy metals count metals. Nothing in the standard panel is chiral, so nothing in the standard panel constrains the enantiomeric composition. This is not a defect in the methods; it is a consequence of choosing methods that were never asked to look.
Whether a given batch is enantiomerically pure is a question about its route. Fermentation and enzymatic routes build the molecule with the biological stereochemistry and give the L-form. Chemical synthesis may or may not be stereoselective depending on the route and on whether a resolution step is included — which is a question about that specific process, and not something you can infer from the words "synthetic" or "natural" on a datasheet. Ask about the route, then verify independently.
What to put on the specification
Three lines rather than one. First, identity: "L-ergothioneine, C9H15N3O2S, CAS 497-30-3" — the name carries the stereochemistry and matches the EU designation. Second, assay: "≥ 98.0% by HPLC, on the dried basis", which is the total. Third, and this is the one usually missing: "Enantiomeric purity: D-ergothioneine ≤ x%, by chiral HPLC", with the chiral stationary phase named in the method and the chromatogram supplied.
Add specific optical rotation as a cheap corroborating check — but require the supplier to state the concentration, solvent and temperature alongside the value, because a rotation figure without those three is not reproducible and therefore not a specification. We deliberately do not quote a reference rotation value here; ask for the supplier's, ask which literature value they are working to, and confirm both against your own laboratory.
Then ask for the manufacturing route in writing, and keep it in the supplier file. If the route changes, the enantiomeric purity is the first thing that can change with it, and a route change is exactly the kind of event that does not generate a new certificate.
The same three-line structure fixes the same problem in theanine, where the failure has actually been measured in commercial products, and in L-carnitine, where the pharmacopoeial assay is a titration and cannot see chirality at all.
Frequently asked
Is D-ergothioneine authorised for sale in the EU?
The Union list of novel foods designates the authorised substance as 'L-ergothioneine' and requires that name on the label of foods containing it (Commission Implementing Regulation (EU) 2017/2470, as amended by (EU) 2018/462). A racemic material is therefore not the authorised novel food as designated. This page states the regulatory position; it does not make any claim about the biological behaviour of the D-form, for which we did not locate published transport data.
How much L-ergothioneine may a food supplement contain in the EU?
30 mg per day for the general population and 20 mg per day for children over three years, under the Union list conditions of use. Other authorised categories carry per-kilogram limits: 0.025 g/kg in alcohol-free beverages and milk-based drinks, 0.040 g/kg in fresh milk products, 0.2 g/kg in cereal bars and 0.25 g/kg in chocolate confectionery.
Can a normal HPLC assay detect D-ergothioneine?
No. Enantiomers have identical retention on achiral stationary phases, so a conventional reversed-phase method reports total ergothioneine regardless of enantiomeric composition. Detecting the D-form requires a chiral method — a chiral stationary phase, or derivatisation with a chiral reagent — specified as a separate test with its own limit.
What is the ergothioneine transporter and why does it matter for sourcing?
ETT, encoded by SLC22A4 and previously catalogued as OCTN1, is the carrier that accumulates ergothioneine in cells; saturation analysis gives a Km of about 21 µmol/L (PNAS 2005;102(14):5256-5261, PMID 15795384). For a buyer the relevance is that a compound whose distribution depends on a specific binding site is one whose stereochemistry is a functional variable, which is why the specification should constrain it explicitly.
This page describes raw-material sourcing and specification. It is not a health claim and not medical advice. Last updated 2026-08-07.