Last reviewed: 11 May 2026
The classic savoury flavour enhancer. Naturally present in tomatoes, parmesan and soy sauce.
E621 is monosodium glutamate — the sodium salt of glutamic acid, the most abundant excitatory amino acid in the human body and the molecule responsible for the umami taste. It is added to savoury snacks, ready meals, soups and stocks to deepen savoury flavour and reduce the salt required for the same taste impact. EFSA's 2017 re-evaluation set a group ADI of 30mg/kg body weight per day across the glutamate-additive family. The historical concept of "Chinese Restaurant Syndrome" — a 1968 letter (not a study) that triggered decades of MSG fear — has not been reproducibly demonstrated in double-blind challenge trials, and the clinical literature has largely moved past it. A small minority of people report consistent symptoms after MSG-rich meals; the mechanism is unclear and the effect is not predicted by standard MSG-challenge methodology.
Monosodium glutamate is the sodium salt of glutamic acid (glutamate), a non-essential amino acid that is one of the 20 building blocks of human protein. Glutamate is also the most abundant excitatory neurotransmitter in the central nervous system — but in food, what matters is its function as a taste molecule. Glutamate binds to T1R1/T1R3 umami receptors on the tongue and signals "savoury" or "brothy" in the same way that sucrose binds sweet receptors and signals "sweet". When you add MSG to a soup, you are not adding a flavour; you are amplifying the perception of the savoury notes already present.
The molecule is chemically identical whether it is fermentation-derived (the modern production route, using Corynebacterium glutamicum on sugar substrate) or extracted from a natural source. The free glutamate in a ripe tomato, a piece of parmesan, dried mushrooms, soy sauce, anchovies or kombu is the same molecule. Breast milk is one of the highest natural sources of free glutamate, at around 20mg per 100ml — orders of magnitude above the amount the human body produces from any individual added portion of MSG-seasoned food.
UK labelling rules require MSG to be declared by name or E-number — it cannot be hidden under "natural flavouring". Related glutamate additives in the same E-number family (E620 glutamic acid, E622 monopotassium glutamate, E623 calcium diglutamate, E624 monoammonium glutamate, E625 magnesium diglutamate) appear less commonly but are governed by the same group ADI.
EFSA reviewed the glutamate additive family in 2017 (EFSA Journal 2017;15(7):4910) and set a group ADI of 30mg/kg body weight per day — a numerical figure replacing the previous "not specified" status. The opinion noted that estimated dietary intakes for certain population groups (notably high consumers of seasoned snacks and ready meals) could approach or exceed the ADI when added-glutamate exposure was combined with naturally occurring free glutamate in food. The opinion did not identify carcinogenicity, genotoxicity or reproductive concerns at the levels permitted.
The term originates in a 1968 letter to the New England Journal of Medicine by Dr Robert Ho Man Kwok, describing symptoms (numbness at the back of the neck, weakness, palpitations) experienced after eating in Chinese-American restaurants. The letter speculated about possible causes including MSG, sodium and cooking wine. The phrase "Chinese Restaurant Syndrome" entered the literature and triggered decades of public concern centred on MSG. The published clinical-trial response across the following half-century — including double-blind, placebo-controlled challenge studies — has not reproducibly demonstrated a consistent MSG-specific symptom complex in the general population at typical dietary doses. When people self-identify as MSG-sensitive and are then challenged with MSG and placebo without knowing which they have received, the responses are not consistent with a specific MSG sensitivity. The cuisine-specific framing of the term has been criticised as stigmatising and the original case-series description has not been replicated in controlled studies.
The honest residual position: a small minority of people report consistent symptoms (headache, flushing, sweating, chest tightness) after meals high in added glutamate, and a smaller minority have reproducible symptoms in some challenge studies particularly at high doses (3g or more on an empty stomach). The mechanism is unclear. If you have personally observed a consistent reaction to specific MSG-rich foods, logging the pattern — what you ate, what dose, what symptom — and taking the record to a GP is more useful than relying on the published trial literature, which has been built around populations without a confirmed individual reactor pattern.
Glutamate's role as a central-nervous-system excitatory neurotransmitter raised the concern that dietary glutamate might over-stimulate neurons (the "excitotoxicity" hypothesis). The blood-brain barrier substantially restricts the passage of glutamate from systemic circulation into brain tissue, and the gut metabolises a large proportion of dietary glutamate before it reaches the systemic circulation at all. Animal models that have produced neurological effects with glutamate did so by injection (bypassing the gut and barrier) at doses far exceeding any plausible dietary route. The dietary-route excitotoxicity hypothesis has not been supported in human-relevant exposure studies, and EFSA's 2017 opinion did not identify a neurological concern at the ADI.
MSG contains about 12% sodium by weight, compared with about 39% in table salt. When MSG is used to partially replace salt in a recipe — adding equivalent savoury impact at lower total sodium — it can lower the sodium content of the food. In practice, MSG is more often used alongside salt rather than as a replacement, so a snack with both adds sodium from both sources; the package nutrition declaration is the practical signal for total sodium.
MSG appears in the ingredients list as monosodium glutamate, flavour enhancer (E621), or E621. Related glutamate-family additives (E620–E625) sit in the same regulatory bracket. You will also see the function declared — UK labelling requires a category designation such as "flavour enhancer" alongside the specific name or E-number. If you are sensitive to high-glutamate meals (whether the mechanism is added MSG specifically or the broader picture of fast-absorbed sodium and savoury fat), the higher-yield categories to look at are stock cubes, packet soups, savoury snacks and ready meals — the four food groups where added glutamate intake stacks up fastest in a UK diet.
UK and EU: authorised as E621 with a group ADI of 30mg/kg body weight per day (EFSA 2017 re-evaluation), across the E620–E625 glutamate-additive family. Listed in Annex II of Regulation (EC) No 1333/2008.
United States: FDA Generally Recognized as Safe (GRAS). FDA labelling requires "monosodium glutamate" to be named on the ingredients list and does not permit hiding it under "natural flavour".
Australia/New Zealand: authorised under additive code 621.
Scan any UK pack in NutraSafe and we surface MSG and the wider glutamate family alongside the rest of the additives, so if you are tracking suspected dietary triggers you have the data point in your log.
Get NutraSafe on the App StoreLast updated: 11 May 2026
Free to log up to 25 foods/day · NutraSafe Pro £3.99/month for AI Coach, allergen warning detail and full reaction history.