Sodium metabisulphite
A sulphite preservative that stops browning and bacterial growth in dried fruit, wine, and seafood. Must be declared on UK labels when present above 10mg/kg.
Sulphites trigger asthma attacks and breathing difficulties in sensitive people, particularly those with asthma. At high dietary intake, EFSA found the margin between real-world exposure and doses that impaired nerve function was too narrow to set a safe daily limit.
What is it?
Sodium metabisulphite is an inorganic sodium salt of metabisulphurous acid. In food it releases sulphur dioxide, the active preserving agent. It is synthetically manufactured and carries no animal-derived ingredients.
What does it do?
Releases sulphur dioxide, which inhibits bacteria, yeasts and moulds and blocks the enzymes responsible for enzymatic browning. It also acts as an antioxidant, preventing colour loss in dried and processed fruit and slowing oxidative spoilage in wine and beer.
Where you will see it
Common in dried apricots and other bright-coloured dried fruit, wine, cider and beer, pickled onions, prawns and other crustaceans (to prevent shell darkening after harvest), frozen and dehydrated potato products, and some baked goods. On a UK label it appears as 'sodium metabisulphite', 'sodium metabisulfite', or E223. Sulphur dioxide and sulphites collectively appear as E220-E228 and must be listed in bold when present above 10mg/kg.
What the science says
Asthma and respiratory reactions
Sulphites release sulphur dioxide gas in the stomach, which can rise into the airway and trigger bronchoconstriction, particularly in people whose airways are already sensitive. The mechanism is not fully resolved but may involve cholinergic nerve reflexes, histamine release, and leukotriene and prostaglandin pathways. Sensitivity in the general population is thought to be below 2%, but rises to between 4% and 10% in people with asthma, and higher still in those with severe or steroid-dependent asthma.
Sulphite sensitivity affects fewer than 2% of the general population but rises to 5-13% in people with asthma, with steroid-dependent asthmatics at greatest risk.
Sulphite-induced bronchoconstriction is thought to involve cholinergic reflex activation, histamine release from mast cells, and leukotriene and prostaglandin pathways; the precise mechanism remains unresolved.
Prevalence of sulphite sensitivity in steroid-dependent asthmatic patients has been estimated at approximately 8.4%, and at around 20% in steroid-dependent asthmatic children.
Declarable allergen under UK food law
Sulphur dioxide and sulphites form one of the 14 major allergens recognised in UK food law. Manufacturers must declare them in bold on the ingredient list whenever the total sulphite content exceeds 10mg/kg or 10mg/L in the product as sold. This threshold is unique among the 14 allergens in having a quantity below which declaration is not required.
Sulphur dioxide and sulphites (E220-E228) are listed as one of the 14 major food allergens in UK law and must be emphasised on labels when present above 10mg/kg or 10mg/L.
EFSA 2022: safety concern at high dietary intakes, no ADI can be set
In 2022 EFSA completed a follow-up review of sulphites that had been triggered by gaps in the 2016 assessment. The panel found that industry did not supply the missing toxicological studies. Using a margin of exposure approach based on a neurotoxicity endpoint in animal tests, EFSA concluded that high consumers across most age groups had margins below the level considered safe. The panel withdrew the temporary acceptable daily intake set in 2016, concluding the toxicity database was inadequate to set any ADI at all.
The EFSA panel withdrew the 2016 temporary group ADI of 0.7mg SO2 equivalents/kg body weight/day, concluding the available toxicity database was inadequate to derive an ADI.
The margins of exposure calculated from dietary intake data were below 80 (the threshold indicating a safety concern) for high consumers in all population groups except adolescents, with children aged 3-10 exceeding the reference point by up to 12.5% and adults by up to 60%.
The neurotoxicity reference point used was a benchmark dose lower confidence limit of 38mg SO2 equivalents/kg body weight/day, based on prolonged visual evoked potential latency in animal studies, indicating delayed nerve responses.
Thiamine (vitamin B1) degradation
Sulphites chemically break down thiamine (vitamin B1), reducing the nutritional content of foods they preserve. This effect is well established and is why sulphites are not permitted in foods considered an important source of thiamine, such as meat products in some markets. The practical significance in a varied diet depends on how much thiamine comes from sulphite-treated foods.
Sodium bisulphite destroys thiamine in solutions at rates strongly dependent on pH; at pH 6.5 only 3-8% of added thiamine remained after 24 hours at room temperature.
The FDA notes that sulphite preservatives are known to destroy vitamin B1 and are not permitted in foods recognised as important sources of that nutrient.
Where it stands with the regulators
Who should be careful
People with asthma, particularly those on steroid treatment or with severe disease, face the highest risk of sulphite-triggered breathing reactions. People with known sulphite sensitivity should also avoid them. Look for E220-E228, 'sulphur dioxide', 'sulphites', or 'sulphiting agents' in the ingredients list, printed in bold.
The honest read
There are two distinct issues with sulphites, and they affect different people. The respiratory risk is well established clinically: sulphites trigger genuine asthma attacks in a meaningful minority of people who have asthma. That is not a theoretical concern. The broader population-level safety question is less resolved. EFSA's 2022 review found that high consumers exceed the dose associated with early signs of nervous system impairment in animal tests, but industry did not conduct the human studies needed to set a firm safe limit. The panel could not set an ADI at all. That does not mean everyone eating dried apricots is at risk. The neurotoxicity reference point comes from animal data, and real-world exposure for most people is far below high-consumer levels. But the science is genuinely unfinished: regulators have flagged a concern they cannot yet quantify.
Related additives
Common questions
Is E223 banned in the UK?
No, sodium metabisulphite (E223) is approved for use in the UK under the assimilated version of EU Regulation 1333/2008, authorised by the UK Food Standards Agency.
Can E223 trigger an asthma attack?
Yes. Sulphites including sodium metabisulphite are a recognised trigger for asthma attacks. Sensitivity affects fewer than 2% of the general population but rises to between 5% and 13% in people with asthma, with steroid-dependent asthmatics most at risk. Reactions can include wheezing, chest tightness, coughing, and in rare cases severe bronchospasm.
What foods contain E223?
Sodium metabisulphite is most commonly found in dried apricots and other brightly coloured dried fruit, wine, cider, beer, prawns and other crustaceans, frozen potato products, and some pickled foods. It must be listed in bold on pre-packed UK food labels when present above 10mg/kg.
Is E223 vegan?
Yes. Sodium metabisulphite is a synthetically produced inorganic compound with no animal-derived ingredients. It is accepted as vegan, vegetarian, halal, and kosher.
Sources
- UK FSA: Authorised Regulated Food Products, E-223 Sodium Metabisulphite
- EFSA Journal 2016;14(4):4438 - Scientific Opinion on the re-evaluation of sulfur dioxide (E220) and sulphites (E221-E228) as food additives
- EFSA Journal 2022;20(12):7594 - Follow-up of the re-evaluation of sulfur dioxide (E220) and sulphites (E221-E228)
- PubMed - Follow-up of the re-evaluation of sulfur dioxide and sulphites (EFSA 2022)
- PMC full text - EFSA 2022 sulphites follow-up review
- Allergy UK: Sulphites and Airway Symptoms factsheet
- Anaphylaxis UK: Sulphites fact sheet
- UK FSA: Food allergen labelling technical guidance: the 14 allergens
- American Journal of Health-System Pharmacy: Thiamine destruction by sodium bisulfite in infusion solutions (1981)
- Food Navigator: Dietary intake of sulfites 'could be a safety concern': EFSA (2022)
- Gastroenterol Hepatol Bed Bench 2012;5(1):16-23 - Adverse reactions to the sulphite additives
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