Riboflavin-5'-phosphate
The sodium salt form of vitamin B2, used as a yellow-orange food colouring in drinks and processed foods.
Judged unlikely to be of concern at authorised uses; no ADI set due to data gaps.
What is it?
Riboflavin-5'-phosphate (also called riboflavin-5'-phosphate sodium, or E101(ii)) is the water-soluble phosphate ester form of riboflavin, a naturally occurring vitamin. It appears as a fine orange-yellow crystalline powder. The phosphate group makes it considerably more soluble in water than plain riboflavin (E101(i)), which is why manufacturers favour it for beverages and liquid products. Intestinal enzymes rapidly strip the phosphate group, releasing free riboflavin that is then metabolised through standard vitamin B2 pathways.
What does it do?
It acts as a yellow to orange-yellow food colouring, giving products a bright, consistent colour. The colour comes from the flavin ring system, which absorbs light strongly in the blue range. Because the body treats it identically to vitamin B2 once absorbed, it also contributes to riboflavin intake, though amounts from food colouring use are typically small compared to dietary sources.
Where you will see it
Found in energy and sports drinks, flavoured dairy products, processed cheese, instant soups, desserts, confectionery, bakery mixes and some sauces. On UK ingredient labels it appears as 'E101', 'E101(ii)', 'riboflavin-5'-phosphate', or 'riboflavin-5-phosphate sodium'.
What the science says
How the body handles it
After ingestion, intestinal enzymes rapidly convert riboflavin-5'-phosphate into free riboflavin, which the body absorbs and uses as vitamin B2. Excess riboflavin is excreted in urine, causing it to appear bright yellow at higher intakes. The body's ability to absorb riboflavin saturates at higher doses, limiting how much can accumulate from any single exposure.
Riboflavin-5'-phosphate sodium is rapidly dephosphorylated to free riboflavin in the intestinal mucosa and then metabolised through standard vitamin B2 pathways, with no unique toxic properties introduced by the phosphate salt form.
Data gaps identified by regulators
When EFSA re-evaluated this additive in 2013, it noted that long-term carcinogenicity studies and reproductive and developmental toxicity studies were absent from the database. Because of those gaps, EFSA considered it inappropriate to set a numerical ADI for riboflavin used as a food additive, despite concluding that use at currently authorised levels is unlikely to raise concern. This is a procedural position reflecting incomplete data, not an identified hazard.
Due to the absence of carcinogenicity and chronic toxicity studies, and the lack of relevant reproductive and developmental toxicity studies, the EFSA Panel considered it not appropriate to allocate an ADI for riboflavin food additives, while noting the database was insufficient to assess potential effects of high intakes from all combined sources.
JECFA at its 92nd meeting (2021) established a group ADI of 'not specified' for riboflavin and riboflavin-5'-phosphate, withdrawing the previous numerical group ADI of 0-0.5 mg/kg body weight, indicating the committee considered a numerical limit unnecessary at intended use levels.
Genotoxicity
Laboratory studies found no evidence that riboflavin or its phosphate form damages DNA. EFSA's 2013 review of available in vitro and in vivo data concluded that neither substance raises concern for genotoxicity.
Riboflavin and riboflavin-5'-phosphate do not raise concern with respect to genotoxicity based on available in vitro and in vivo data.
Rare allergy
A small number of case reports describe allergy to riboflavin, including one case of anaphylaxis in an adolescent after exposure from both a soft drink and a multivitamin. These reactions are uncommon and not a general population risk, but anyone who has experienced a reaction to riboflavin or vitamin B2 supplements should check ingredient labels carefully.
A case report described recurrent anaphylaxis in a 15-year-old boy linked to riboflavin in a soft drink and a multivitamin, confirmed by positive intradermal skin tests; this was described as the first reported case of anaphylaxis to riboflavin at the time.
Where it stands with the regulators
Who should be careful
People who have had allergic reactions to riboflavin or vitamin B2 supplements should look for 'riboflavin-5'-phosphate', 'riboflavin-5-phosphate sodium', 'E101' or 'E101(ii)' on ingredient lists. Such reactions are uncommon but have been documented.
The honest read
E101a is vitamin B2 in a more water-soluble form. Both regulatory bodies that have most recently assessed it, EFSA in 2013 and JECFA in 2021, concluded it does not raise concern at food-additive use levels. EFSA declined to set a numerical daily intake limit, noting the absence of long-term animal carcinogenicity studies from the dataset; this is an honest statement about an evidence gap rather than a finding of harm. JECFA's position is 'ADI not specified', meaning a numerical ceiling is considered unnecessary. There is no IARC classification, no UK or EU restriction or ban, and no identified hazard from normal dietary exposure. The science here is stable and consistent across the organisations that have reviewed it.
Related additives
Common questions
Is E101a banned in the UK?
No. E101a (riboflavin-5'-phosphate sodium) is approved for use in the UK as a food colour under the UK FSA's approved-additives list and the assimilated EU Regulation 1333/2008, Annex II.
Why did EFSA not set a daily intake limit for E101a?
EFSA's 2013 re-evaluation found that long-term carcinogenicity studies and reproductive toxicity studies were missing from the available database. Because those studies had not been done, the panel considered it procedurally inappropriate to calculate a numerical ADI. The panel nonetheless concluded that use at currently authorised levels is unlikely to raise concern. JECFA in 2021 took the position that a numerical ADI was unnecessary, designating it 'not specified'.
What foods contain E101a?
E101a appears most commonly in energy and sports drinks, flavoured dairy products, processed cheese, instant soups, confectionery, bakery mixes and sauces. Manufacturers prefer E101a over plain riboflavin (E101(i)) in liquid products because the phosphate form dissolves more readily in water.
Is E101a vegan?
Yes. Industrial production of riboflavin-5'-phosphate relies on microbial fermentation using organisms such as Bacillus subtilis and Ashbya gossypii. No animal-derived source material is used in commercial production. It is also suitable for vegetarians.
Sources
- EFSA ANS Panel, Scientific Opinion on the re-evaluation of riboflavin (E101(i)) and riboflavin-5'-phosphate sodium (E101(ii)) as food additives, EFSA Journal 11(10):3357
- UK FSA Approved Additives and E Numbers
- JECFA database entry: riboflavin 5'-phosphate sodium
- International Association of Color Manufacturers: Riboflavins
- Anaphylaxis to riboflavin (vitamin B2), Annals of Allergy, Asthma and Immunology, vol. 87, pp. 430-433
- Regulation (EC) No 1333/2008 Annex II, UK legislation database
- Wikipedia: Flavin mononucleotide
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