Silicon dioxide and anti-caking agents: what E 551 is doing there

A supplement label makes an arithmetic promise. Ours reads Ginseng 25 mg, Maca 25 mg, Tribulus 20 mg, Guarana 10 mg, Ashwagandha 10 mg, Catuaba 10 mg - six numbers, 100 mg of plant material, one tablet, a maximum of one in any 24-hour period. Keeping a promise like that is not a matter of good intentions. It is a mechanical problem, solved or lost in a factory, in a steel hopper, weeks before anyone argues about the wording on the carton.

Anti-caking agents exist because of that problem. Silicon dioxide - E 551 - is the one people look up by name, usually after finding it in an ingredient list and wondering why it is there. So: what it is doing in the powder, what the number beside it certifies, and why a pack claiming to contain nothing of the sort may be telling you less than it seems to.

What does an anti-caking agent actually do?

Tablets are not weighed one at a time. A press works by volume. The blended powder sits in a hopper, feeds down into a row of small steel cavities called dies, and each cavity is filled by gravity and then struck flat. Whatever fits in the cavity is what gets compressed. The machine has no idea what is in the powder. It knows how much space the powder occupies, at thousands of tablets an hour.

Milled plant material is a difficult thing to feed into that. It is not sand. The particles are irregular, they carry moisture, and they are slightly sticky. Under the weight of the powder above them they cling to each other and to the steel, and the blend forms bridges and channels instead of flowing evenly. That is caking. It is a physical process, not a spoilage one. The consequence is that one die fills heavy, the next fills light, and the tablets coming off the press stop matching the label. Nobody sees it happen. They look identical.

An anti-caking agent is one answer to that. Silicon dioxide is used in an extremely fine form: particles small enough to coat the larger plant particles, hold them slightly apart, and cut the friction between them, so the blend behaves less like damp flour and more like something that pours. Its job is not with the person swallowing the tablet. It is with the powder, upstream, so that the six numbers on the back stay true from the first tablet of the run to the last.

What silicon dioxide (E 551) is made of

The regulated material has a definition. E 551 is synthetic amorphous silica, made by one of two routes: a vapour-phase process at high temperature, giving fumed silica, or a wet process, giving precipitated silica, silica gel or hydrous silica. Both end at the same chemical composition. Colloidal silica - silica sol - is a different material and is not authorised as E 551. It has relatives in the same functional class, mostly silicates: calcium silicate is E 552, the magnesium silicates E 553a, talc E 553b. Anti-caking agent is a functional class in the general food additive list, not a supplement-industry invention, and it turns up wherever a dry powder has to stay loose.

What is an E number, and what does it prove?

The number is the part most people misread. An E number is a reference code for a substance authorised for use in food. It is closer to a catalogue number than to a hazard symbol.

What sits behind it is a set of conditions. Under assimilated Regulation (EC) No 1333/2008, an additive may only go on the list if, on the scientific evidence available, it does not pose a safety concern to the health of the consumer at the level of use proposed; if there is a reasonable technological need that cannot be met by other economically and technologically practicable means; and if its use does not mislead the consumer. Separately, a specification defines what may be sold under that number: the identity of the material, how much of it must be the substance named, and maximum limits for contaminants such as lead, mercury and arsenic.

Where E numbers are listed in Great Britain

The British paperwork moved recently, worth knowing if you go looking. The Food Standards Agency publishes a plain list, Approved additives and E numbers, and silicon dioxide sits in the section covering anti-caking agents. On 1 April 2025 the annexes of assimilated Regulation 1333/2008 and the assimilated specifications regulation, 231/2012, were revoked as free-standing instruments and authorisations moved to the FSA's Register of Food Additive Authorisations, which links back to the pre-reform text for conditions of use and specifications. Nothing about E 551 changed. The filing cabinet changed.

So a number on a pack tells you something specific and limited: this substance went through that process, and there is a published specification for what it has to be. It does not tell you the substance is good for you. Nothing on the additive list is there for your benefit - it is there because someone demonstrated a technological need, which is a different argument entirely.

The reverse is where the confusion lives. No number does not mean nothing was used. Some materials doing similar jobs are ordinary foods, declared as ingredients with no code at all. More importantly, additives used as processing aids do not have to appear in the ingredients list, and neither do substances carried over from an ingredient if they serve no technological function in the finished product. An E number is a filing reference. Its presence tells you a process happened and left a trail. Its absence tells you either that nothing was used or that nothing had to be written down, and the label looks the same either way.

How much silicon dioxide is in a supplement tablet?

The obvious next question is how much, and this is where most articles on the subject quietly invent a figure. For food supplements in solid form - capsules and tablets - silicon dioxide and the silicates are authorised at quantum satis. That is a defined term, and its definition is that no maximum numerical level is specified. The substance must be used in accordance with good manufacturing practice, at a level no higher than is necessary to achieve the intended purpose, and without misleading the consumer. In other words, the ceiling is a technological argument rather than a number: use what the powder needs and no more, and be able to justify it.

Nor does the label carry a weight. Additives in the listed functional classes are declared by the name of the class followed by the specific name or, where appropriate, the E number. That is why the line reads Anti-caking agent: silicon dioxide, or Anti-caking agent (E551), with no milligram figure beside it. The milligram figures on a supplement pack belong to the substances with a nutritional or physiological effect. The additives get a name and a position in the list, which is ranked by weight. So if you want to know how much is in a given tablet, the pack cannot tell you and neither can arithmetic. Only the manufacturer knows.

What EFSA has assessed, and what it recommended

On the assessment itself, the record is public and it has two parts. In 2018 the European Food Safety Authority re-evaluated E 551 and could not confirm the previous acceptable daily intake of 'not specified' on the database then available. It recommended amending the specification to characterise particle size distribution properly, including the percentage - by number and by mass - of particles in the nanoscale, meaning at least one dimension below 100 nanometres. A call for data followed. In October 2024 EFSA's Panel on Food Additives and Flavourings published the follow-up opinion, whose stated conclusion was that E 551 does not raise a safety concern in any population group at the reported uses, including infants under 16 weeks of age, and which recommended tightening the specification by lowering the maximum limits for lead, mercury and arsenic and setting one for aluminium.

That is the regulatory record, and it is all this article will say on the subject. What silicon dioxide does inside a human body is not a question a supplement brand should answer, in either direction. The paragraph above is a summary of somebody else's opinion, with the citation printed underneath.

Is a 'no additives' or 'clean label' claim worth anything?

Phrases like 'clean label', 'no nasties' and 'no additives' have no legal definition in the United Kingdom. There is no standard to meet before printing them. What does exist is a rule against misleading information, and one part of it is unusually useful here: food information must not suggest that a product has special characteristics when in fact all similar foods have them, particularly by emphasising the presence or absence of certain ingredients. A pack boasting that it contains no artificial colours, in an aisle where nobody uses colours, is describing the category and taking credit for it.

A four-step test for the back of any pack

Which gives you a test you can run in a shop, in under a minute, on any pack including ours. First, turn it over and find the ingredients list, which is mandatory and which no front-of-pack design can remove. Second, read it for functional-class words: anti-caking agent, bulking agent, emulsifier, glazing agent, humectant, colour. Those words are the tell, more than the E numbers, because the class is the job. Third, count them. Fourth, hold that count against whatever the front of the pack said. A front claim that the back can contradict is a claim. A front claim the back cannot touch is decoration. This is the same exercise as reading the back of the pack in the order the information turns up, applied to one line of it.

The test has a hard limit, and it is the point of this whole piece. Absence from the ingredients list is not absence from the process. Processing aids and carried-over substances sit legitimately outside the list, so a short list is evidence of a short list and nothing more. Past that point, reading stops working and asking starts. The batch code printed on the carton is what makes asking answerable, because it identifies one specific run of tablets rather than a product in general.

Our own tablet is pressed in the UK on a GMP-registered line, vegetarian-friendly, allergen-free and sealed into blister foil rather than tipped into a bottle - another moisture decision, made for the same reason caking is a moisture problem. Whatever the tablet contains beyond the six botanicals is declared on the carton in the format described above, by functional class and name, because the labelling rules require it of us and of everyone else. If you want the full breakdown before you buy rather than after, quote the batch code to hello@maleup.co.uk and ask.

The conclusion is a dull one. An anti-caking agent is a manufacturing decision that appears on a label because the law says manufacturing decisions have to appear on labels. The E number beside it is the receipt for that disclosure, not a verdict on it. A pack with three functional-class words on the back and nothing on the front has told you more than a pack with a promise on the front, and the difference takes about a minute to check.

Common questions

† Describes the traditional use of these botanicals (UK 'on-hold' botanical claims under retained Regulation (EC) 1924/2006). Not authorised EFSA health claims. AllMaleUp is a food supplement, not a medicine.