What do the fillers in your supplement actually do?

Turn a supplement over and the interesting part is usually the second list. After the actives comes a short line of substances nobody markets: a capsule shell, a cellulose, a stearate, a silica. The internet has decided these are cheap padding at best and quietly harmful at worst. They are neither. Most of them exist because a powder has to survive being weighed, poured, pressed and stored, and then has to fall apart again in your stomach within about a quarter of an hour. This article explains what each job is, what the evidence actually says about the two most accused ingredients, and how to read that second list in under a minute.

The short answer

Fillers are not one thing. On a European label the substances after the actives are doing one of five jobs: adding bulk so a small dose can be handled at all, keeping powder flowing so every capsule gets the same amount, stopping the powder welding itself to the machine, making the finished dose break apart again in water, and forming the shell that holds it together until then. Almost all of them are regulated food additives with published safety assessments. The real quality question is not whether they are present. It is whether the manufacturer used the right amount, because the same substance that makes a tablet possible can, in excess, slow down the moment it dissolves.

What the second ingredient list legally is

There is no legal category called "filler". Under Regulation (EU) No 1169/2011 a food, and a supplement is a food, must carry a list of all its ingredients "in descending order of weight, as recorded at the time of their use in the manufacture of the food" [1]. That single rule tells you more than any marketing line: whatever sits at the end of the list is present in the smallest quantity. Ingredients that are engineered nanomaterials must be named with the word "nano" in brackets [1], which is why the nanoparticle argument you may have read about is, on a European label, a visible question rather than a hidden one.

Additives are named in a specific way. Annex VII Part C of the same regulation requires them to be shown by the name of their functional category followed by their specific name or E number, and lists those categories verbatim: acid, acidity regulator, anti-caking agent, anti-foaming agent, antioxidant, bulking agent, colour, emulsifier, firming agent, humectant, raising agent, stabiliser, sweetener, thickener and others [1]. Notice what is absent: there is no category called lubricant, which is why a tableting lubricant usually appears either under its own name or as an anti-caking agent.

Whether an additive may be there at all is decided upstream. Regulation (EC) No 1333/2008 permits only additives that are authorised for the food category in question [2], and an additive can only be authorised if, on the available evidence, it poses no safety concern at the proposed level of use, if "there is a reasonable technological need that cannot be achieved by other economically and technologically practicable means", and if its use does not mislead the consumer [2]. Technological need is the whole point. Under that regulation an additive is not allowed to be in your capsule simply to make it cheaper to produce a nice round number of grams.

One more layer sits on top for supplements specifically. Directive 2002/46/EC requires the product to be sold under the name "food supplement" and to carry the recommended daily portion, a warning not to exceed that stated dose, a statement that supplements are not a substitute for a varied diet and a note to keep them out of reach of young children, and it forbids labelling that attributes to the product the property of preventing, treating or curing a human disease [15]. That is why a well made supplement label looks repetitive. Most of it is compulsory.

The five jobs

1. Bulk

Some active doses are tiny. A daily amount of biotin is measured in micrograms, and you cannot fill a capsule, or accurately dose one, with a speck. A bulking agent gives the machine something to work with. Celluloses are the usual choice. EFSA re-evaluated the whole cellulose family in 2018 and concluded that celluloses are not absorbed and are excreted intact, and that an acceptable daily intake "not specified", the most permissive category the system has, remained appropriate [3].

2. Flow

Powder is not a liquid. It bridges, clumps and sticks, and a powder that does not flow evenly produces capsules that are not evenly filled. Silicon dioxide is the standard fix. Its function depends on physical size: the particles act as spacers between food particles to prevent caking, and the aggregates specified for food use are typically larger than 100 nanometres precisely because that size is needed to do the job [4].

3. Lubrication

Tablets are formed under enormous pressure inside a steel die, and capsules are filled by machinery moving at speed. Without a lubricant the powder welds to the tooling. Magnesium stearate and calcium stearate are the industry answer, typically at well under one percent of the mass.

4. Falling apart again

A dose that survives manufacturing must still break up in the stomach. That is the job of a disintegrant, which swells or wicks water into the compacted mass. Measured with terahertz imaging, a disintegrant and a lubricant pull in opposite directions, and both were found to accelerate and to retard different stages of the same disintegration process [5]. Formulation is a balance, not a checklist.

5. The shell

The capsule is an ingredient too, and it is usually either gelatin or hydroxypropyl methylcellulose, which appears on labels as HPMC or hypromellose. It is not a trivial share of the weight. On our own magnesium label the plant based shell sits fifth in the ingredient list, which under the descending order rule means it outweighs three of the seven magnesium compounds inside it.

Magnesium stearate: the accusation and the evidence

Magnesium stearate is the most attacked excipient on the internet, and almost every attack traces back to one paper. In 1990, Tebbey and Buttke reported in Immunology that stearic acid selectively inhibits T dependent immune responses. Their measurements were made by incubating mitogen activated mouse B and T cells with stearic acid in culture, where the saturated fatty acid accumulated in the T cell membranes and caused membrane integrity to collapse within about eight hours in a time and dose dependent way [6].

Read what that study was. It is an in vitro experiment on isolated immune cells bathed directly in a fatty acid, not a study of anyone swallowing a few milligrams of a magnesium salt. Our article on how to spot a weak study behind a supplement claim exists for exactly this move: taking a cell culture result and quoting it as if it described a human eating a capsule.

What happens when the substance itself is assessed for eating? EFSA re-evaluated magnesium salts of fatty acids, E 470b, alongside the sodium, potassium and calcium salts. The panel noted that these salts are expected to dissociate in the gastrointestinal tract into fatty acid carboxylates and their cations, found no concern for mutagenicity for magnesium stearate, calculated that the fatty acid moieties contribute at most about 5 percent of total dietary saturated fat intake, and concluded that there was no need for a numerical acceptable daily intake and that these additives were of no safety concern at the reported uses and use levels [7].

The genuine issue with magnesium stearate is mechanical, not toxicological. It is hydrophobic, so overdoing it can slow the moment the dose wets and dissolves. When one industrial capsule blend was deliberately over blended, processes that induced shear stress produced less permeable blends and retarded capsule dissolution by at least 35 percent at the tested time points [8]. A separate study found that even the fatty acid composition of the lubricant matters: magnesium stearate with a lower stearate content produced tablets with longer disintegration times and slower drug release [9].

So the honest version is the opposite of the scare story. Magnesium stearate is not attacking your immune system. It is a manufacturing decision whose amount and blending time determine whether your capsule opens promptly, which is a question about the factory, not about the molecule.

Silica, cellulose and the nano question

Synthetic amorphous silica, E 551, is the second most produced food additive and has been studied heavily. A detailed review of the toxicological and intake data concluded that silica products meeting the E 551 specification do not cause adverse effects in oral repeated dose studies, including at doses exceeding current guideline recommendations, with no evidence of liver toxicity after oral intake and no indication of immunotoxic or neurotoxic effects in vivo [4]. The same review makes an important caveat that applies to any ingredient: a silicon dioxide product that does not meet the established specification, or that is engineered to provide new functionality, needs its own assessment [4].

That is where the nano debate belongs. When EFSA re-evaluated E 551 in 2018 it considered the available data on oral toxicity insufficient for a reliable risk assessment, which prompted further work. One such study exposed an advanced human intestinal co-culture model to six food grade silicas at doses relevant to dietary exposure and detected no effects on cell viability, barrier integrity, microvilli function or inflammatory cytokine release after acute exposure, with only slight biological responses in a few readouts [10]. Uncertainty is not the same as harm, and it is not the same as safety either. It is a live question, which is why the nano labelling rule in Article 18 matters to you as a reader [1].

The one that actually got removed

If you want evidence that the European system is capable of taking something off the shelf, look at titanium dioxide, the whitener E 171. In 2021 EFSA assessed it again and concluded that titanium dioxide particles have the potential to induce DNA strand breaks and chromosomal damage but not gene mutations, that a concern for genotoxicity could not be ruled out, that no threshold could be assumed and no cut off particle size identified, and therefore that "E 171 can no longer be considered as safe when used as a food additive" [11]. It was subsequently removed from the authorised list for food in the EU.

Two lessons follow. The first is that "it is approved" is a statement about a moment in time, not a permanent verdict. The second is that the substances people argue about online are rarely the ones the regulators end up acting on.

Gelatin or HPMC: does the shell change anything?

For most people, less than the marketing on either side suggests. Both shells are designed to open in the stomach within minutes. Using standardised equipment to model the fed state, initial disintegration of hard gelatin capsules matched a previously estimated mean intragastric initial disintegration time of about 11 minutes, while complete disintegration of a reference immediate release tablet was close to the previously estimated 14 minutes [12]. That is the timescale you are dealing with.

Where they differ is in how they respond to their environment. A clinical study in 12 healthy volunteers compared gelatin and HPMC capsules taken with warm water, cold water and warm black tea. In vitro, temperature strongly affected gelatin capsules, which opened rapidly in warm media and slowly in cold, and in vivo the opening time of gelatin capsules in warm black tea was slightly delayed compared with warm water. HPMC capsules showed no significant differences in opening time in vivo. The authors titled the paper "Much ado about little", and gastric emptying itself was not affected by the fluid [13]. The other practical difference is moisture: HPMC shells absorb less water than gelatin or pullulan and protect hygroscopic contents more effectively [14], which is the same reason the storage advice in our article on whether storage ruins your supplements keeps returning to humidity rather than heat.

How to read the second list in 60 seconds

• Read the order. Weight descends. Anything after the actives is present in smaller amounts than the actives, and the last name on the line is the smallest of all [1].
• Name the job. Bulking agent, anti-caking agent, capsule shell, sweetener. If you cannot work out which of the five jobs an ingredient is doing, that is a fair question to ask the manufacturer.
• Check for a functional category plus an E number. That format is what the law asks for, and its presence is a sign the label was written properly, not a sign of a cheap product [1].
• Look for the word nano in brackets. Its absence on an EU label means the manufacturer is declaring that no ingredient is present as an engineered nanomaterial [1].
• Count what the format needs. A powder can be a single ingredient. An effervescent tablet cannot: the fizz is chemistry, and the acids, the sugar alcohol and the flow agents are the mechanism, not padding.
• Ignore "no fillers" as a claim. It has no legal definition. A product with a capsule shell and an anti-caking agent may be better made than one that avoids both and arrives compacted at the bottom of the pot.

What is in ours, honestly

Applying the same test to our own labels gives a mixed and, we think, defensible picture.

Two products carry nothing at all. Our creatine monohydrate is an unflavoured powder with one ingredient, and our shilajit resin declares no additives or preservatives. A powder and a resin do not need help holding a shape.

The capsules need a little. Our zinc picolinate lists zinc picolinate, acacia fibre and a plant based HPMC shell. Our bioactive B complex lists its eight B vitamins plus choline bitartrate and inositol inside an HPMC shell. Our magnesium 7 in 1 lists seven magnesium compounds, the HPMC shell and magnesium stearate of vegetable origin. Yes, the ingredient this article spent a section defending is on our own label, in last place, which is exactly where the descending order rule says the smallest quantity sits.

The formats that need more, get more. Our molecular hydrogen tablets list glucose, mannitol, citric acid, malic acid, polyethylene glycol and silicon dioxide alongside the mineral content. By weight, most of that tablet is not the active: it is the reaction that releases hydrogen gas into your glass, plus what keeps the powder pressable and stable until you drop it in water. Our FocusFuel lozenges contain calcium stearate, gum arabic, spirulina extract, natural flavours, sucralose and steviol glycosides, because a lozenge you hold in your mouth has to taste of something. If you object to sweeteners on principle, that is a legitimate reason to choose a capsule instead, and we would rather say so than pretend the line is not there.

One thing you will not find on any of our labels is titanium dioxide, and that is not a marketing achievement. It is no longer authorised in food in the EU [11].

Frequently Asked Questions

Is magnesium stearate bad for you?

The evidence does not support that. The study everyone cites incubated isolated mouse immune cells with stearic acid and watched their membranes collapse in culture [6], which says nothing about swallowing a few milligrams of a magnesium salt. EFSA re-evaluated magnesium salts of fatty acids, found no mutagenicity concern, noted they dissociate in the gut into fatty acids and their cations, and concluded no numerical acceptable daily intake was needed and that they are of no safety concern at reported uses [7].

Do fillers reduce how much of the active I absorb?

Not by displacing it, and the amounts are small anyway. The mechanism worth knowing is different: a hydrophobic lubricant used in excess, or blended for too long under shear, can slow how fast the dose wets and dissolves, with retardation of at least 35 percent measured in one over blended industrial batch [8]. That is a manufacturing quality issue rather than a property of the ingredient.

What does "no fillers" or "clean label" mean legally?

Nothing. Neither phrase is defined in EU food law. What is defined is the ingredient list itself: every ingredient must be named, in descending order of weight, with additives shown by functional category plus their specific name or E number [1]. Read that list instead of the claim.

Are vegetarian HPMC capsules better than gelatin ones?

They behave slightly differently rather than better. In a study in 12 volunteers, gelatin capsules opened faster in warm fluid and slower in cold, and slightly later in warm black tea than in warm water, while HPMC capsules showed no significant in vivo differences between fluids [13]. HPMC shells also take up less moisture, which helps protect hygroscopic contents [14]. For most people the meaningful reasons to choose HPMC are dietary and storage related.

Why does an effervescent tablet have so many other ingredients?

Because the fizz is the delivery system. The acids and the bulking agents are what make the tablet disperse and release its gas in the glass, so on that format a long second list is a sign that the product works as intended rather than a sign of padding.

Should I worry about nanoparticles in silicon dioxide?

Watch the label rather than the headline. Food grade silica works precisely because its aggregates are typically larger than 100 nanometres, and reviews of oral studies have not found relevant systemic toxicity [4], though EFSA judged the available data insufficient for a full risk assessment in 2018 and further work followed, including human intestinal models that found no effects at dietary relevant doses [10]. In the EU any ingredient present as an engineered nanomaterial must be labelled with the word nano in brackets [1].

The Bottom Line

The second ingredient list is not where corners are cut. It is where the format is made possible: bulk so a microgram dose can be handled, flow so every capsule holds the same amount, lubrication so the press does not jam, a disintegrant so the whole thing falls apart again, and a shell to hold it together in between. The ingredients that attract the most fear online, magnesium stearate and silicon dioxide, are among the better characterised substances in the food supply, while the one European regulators actually removed, titanium dioxide, rarely came up in those conversations at all. Judge the second list by whether each name has a job, whether the amount is plausible from its position in the order, and whether the format needs it. And treat "no fillers" for what it is: a phrase with no legal meaning, printed on a label whose every other line is defined by law.

Sources

1. Regulation (EU) No 1169/2011 on the provision of food information to consumers, Article 18 and Annex VII Part C (ingredients listed in descending order of weight; engineered nanomaterials followed by the word "nano" in brackets; additives designated by category name followed by specific name or E number, with the category list given verbatim).
2. Regulation (EC) No 1333/2008 on food additives, Articles 4 and 6 (only authorised additives may be used in the food categories specified; authorisation requires no safety concern at the level of use, a reasonable technological need that cannot be met by other practicable means, and no misleading of the consumer).
3. EFSA Panel on Food Additives and Nutrient Sources added to Food. Re-evaluation of celluloses E 460(i), E 460(ii), E 461, E 462, E 463, E 464, E 465, E 466, E 468 and E 469 as food additives. EFSA Journal, 2018 (celluloses are not absorbed and are excreted intact; acceptable daily intake "not specified").
4. Fruijtier-Polloth C. The safety of nanostructured synthetic amorphous silica (SAS) as a food additive (E 551). Archives of Toxicology, 2016 (aggregates typically larger than 100 nm, which is required for the anti-caking function; no adverse effects in oral repeated dose studies; products outside the specification need their own assessment).
5. Lee J, et al. Terahertz-based analysis of immediate-release tablet hydration and disintegration: effects of croscarmellose sodium and magnesium stearate. International Journal of Pharmaceutics, 2025 (disintegrant and lubricant exert contradictory effects, both accelerating and retarding different stages of disintegration).
6. Tebbey PW, Buttke TM. Molecular basis for the immunosuppressive action of stearic acid on T cells. Immunology, 1990 (in vitro study; mitogen activated mouse lymphocytes incubated with stearic acid; time and dose dependent collapse of T cell membrane integrity within eight hours).
7. EFSA Panel on Food Additives and Nutrient Sources added to Food. Re-evaluation of sodium, potassium and calcium salts of fatty acids (E 470a) and magnesium salts of fatty acids (E 470b) as food additives. EFSA Journal, 2018 (salts dissociate in the gastrointestinal tract; no concern for mutagenicity of magnesium stearate; fatty acid moieties contribute at most about 5 percent of dietary saturated fat; no numerical ADI needed and no safety concern at reported uses).
8. Barrocas P, Vieira A, Almeida H, et al. Over-blending effect of lubricants on capsules manufacturing. Pharmaceutical Development and Technology, 2023 (shear inducing processes produced less permeable blends and retarded capsule dissolution by at least 35 percent at the tested time points).
9. Veronica N, Heng PWS, Liew CV. Magnesium stearate fatty acid composition, lubrication performance and tablet properties. AAPS PharmSciTech, 2024 (lower stearate content produced longer disintegration times and slower drug release).
10. Investigating the effects of differently produced synthetic amorphous silica (E 551) on the integrity and functionality of the human intestinal barrier using an advanced in vitro co-culture model. Archives of Toxicology, 2021 (notes EFSA judged the 2018 data insufficient for reliable risk assessment; no effects on viability, barrier integrity, microvilli function or cytokine release at dietary relevant doses).
11. EFSA Panel on Food Additives and Flavourings. Safety assessment of titanium dioxide (E171) as a food additive. EFSA Journal, 2021 (potential to induce DNA strand breaks and chromosomal damage; genotoxicity concern could not be ruled out; E 171 can no longer be considered safe as a food additive).
12. Vertzoni M, et al. Estimating intragastric disintegration times of immediate release dose units administered after a high-calorie, high-fat meal. The AAPS Journal, 2026 (initial intragastric disintegration of hard gelatin capsules about 11 minutes; complete disintegration of the reference tablet about 14 minutes).
13. Sarwinska D, et al. The effect of black tea and water temperature on the disintegration of gelatine and HPMC capsules. International Journal of Pharmaceutics: X, 2025 (clinical study in 12 volunteers; gelatin capsule opening sensitive to temperature and slightly delayed in warm black tea; no significant in vivo differences for HPMC capsules; gastric emptying unaffected).
14. Yang N, Chen H, Jin Z, et al. Moisture sorption and desorption properties of gelatin, HPMC and pullulan hard capsules. International Journal of Biological Macromolecules, 2020 (HPMC capsules show lower moisture sorption and protect hygroscopic contents more effectively than gelatin).
15. Directive 2002/46/EC on food supplements, Article 6 (sold under the name "food supplement"; must state the recommended daily portion, a warning not to exceed it, that supplements do not replace a varied diet and that they should be kept out of reach of young children; must not attribute disease prevention, treatment or cure).

A note on the legal sources: the official EUR-Lex service returns an empty response to automated requests, so the wording of Regulation (EU) No 1169/2011, Regulation (EC) No 1333/2008 and Directive 2002/46/EC was verified on the publicly accessible legislation.gov.uk mirror of those instruments. The EU instruments are named in full above.