Explainer Published 16 Sep 2026 By Logan King ~10 min read

Are supplement fillers toxic? I made capsules without them to find out

Have you ever wondered what the extra ingredients in your supplements are? Perhaps you've seen the articles online about how they're toxic, or watering down your supplements. So I ran the experiment: capsules and tablets made in my own facility with no added ingredients at all, then with them, weighed and compared. And my research team and I spent weeks going through every toxicity study we could find on the two most-attacked excipients. Here's what happened, and what the numbers say.

Quick answers

754mglightest capsule with no flow agent, vs 900mg target
42,000capsules a day to reach the magnesium stearate toxicity dose
8,000mgstearic acid the average American already eats daily from food
6,000tablets a day to reach the MCC toxicity dose
~4mgmagnesium stearate in a size 0 capsule
0studies the "clean" brand could cite when asked for evidence of harm

1. What happens when you make capsules with no added ingredients?

Short answerThey come out underdosed and inconsistent. Sticky powder doesn't flow through the filling machine fast enough to fill each capsule before it closes.

This is collagen powder, and this is a machine that makes capsules. It's set up to make collagen capsules, and there are no added ingredients in this mix: it is 100% collagen in the capsules. Shut the doors and run.

The total weight of each capsule should be 900mg. 130mg is the weight of the empty capsule and the rest should just be collagen. So let's weigh a load of them, aiming for 900mg per capsule: 808. Under. 783. Under. 880. Okay, that's not far. 754. Under. 785.

So it's pretty inconsistent. This is because the powder is too sticky, and therefore not flowing through the machine fast enough to fill the capsule before it closes, giving us both underdosed and inconsistently dosed capsules. This is why magnesium stearate is used: to help the powder flow through the machine and give you consistently dosed capsules.

Collagen capsule weights, with and without a flow agent

Target 900mg per capsule; the usual manufacturing tolerance is ±10%

Dot plot of five capsule weights made without magnesium stearate: 808, 783, 880, 754 and 785 milligrams, four of them below the 810 milligram lower tolerance limit, against a 900 milligram target; with magnesium stearate every capsule sat at or slightly above 900 milligrams, inside tolerance 700 750 800 850 900 950 1,000mg ±10% tolerance: 810 to 990mg No flow agent 100% collagen 754mg: 16% under target 783mg: 13% under 785mg: 13% under 808mg: 10% under, just outside tolerance 880mg: 2% under, inside tolerance 754 783 785 808 880 With magnesium stearate added Every capsule at or slightly over 900mg, inside tolerance; in a real run the fill would be turned down a touch all "slightly over 900mg" Without the flow agent, 4 of 5 capsules fell outside tolerance, up to 16% under. With it, all were inside.
Five capsules weighed on camera from the no-excipient run. The with-stearate run's individual weights weren't read out on camera; every one was at or just over 900mg, "pretty much bang on". The 130mg empty capsule shell is included in all weights.

2. What is magnesium stearate?

Short answerA salt made from magnesium and stearic acid. Stearic acid is a saturated fat that occurs naturally in olive oil, beef and some vegetables.

But what is magnesium stearate, and is it safe to consume? To answer this, my research team and I spent weeks scouring every database for research on these excipients. We've compiled the studies into a single spreadsheet, and you can read it here, free: each study's type, dose, what it measured, who it was done on, what it found, and a link to the paper, including the ones reporting negative effects. Here's what we found.

Magnesium stearate is a salt from magnesium and stearic acid. Stearic acid is a saturated fat found in things like olive oil, beef and some vegetables. In other words, it's naturally occurring.

3. Is magnesium stearate toxic?

Short answerOnly at doses no human could reach from supplements: the studies needed up to 2,500mg per kilo of body weight, which is about 170,000mg a day for a 68kg adult, or roughly 42,000 capsules. Food already gives the average person about 8,000mg of stearic acid a day, and even that plus supplements is nowhere near the threshold.

For it to be toxic, the research shows you'd have to consume up to 2,500mg per kilo of body weight. For a 68 kilo adult, that equals 170,000 milligrams per day. And such small amounts of it are used in capsules or tablets. For a size 0 capsule, you'll be getting about 4mg of magnesium stearate per capsule. For a tiny 6mm tablet, you'll be getting 0.2mg. So basically nothing. Unless you're consuming 42,000 capsules per day, the research says you're fine.

Another thing to mention is that people are already consuming magnesium and stearic acid from foods. The average American consumes 8,000 milligrams of stearic acid per day from food sources, making supplements a drop in the ocean. And even when those numbers are combined, it's way below the toxic levels.

Magnesium stearate: the dose in a capsule vs the dose in the toxicity studies

Milligrams per day, on a log scale, because the gap is too big to draw any other way

Log-scale bar chart: 0.2 milligrams of magnesium stearate in a 6 millimetre tablet, 4 milligrams in a size 0 capsule, 8,000 milligrams of stearic acid a day from food, and 170,000 milligrams a day as the toxicity threshold for a 68 kilogram adult 0.1 1 10 100 1,000 10,000 100k 1M mg 6mm tablet About 0.2mg per tablet 0.2mg Size 0 capsule About 4mg per capsule 4mg Stearic acid from food About 8,000mg a day for the average American 8,000mg/day Toxicity threshold 2,500mg per kg: about 170,000mg a day for a 68kg adult 170,000mg/day Each gridline is ten times the last. From one capsule to the toxic dose is a gap of about 42,000 capsules.
Threshold from the toxicity literature at 2,500mg/kg body weight, scaled to a 68kg adult. Capsule and tablet amounts are typical for those sizes. Food intake figure is the average American's daily stearic acid from diet.

4. What happens when you add it?

Short answerEvery capsule hits the target. The weights went from scattered and under to consistently at or just over 900mg, close enough that a real run would just be turned down a fraction.

So now we add some magnesium stearate to the mix, turn the machine on, and see what sort of weights we get. We should be getting 900mg. That's nice and over 900mg. As you can see, these weights are all slightly over. If this was a real run we'd just turn it down ever so slightly; you want it to be within 10%. So that's pretty much bang on.

The weights are much more consistent now, simply by adding a bit of magnesium stearate. It also gives you an idea of how much better the capsules feel, just by adding this one ingredient.

5. Does magnesium stearate water down the active ingredient?

Short answerNo. It's the reason the capsule contains the dose on the label. Without it, my capsules were up to 16% under.

You might think that by adding in this extra ingredient, it's reducing the amount of collagen in the capsules. But this is not the case. By helping the collagen fill the capsule, magnesium stearate ensures that you get the amount of active ingredient that's advertised on the label.

6. What is microcrystalline cellulose, and what does it do?

Short answerMCC is plant fibre that works like a glue in tablets, so the press can crush powder into something solid. Without it, tablets fall apart in your hand; you'd receive a bottle of powder.

The next excipient is called microcrystalline cellulose, or MCC. MCC is very commonly used in tablets, because it's very good at helping machines like this crush powder together. It's essentially like a glue for the powder. It also dissolves very easily in the stomach, so it's very good for absorption.

Let's demonstrate what happens with just 100% active ingredient: no MCC, no added ingredients. Because there's no binder, first off, there's mostly powder coming out of the machine. And look: if I literally just touch them, they all just turn to powder. They're extremely weak. If you bought these tablets, by the time they arrived after shipping you'd just have a bottle of powder, basically. That is why you add something like MCC. It helps the powder stick together and actually gives you a tablet rather than just powder.

Now we've added some MCC to the mix. These pills are way more solid. I can scrunch them in my hands, press them: they are 100% solid. You'd have a tough time cracking these.

7. Is MCC safe?

Short answerIt isn't absorbed by the body at all. The safety threshold is 9,000mg per kilo, or 612,000mg a day for a 68kg person; an 8mm tablet carries about 100mg, so you'd need around 6,000 tablets a day.

MCC is extracted from plant fibre. It's unique because it's not actually absorbed by the body. MCC is safe at doses of 9,000mg per kilo of body weight, which would be 612,000mg per day for a 68 kilo person. An 8mm tablet, about this size, will contain about 100mg of MCC, so you would need about 6,000 tablets per day to get anywhere close to being toxic. A 6mm tablet, about half that size, will contain about 50mg of MCC, so you'd need even more.

Both excipients, side by side
ExcipientWhat it doesWhere it comes fromPer doseToxicity threshold (68kg adult)Doses per day to reach it
Magnesium stearateFlow agent: powder fills every capsule evenlyMagnesium + stearic acid (a fat in olive oil, beef, vegetables)~4mg per size 0 capsule
~0.2mg per 6mm tablet		2,500mg/kg = ~170,000mg/day~42,000 capsules
Microcrystalline cellulose (MCC)Binder: holds a tablet together; dissolves in the stomachPlant fibre; not absorbed~100mg per 8mm tablet
~50mg per 6mm tablet		9,000mg/kg = ~612,000mg/day~6,000 tablets

8. Why do brands say fillers are toxic?

Short answerBecause a headline without a dose is a marketing tool. The toxicity studies exist, at absurd doses in rats. Screenshot the headline, skip the dose, and you can sell "clean" supplements against a "toxic" industry. When I asked one such UK brand for evidence, as a customer, they conceded there isn't any at supplement doses.

Because there are studies that show these ingredients are toxic at super high doses, technically you could just screenshot the study headline, not mention the dosage part, and essentially trick people into thinking these ingredients are toxic. But these are usually rat studies, at crazy high doses. And brands, at least here in the UK, have been jumping on this as a marketing opportunity to differentiate themselves from a "toxic" industry: selling "clean" supplements, without fillers or nasty ingredients, without citing any real scientific research.

I actually contacted one of these companies as a hidden customer and asked them for any evidence that these nasty ingredients cause any harm. Here's what they said:

"Some online sources claim magnesium stearate affects digestion, immunity, or nutrient absorption. But these claims are not supported by scientific evidence at the levels used in supplements. There isn't reputable clinical evidence showing harm from magnesium stearate at supplement doses." Customer-service reply from a UK brand marketing "clean" supplements without fillers, when asked for evidence of harm

So I guess that kind of says it all, doesn't it?

9. Do "no fillers" supplements actually dose properly?

Short answerThat's the obvious next test. My no-excipient capsules were up to 16% under. If a brand sells capsules with no flow agent, the question is whether every capsule you buy contains what the label says.

Leave a comment on the video if you want me to buy a load of these "clean" supplements and weigh every single capsule to see if they're actually ripping you off, and subscribe to know when it comes out.

The research spreadsheet The free resource promised in the video: every magnesium stearate study we found, in one Google Sheet. Study type, dose, outcome measured, participants, duration, results and a link to each paper. The studies that reported negative effects are in there too, with their doses, so you can judge them for yourself.
More from this series Shorter clips from the same session: What is magnesium stearate?, Making capsules without extra ingredients and Investigating extra ingredients in capsules. And if you want to see what third-party testing finds inside finished products, start with the nattokinase test.
Disclosure and limits I run NF Supplements, which manufactures in a GMP-certified facility in Kidlington, Oxford, and uses these excipients where they're needed. That's a reason to check my numbers, so they're all here. The capsule weights are from one demonstration run of five capsules each way, not a validated study. Toxicity thresholds are from the animal-study literature; "safe at supplement doses" is the conclusion of that literature and of the brand quoted above, not medical advice for your situation.

Sources and further reading

  1. VideoI Tried To Make Supplements Without "Extra" Ingredients - the demonstration this post is written from (published 22 Jan 2026)
  2. ResearchThe excipient research spreadsheet - every study my research team found on magnesium stearate, with dose, outcome, participants, results and a link to each paper; the free resource promised in the video
  3. FacilityNF Supplements, GMP-certified manufacturing facility, Kidlington, Oxford - where the capsules and tablets were made; every batch is tested and published at testing.nfsupplements.co.uk
  4. RelatedProf. Vormann on magnesium supplements - the same "label number vs actual dose" theme, from the other direction
  5. ChannelLogan Talks Health on YouTube