The ingredients on your supplement label that aren't the vitamin? They're not "fillers" and they're not there to cut corners. Most people don't know what these extra ingredients actually do, or why they're necessary.

In this episode, Jen, Chris, and Matt tackle one of the most debated topics in the supplement world. With Chris's 30+ years as a formulation scientist, the team explains why supplements physically cannot be made from pure nutrients alone, what common excipients actually do, and where the real concerns in this industry lie. Along the way, they bust myths about magnesium stearate, silicon dioxide, maltodextrin, and carrageenan, and reveal some genuinely questionable practices hiding behind "filler-free" marketing.

In this episode:

1. 00:00 - Introduction
2. 01:32 - Why supplements can't just be pure nutrients
3. 04:01 - Magnesium stearate and why it's safe
4. 06:06 - Silicon dioxide and the nanoparticle question
5. 07:57 - Maltodextrin and the dose that makes the poison
6. 10:12 - Carrageenan and research misinterpretation
7. 13:02 - The salt shaker situation
8. 16:42 - What "filler-free" labels actually mean
9. 18:39 - What to actually look for on a label

Why Supplements Can't Be Pure Nutrients

(01:32)

A typical vitamin B12 dose is around 100 micrograms, a millionth of a gram. That's too small to see with the naked eye, let alone measure consistently or form into a tablet. Without excipients, you'd have a pile of powder too tiny and too clumpy to work with.

Chris uses a beach sand analogy to explain the manufacturing challenge. Fine, dry beach sand flows through your fingers almost like liquid and pours easily into a capsule. Wet sandcastle sand? Great for building, but impossible to get through a machine. Most nutrient powders behave like the wet sand. Excipients give them the flow they need.

"Supplement companies don't actually financially benefit by adding these things. These ingredients add cost. So if we could make a tablet with just pure vitamin D3 powder, we would. But the problem is it's often physically impossible." — Chris
The Safety of Common Excipients

(04:01)

The team works through four of the most questioned ingredients on supplement labels.

Magnesium stearate is a salt of stearic acid, a natural fatty acid found in cooking oil, chocolate, and any oily food. It's used in milligram quantities as a flow agent. The FDA considers it safe at up to 2,500 milligrams per kilogram of body weight daily. For an average person, that's 175,000 milligrams. A typical capsule contains around 50 milligrams, or roughly 0.03% of the safety threshold.

"It's kind of ironic, right? Avoiding supplements with magnesium stearate whilst eating everyday foods like chocolate that contain far more of the ingredient seems a bit odd." — Matt

Silicon dioxide (silica) prevents clumping and absorbs moisture. There is a legitimate conversation around nanoparticles, and the European Food Safety Authority has been reviewing this. But most silica used in supplements is non-nano, meaning the particles are larger and raise no safety concerns.

Maltodextrin is derived from starch (potatoes or maize) and serves as a binder and natural sweetener in chewable tablets. Online concerns about its glycaemic index come from studies using many grams per day, compared to the 100–200 milligrams in a supplement tablet. At these levels, maltodextrin does its structural job without any measurable effect on the body.

Carrageenan is a natural gum from seaweed, used to form plant-based soft gel capsules. The inflammation concerns originate from studies on poligeenan, a completely different compound not even permitted for use in foods.

"It's literally like saying water is dangerous because of studies about hydrogen peroxide." — Chris
The Salt Shaker Situation and Hidden Ingredients

(13:02)

The genuinely concerning practices in the supplement industry aren't about which excipients are present. They're about transparency.

Some companies, when their vitamin mix won't flow into capsules, use what Chris describes as "a big salt shaker" full of a flow agent like magnesium stearate, pouring it over the mix at the machine. Because they classify this as a "processing aid," they don't declare it on the label. Some of these companies are simultaneously marketing their products as "filler-free."

The team tested a set of "clean" capsules from another company. The plant extract was beige, but there were visible white specks and white residue in the bottle. Nothing white was listed on the ingredients.

"To forget to list flow aids or sub-ingredients is one thing, but to then be making a big noise about how clean or filler-free the product is, is completely unacceptable." — Chris
What to Actually Look for on a Label

(18:39)

The team offers three things to check when choosing a supplement:

1. Bioavailable forms of active ingredients. Vitamin D3 rather than D2, folate rather than folic acid, EPA and DHA for omega-3, and chelated mineral forms.
2. Full transparency. Every ingredient should be listed, including sub-ingredients and processing aids. If something seems to be missing, ask the company directly.
3. Formulation expertise. Is the company run by formulation scientists, or by marketers importing cheap multivitamins?

"What ultimately matters is whether the product contains the active ingredients that are in the right forms and effective doses, and that the product's been formulated properly." — Chris
About So That's Why

So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.