What to look for in supplements: Fillers, additives, and more

Before you spend money on a supplement at all, it helps to know what you're actually low in. You can check levels like vitamin D, iron, and folate with a home blood test.

Learn more about home blood testing with Thriva.

Fillers and bulking agents in supplements

Micronutrients, such as vitamins and minerals, are needed in tiny amounts. On their own, they wouldn't come close to filling a capsule or tablet. Some capsules would look almost empty.

Fillers make up the rest of the space. You might also see them called bulking agents, diluents, or carriers, but they do the same job. Common ones include:

• Rice flour: A common plant-based filler, and one of the easier ones to recognise on a label.
• Calcium: Adds bulk and a small mineral contribution.
• Salt, sugar and lactose: Inexpensive bulking agents, but handy to know about if you're cutting back on any of them.

There's nothing inherently wrong with fillers. They're mostly there to make a supplement the right size and easy to handle, and a recognisable ingredient like rice flour is nothing to be concerned about.

Binders in supplements

Binders hold a supplement together so it doesn't crumble, and they're most common in compressed tablets.

They're usually made up of sugar derivatives or cellulose, which is an insoluble plant fibre. Like fillers, these aren't usually cause for concern, and they're needed to hold a compressed tablet together.

Coatings: Gelatin and vegetable cellulose

Coatings make capsules and tablets easier to swallow. Many are made from gelatin, which is animal-derived. If you're plant-based, it's worth checking the label, though many supplements will state on the packaging if they're safe for vegetarians or vegans.

Vegetable cellulose capsules are a common plant-based alternative and a good vegan option. Cellulose is a plant fibre, so it's suitable for vegetarians and vegans and passes through your body without being absorbed.

Colourings and flavourings

Some supplements, often cheaper ones, add colourings or flavourings. Natural versions are generally preferable to artificial ones, so this is a good place to read the label if you'd rather keep the ingredient list short.

Flow agents and magnesium stearate

Supplements are mostly made by machines, and finely powdered ingredients need to flow smoothly through the equipment. Flow agents, also called lubricants, stop the powder from sticking. The most common is magnesium stearate.

You'll often see magnesium stearate listed as an ingredient to avoid. That reputation comes largely from older laboratory studies showing it can slow how quickly a tablet dissolves. At the small amounts used in supplements, typically around 1% of a tablet, there's little good evidence that it meaningfully reduces how much of a nutrient you absorb.

You may also come across magnesium silicate (talc), used as an anti-caking and flow agent. The health concerns people have heard about talc mainly relate to inhalation or past contamination concerns, not to the small amounts used in tablets. If you'd still rather avoid it, it's fairly easy to choose a product without it.

How well your body absorbs a supplement

Bioavailability is the amount of a nutrient that your body is actually able to use. Taking in a certain amount doesn't mean you absorb all of it, which is why this matters.

A few things affect how much you absorb:

• The form of the nutrient: Different forms absorb differently. Sometimes that's clear-cut, like with vitamin D. But in other cases, it's less clear, which is why it's a good idea to do your research.
• Other nutrients present: Nutrients interact with each other. Vitamin D helps you absorb calcium, while iron can interfere with zinc absorption.
• Your diet: Some foods and drinks affect your body's uptake of nutrients. The tannins in tea, for example, can reduce how much iron you absorb.
• Individual variation: Your age, sex, health, and existing nutrient levels all play a part. In pregnancy, for instance, your body absorbs more calcium and iron.

If you're not sure which form to choose or what to take it with, ask your doctor or a pharmacist. They should be able to answer any questions you may have.

How much should you take?

There's no single rule for supplement doses, but the UK recommended daily allowances (RDAs) are a useful reference point. Most labels show the percentage of the RDA a product provides, so look for one that covers a good proportion of what you need.

If you're unsure about a dose, check with your doctor or pharmacist.

How to choose the right supplement

In the UK, the Food Standards Agency sets strict rules on what can go into a food supplement.

Here are a few tips to help you when choosing one:

• Read the label: Non-GMO, allergen-free, and vegetarian or vegan certifications mean a product has been through extra testing to verify these claims.
• Check the dose: Use the RDA percentage on the label to make sure you're actually getting a useful amount.
• Count the ingredients: A shorter list usually means fewer unnecessary additives.
• Favour pure forms: Capsules, liquids, and powders tend to need fewer additives than compressed tablets.

And remember, some additives are there for a good reason: to bind, fill, or stabilise the nutrients. When you see an ingredient you don't recognise, it's a good idea to research or ask a pharmacist for advice.

Takeaway

Most of the extra ingredients in a supplement are harmless and there to make it work, so fillers, binders, and coatings rarely need a second thought. Focus on a short ingredient list, a sensible dose, and, ideally, testing your levels first so you only supplement what you actually need.

Disclaimer: This information is for general educational purposes only and isn’t a substitute for professional medical advice, diagnosis, or treatment. Always speak with a qualified healthcare professional first if you have concerns about your health.