How to Read a Supplement Label -- and the Four Things It Does Not Tell You

What the label genuinely tells you

Start with the honest half, because there is one, and a lot of it is useful. The panel has to give you the serving size and how many servings are in the container, which is how you catch the product that looks cheap until you notice a serving is four capsules. It has to list every dietary ingredient and the amount per serving. It gives you a percent Daily Value where one exists. It has to list the other ingredients -- fillers, binders, capsule material, flow agents -- and it has to declare major allergens. It carries a lot number and an expiry or best-before date. That is a real amount of information, and reading it carefully already puts you ahead of most buyers.

• Serving size and servings per container -- the only way to compare two prices honestly.
• Each dietary ingredient and its amount per serving.
• Percent Daily Value, where a Daily Value has been set for that nutrient.
• Other ingredients: fillers, binders, capsule material, anti-caking agents.
• Lot number and expiry date, which are what a recall or a complaint is traced through.

The first thing it does not tell you: how much of the active compound is in there

A line reading 500 mg turmeric tells you the weight of plant material. It does not tell you the weight of the compounds the research was actually run on. Dried turmeric root is only a few percent curcuminoids; a standardised turmeric extract is commonly sold at around ninety-five percent. Both are legitimately labelled turmeric, and there is more than an order of magnitude between them. The same problem runs through every botanical on the shelf. Unless a label states a standardised percentage -- standardised to 24 percent, say -- the number in milligrams is telling you about the plant, not about the dose. A sibling post in this series takes that apart in full.

The second: whether the form is one your body can take up

Minerals are the cleanest place to see this, because you can catch it with arithmetic. Magnesium is never magnesium on its own; the metal is bound into a salt, and the metal is only a fraction of that salt's weight. Magnesium oxide is roughly sixty percent elemental magnesium, while the citrate forms sit somewhere around ten to sixteen percent. So 500 mg of magnesium citrate and 500 mg of magnesium oxide are not the same amount of magnesium, and neither of them is 500 mg of magnesium. Then the twist that makes the point: oxide carries the most magnesium per gram and is among the worst absorbed of the common forms. The bigger number on the front can belong to the weaker product. A label reporting compound weight rather than elemental weight is not lying. It is also not answering the question you were asking.

The third: whether what went in survived until you swallowed it

Many plant actives are reactive by nature, which is exactly why they are biologically interesting and exactly why they do not sit quietly on a shelf. Light, heat, oxygen and time all take a toll, and none of that shows up on a panel that was printed before the bottle was filled. The clearest consumer-checkable version is a probiotic: ask whether the colony count is guaranteed at manufacture or at expiry. If it is guaranteed at manufacture, you have been told what was in there at the factory. A study of expired probiotic products found that most retained some viable cells, but that total counts were mostly well below the level the manufacturer itself said was needed for the product to do anything [1]. The third post in this series is about that whole journey.

The fourth: who made it and who tested it

The name on the front of a bottle is often not the company that made what is inside it. An ingredient grower sells to an extractor, who sells to a supplier, who sells to a contract manufacturer, who blends and encapsulates and bottles it for a brand. What travels alongside the powder is paperwork: a certificate of analysis, passed hand to hand. Whether anyone in that chain independently tested the finished capsule, as opposed to accepting the drum it arrived in, is not on the label and is not required to be. That is the subject of the fourth post here, and it is the single question that separates brands most sharply.

The sugar trick, in a different aisle

There is a useful parallel from the food world. The sugar industry has dozens of names for sugar -- cane juice evaporate, dextrose, maltodextrin, fruit juice concentrate, rice syrup -- and a label carrying six of them, each far down the ingredient list, reads very differently from one that says sugar near the top. Nothing there is false. The information is simply distributed in a way that is hard to add up. Supplements have the same structure with a different vocabulary: dozens of ways one ingredient can be grown, extracted, concentrated, stabilised and dosed, and a label that names the plant without naming any of it. Same trick, different aisle. Once you have seen it in the food aisle you cannot unsee it in this one.

What independent laboratories actually find

This is the part that must not rest on anyone's opinion, so here is what has been measured. An analysis of thirty commercial melatonin products found wide variability between the labelled and the measured content, and detected serotonin in some of them [2]. Twelve supplements labelled as containing ecdysterone were measured by validated mass spectrometry and generally contained far less than stated [3]. Four independent laboratories testing fifty-three cranberry supplements found that products declaring a proanthocyanidin amount contained significantly less than declared [4]. And a review of United States Food and Drug Administration warnings from 2007 to 2016 identified 776 supplements containing undeclared pharmaceutical ingredients -- concentrated overwhelmingly in the sexual enhancement, weight loss and muscle building categories rather than spread evenly across the shelf [5].

The honest counterweight: it runs both ways

It would be easy, and wrong, to stop there. When adult multivitamin and mineral products were bought under a national sampling plan and chemically analysed, most nutrients were found present above their labelled amounts, with selenium and iodine averaging around twenty-five percent over [6]. Manufacturers add overage on purpose, to cover expected losses across shelf life. So label mismatch is not a one-directional story about being cheated. It is a story about a label being a plan rather than a measurement, and a plan can miss high as easily as low. That distinction matters, because a claim that only ever points one way is a claim that has stopped checking itself.

What to do with all this

None of this argues for giving up on supplements, and none of it means every bottle is suspect. It means the label answers a narrower question than it appears to, and the remaining questions are askable. Ask for the standardised percentage, not just the milligrams. Ask which chemical form. Ask whether potency is guaranteed at expiry or only at manufacture. Ask who ran the test and whether it was on the finished bottle or the incoming raw material. A company that tests its own material can answer all four in a sentence. A company that cannot answer them is not necessarily doing anything wrong -- it may simply be passing on what it was told, which is most of the industry -- but that is a different thing to buy, and you are entitled to know which one you are holding.

• Standardised to what percentage of which compound?
• Which chemical form, and how much elemental mineral does that actually give?
• Is potency guaranteed at expiry, or only at the moment of manufacture?
• Was the finished product tested, or the drum it arrived in? By whom?