Register of Formulation · Observation V
Five years ago, bioavailability was a pharmacology term. You found it in scientific journals, university manuals and technical documents written for pharmacists. Today it appears on half the labels in the supplement aisle.
Highly bioavailable form. Optimal bioavailability. Bioavailability increased x10.
The word became mainstream. And like every word that becomes mainstream, it also became hollow. It now says almost anything about almost anything, as soon as a product needs a scientific varnish. The same thing happened to natural fifteen years ago, to organic ten years ago, to clean five years ago. Now it is bioavailability's turn.
This article tries to put back into the word what it contained at the start.
IWhat the word actually says
At its simplest: bioavailability is the fraction of an active compound that your body absorbs and can actually use. Not just swallowed. Not just passed into the bloodstream. Available to the cells that need it.
You can swallow 500 mg of magnesium and absorb only 20. You can ingest a gram of pure curcumin and have almost none of it in plasma an hour later. What is in the capsule does not arrive unchanged inside the cell.
A food supplement actually hides two numbers: the displayed dose and the dose that acts. Between the two, there is often a gulf. Bioavailability is the name of that gulf. The word was coined to measure it, not to sell.
It is precisely because that gulf exists that it has to be measured. It is also why it can be manipulated.
IIMagnesium, the textbook case
Magnesium is the simplest example, because it exists in dozens of forms sold side by side in the same pharmacy.
Magnesium oxide is highly concentrated, around 60% elemental magnesium by mass. On the label, that gives big numbers: 400 mg, 500 mg. Very readable, very reassuring. Except its absorption is low, and a large part is eliminated without being used.
Magnesium citrate is less concentrated, around 16%. The displayed numbers are smaller. But absorption is clearly better.
Magnesium bisglycinate follows the same logic: less concentrated by mass, better absorbed, and with superior digestive tolerance, meaning fewer laxative effects.
Consequence: a product proudly displaying 400 mg of magnesium as oxide may deliver less effectively used magnesium than a product displaying 200 mg as citrate. The dose, read alone, says nothing. You have to read the form.
The dose, read alone, says nothing. You have to read the form.
IIIIron, where the gap becomes enormous
Iron goes much further than magnesium, because absorption gaps are huge.
Heme iron, found in meat and fish, is absorbed at 15-35% depending on sources. Non-heme iron, found in plants and in most supplements, is absorbed at 2-20% depending on conditions. Inhibitors such as tea tannins can make that fall to negligible levels.
Even among non-heme iron salts, the gaps are significant. Ferrous sulfate, the classic pharmacy form, is poorly tolerated: nausea, gastric pain, constipation. Anyone who has taken it remembers. Iron bisglycinate, at an equivalent elemental iron dose, is better absorbed and better tolerated.
Once again, reading only the displayed milligrams is not enough. That is reading half the information. The other half decides the rest: the form, the context, and what the iron is taken with.
IVCurcumin, where the word is true and false at the same time
Curcumin is probably the clearest example, because it shows an honest scientific use of the word and a distorted marketing use side by side.
Taken alone, curcumin is very poorly absorbed. Only a few percent reach the bloodstream. It is an interesting active compound that has long been frustrating to formulate.
Combined with piperine, the active compound in black pepper, its absorption is multiplied by about twenty. That result has been documented since 1998 in a study by Shoba and collaborators, and has been reproduced since. Bioavailability increased x20 with piperine, in that precise case, is true.
Next to that, the market offers products claiming bioavailability increased x100, x185, x277. The figures become dizzying. These claims almost always rely on patented studies comparing a proprietary form to free curcumin, the least absorbable form available, often in vitro, with protocols that are not comparable to real absorption studies in humans.
Same word. Two uses. One honest, one floating. The word itself does not tell you which one you are facing. The context does, and that context is rarely given on the label.
VFour signs of a floating argument
Whenever you see bioavailability on a label or in a sales argument, look at what comes with the word. Four formulas are worth recognizing.
"Increased x10" without a stated comparator. Increased compared to what? Compared to the least absorbed form on the market? Compared to placebo? Always ask for the comparator. Without it, the number means nothing.
"Highly bioavailable form" without any value. Highly compared to what, again? And how highly: 30% or 60%? The phrase contains no information unless a measurement is given.
"Optimal bioavailability". Pure marketing superlative. Nothing is ever optimal in a rigorous study. It is better than X, equivalent to Y, non-inferior to Z. Optimal belongs to advertising vocabulary, not scientific vocabulary.
Figures extrapolated from in vitro studies. What happens in a test tube almost never happens identically in a human intestine. Absorption measured in a lab on isolated cells can produce spectacular numbers without telling us what happens in a body. In vivo studies, conducted in humans, are what matter.
Four empty formulas to recognize. When the word bioavailability arrives without concrete supports, form, comparator, measurement and study type, it no longer says anything. It is just one more word slipped in to sound serious.
VITo finish
Bioavailability, at bottom, is the opposite of a marketing promise. It is an admission. It recognizes that not everything we ingest nourishes us equally. That matter has resistances, preferences and thresholds. That the body sorts, eliminates, ignores. That the dose printed on a box is a half-empty promise until we know what remains after the journey.
An honest supplement begins by acknowledging the gulf between what it contains and what it delivers. It chooses forms accordingly. It combines active ingredients where documented combinations exist. It says what it does. It does not promise the absolute.
When the word bioavailability is used to erase the gulf instead of naming it, something precious has been lost. A measuring tool has been turned into a sales tool. The word remains, but what it contained evaporated along the way.
Which is exactly what the word was originally meant to describe.
N.B. Shoba G. et al., Influence of piperine on the pharmacokinetics of curcumin in animals and human volunteers, Planta Medica, 1998 · EFSA, Scientific opinion on magnesium salts in food supplements · Hurrell R., Egli I., Iron bioavailability and dietary reference values, American Journal of Clinical Nutrition, 2010.
