What's Actually In That · Part 3 · Supplement Bioavailability
The label on the bottle tells you what went into the capsule. It doesn't tell you how much of that actually reaches your cells — which is the only number that matters. For a significant proportion of supplements sold in the UK, the gap between those two figures is large, invisible at the point of purchase, and the primary reason why "I've tried that and it didn't do anything" is the most common supplement story in clinical practice.
This series applies clinical assessment to consumer health products. Part 1 compared four electrolyte products. Part 2 assessed the supermarket health aisle. This post covers the broader supplement industry — specifically the bioavailability problem that makes two products with identical labels produce completely different results.
I have had clients arrive with supplement bags. Not a supplement, not a handful of supplements — bags. Carrier bags containing thirty, forty, sometimes fifty individual products collected over years from health food shops, online retailers, well-meaning friends, social media recommendations, and previous practitioners. All with impressive labels. Many with very high milligram counts. Almost none chosen on the basis of what form the ingredient was in, how well that form is absorbed, or whether the dose was clinically meaningful after accounting for absorption losses.
This is the norm, not the exception. The supplement industry has spent decades competing on the number people can see — the milligrams on the front of the bottle — rather than the number that actually matters, which is how much of that dose reaches the tissues where it needs to work. The two numbers are often very different. And the difference is invisible unless you know what to look for.
This post is that education. Not a list of supplements to take — the right supplements depend on what your test results show is actually deficient. But a guide to reading the label properly so that when you do take a supplement, you can be reasonably confident it's doing what you're paying for it to do.
Bioavailability is the scientific term for the proportion of an administered dose that reaches systemic circulation in active form. A supplement with 100% bioavailability delivers every milligram on the label to the bloodstream. No supplement has 100% bioavailability. Most have significantly less. And for some commonly sold forms of popular supplements, bioavailability is so poor that the product represents little more than expensive raw material for your excretory system.
To understand why, consider what a supplement has to survive between the moment you swallow it and the moment it reaches the cell that needs it.
The milligram arms race that defines most supplement marketing is not accidental. Dose is visible, comparable, and communicable on a label. A consumer can glance at two magnesium supplements — one at 500mg, one at 150mg — and conclude the first is the better product. Bioavailability is invisible, requires scientific literacy to communicate, and costs money to optimise through better ingredient forms and delivery systems.
So the industry competed on dose. Raw, unformulated magnesium oxide at 500mg is cheap to manufacture and produces an impressive label. Albion-chelated di-magnesium malate at 150mg is more expensive to source and produces a less impressive number — but delivers more elemental magnesium to cells than the oxide at three times the dose, because the oxide form's bioavailability is comparable to a placebo in some studies.
The same pattern repeats across nearly every supplement category. Cyanocobalamin at 1000µg looks more impressive than methylcobalamin at 500µg — but for anyone with MTHFR variants, cyanocobalamin cannot be converted to the active form and the 1000µg is largely irrelevant. Ubiquinone at 200mg competes on milligrams with ubiquinol at 100mg — but ubiquinol is the reduced, active form that cells can use directly, while ubiquinone must be converted in the body, with conversion efficiency declining significantly with age.
The outcome is a market full of products optimised for the label rather than for the cell. And a population of supplement users who conclude that nutrients "don't work for them" — when the more accurate conclusion is that the form they took didn't survive the journey.
Form is only half the story. The other half is dose — whether the amount in the product is clinically meaningful after accounting for bioavailability losses.
Many supplements include ingredients at what might be called compliance doses — enough to appear on the label, enough to allow marketing claims, but not enough to produce the clinical effects demonstrated in the research the marketing references. The 50mg ashwagandha in an adaptogen latte references research conducted at 300–600mg of standardised KSM-66 extract. The 10mg CoQ10 in a multivitamin references research conducted at 200–600mg. The 100µg B12 (as cyanocobalamin) references research conducted with methylcobalamin at doses that bypass the conversion limitation.
Dose adequacy requires knowing two things: what dose the clinical evidence used, and what form the product uses. Only then can you assess whether the milligrams on the label translate to a clinically meaningful amount reaching target tissues.
The quality picture is further complicated by the structural changes in the supplement industry over the past decade. The entry of venture capital and private equity into the sector has produced a predictable pattern: acquisition of brands with established quality reputations, pressure to increase margins via cost reduction in ingredient sourcing and manufacturing, and marketing investment to maintain the brand perception while the underlying formulation quality declines.
The result is that a brand you trusted five years ago may be formulating differently today under new ownership — using cheaper ingredient forms, lower doses, and contract manufacturers chosen on price rather than quality. The label may look similar. The product may not be.
The indicators of maintained quality include: named ingredient forms (Albion chelates, Quatrefolic, MK-7, BCM-95 — brand names for specific quality forms that cost more to source); third-party testing certification (NSF, USP, Informed Sport for sports supplements); transparency about manufacturing and testing protocols; and independence from the private equity ownership pattern that consistently produces quality degradation at scale.
The most valuable clinical reframe in the supplement conversation is this: if a nutrient has solid evidence for an effect you expected and didn't receive, the first question is not "does this nutrient work?" but "did this nutrient actually reach the tissue where it needed to work?"
Many people have abandoned zinc for immune function after trying it without effect — when they were taking zinc oxide. Many have dismissed magnesium for sleep and muscle tension after trying it without benefit — when they were taking magnesium oxide, which has absorption so poor it primarily acts as a laxative. Many have tried vitamin B12 for energy and cognitive function without result — when they were taking cyanocobalamin, which cannot be utilised by those with MTHFR variants regardless of dose.
In each of these cases, the nutrient was right. The form was wrong. The experiment was never properly conducted — because the dose on the label never reached the cell that needed it.
The supplement industry has trained consumers to judge products by the number on the front. The number that actually matters — how much reaches your cells — is not on the label. Learning to read what is there, and what it implies about what isn't, is the single most useful skill in supplement purchasing.
There is a more fundamental point beneath all of this — one that this practice is built on. The most reliable supplement stack is the one built from test results, not from population-level recommendations.
Taking the right nutrient, in the right form, at the right dose is significantly more effective than taking a broad supplement protocol that guesses at what you might need. An OAT (Organic Acids Test) reveals functional B vitamin status, mitochondrial cofactor adequacy, oxidative stress burden, and neurotransmitter precursor availability — more specifically than any symptom assessment. A comprehensive blood chemistry panel reveals ferritin, zinc, vitamin D, omega-3 index, B12 functional status, and inflammatory load — the specific deficiencies and insufficiencies that targeted supplementation addresses.
The supplement audit capability in the DH Clinical Concierge exists precisely for this reason. Bring your current supplement list, paste a label, describe what you're taking — and get an honest clinical assessment of whether you're using the right form, at a meaningful dose, for your specific picture. Not what the packaging says it does. What the evidence and your biochemistry suggest it will actually do.
The DH Clinical Concierge can audit your current supplement protocol — ingredient forms, doses, interactions, and what the evidence says about whether what you're taking is clinically meaningful.
Talk to the Concierge