The Bioavailability Problem: Why Actives Don't Always Absorb

Standardisation answers the question “how much active is in the dose?” Bioavailability answers a different and harder one: “how much of that active actually reaches systemic circulation, and in what form?” The two are routinely confused. A formulator sees “95% curcuminoids by HPLC” on a Certificate of Analysis and reasons about efficacy as if every milligram were available to the body. In reality, many of the most valued botanical actives are absorbed poorly, erratically, or only after extensive chemical modification by the gut and liver. This article maps why that happens, what can be done about it, and — most usefully — how to interrogate a supplier who claims to have solved it.

Throughout, this is a discussion of physicochemical behaviour and pharmacokinetics (PK), not of clinical outcomes. The point is not what an active does in the body but whether it gets there at all — a formulation-science question that any product developer can and should reason about quantitatively.

Label claim is not systemic exposure

Oral bioavailability — conventionally written F — is the fraction of an administered dose that reaches the systemic circulation intact. By definition F is 1.0 (100%) for an intravenous dose and almost always lower, often far lower, for an oral one. For many botanical actives F is in the low single-digit percent. That means a dose carrying a beautifully characterised marker can still present the body with a small and variable amount of circulating compound.

The gap between content and exposure is governed by a sequence of hurdles a molecule must clear after it is swallowed: it has to dissolve in gastrointestinal fluid, survive the chemical environment of the gut, permeate the intestinal wall, escape being pumped back out, evade transformation by gut microbiota, and then survive first-pass metabolism in the gut wall and liver before it ever reaches general circulation. A marker assay measures none of this. It measures what is in the powder. Everything downstream is where bioavailability is won or lost.

Why phytochemicals absorb poorly

Poor oral absorption is rarely caused by one defect. It is usually the product of several overlapping limitations, and useful diagnosis means naming each one because the right enhancement strategy depends on which barrier dominates.

Low aqueous solubility

A molecule can only be absorbed from solution. Many botanical actives — curcuminoids, resveratrol, boswellic acids, many flavonoid aglycones — are highly lipophilic and barely dissolve in the watery contents of the gut. If only a fraction dissolves within the limited transit time of the small intestine, only that fraction is even available to cross the gut wall. Dissolution, not permeation, is the rate-limiting step for these compounds.

Poor membrane permeability

Other actives have the opposite problem: they dissolve readily but cross the intestinal membrane slowly. Large, highly polar molecules — many intact glycosides and saponins — struggle to traverse the lipid bilayer of the enterocyte. Their size and polarity, the same properties that keep them in solution, work against passive diffusion across the gut wall.

These two axes — solubility and permeability — are the basis of the Biopharmaceutics Classification System (BCS), a framework borrowed from pharmaceutical science that is a clarifying lens for botanicals. Class I compounds are well-behaved (high solubility, high permeability). The problem children are Class II (low solubility, high permeability — dissolution-limited) and Class IV (low solubility, low permeability — the hardest case). A large share of difficult botanical actives sit in Class II or IV, which immediately tells the formulator where to aim.

First-pass metabolism

Even a molecule that dissolves and permeates is not home. Absorbed compounds drain via the portal vein straight to the liver before reaching general circulation, and the gut wall itself is metabolically active. Phase II conjugation — glucuronidation and sulfation in particular — rapidly modifies many polyphenols. Quercetin and resveratrol are heavily conjugated; much of what is “absorbed” circulates as metabolites rather than the parent compound. This is why a plasma assay that only measures the parent molecule can dramatically understate total exposure, and why the choice of analyte matters when reading PK data.

Efflux transporters (P-gp)

The enterocyte membrane carries active efflux pumps, most notably P-glycoprotein (P-gp), which recognise certain molecules and pump them back into the gut lumen as fast as they diffuse in. For a P-gp substrate, net absorption is the difference between passive influx and active efflux — and that difference can be small. Efflux also couples with metabolism: a molecule pumped out and back in is exposed repeatedly to gut-wall enzymes, amplifying first-pass loss.

Gut microbiota transformation

The colonic microbiota is a vast and variable metabolic reactor. It cleaves glycosides to aglycones, hydrolyses, reduces and ring-opens many phytochemicals, sometimes producing the species that is actually absorbed and sometimes degrading the active before it can be. Because microbiota composition varies enormously between individuals, microbiota-dependent absorption is a major source of the high inter-subject variability seen in botanical PK studies. It also means an in vitro assay with no microbial component can mispredict what happens in vivo.

Large, polar glycosides

Many actives occur in the plant as glycosides — the aglycone bound to one or more sugars. Glycosylation improves water solubility but adds size and polarity that hinder passive permeation, and the intact glycoside frequently must be deglycosylated (by gut enzymes or microbiota) before the aglycone can be absorbed. The form in the extract therefore shapes both the rate and the route of absorption, which is one more reason two extracts at the same marker percentage can behave differently.

Enhancement strategies and their trade-offs

A family of formulation technologies exists to overcome these barriers. Each works by a definable mechanism and each carries trade-offs in cost, stability, manufacturability and the strength of evidence behind it. The serious formulator matches the mechanism to the diagnosed barrier rather than chasing whichever technology is fashionable.

Particle-size reduction: micronisation and nanonisation

Dissolution rate scales with surface area, so milling a poorly soluble solid to micron (micronisation) or sub-micron (nanonisation, nanocrystals) particles speeds dissolution and can raise absorption for dissolution-limited Class II actives. It is conceptually simple and adds no novel excipient chemistry. The trade-offs: very fine powders are dusty, cohesive and hard to handle; nanocrystals can re-aggregate or grow during storage (Ostwald ripening) unless stabilised; and size reduction alone does nothing for a compound whose true solubility ceiling is low.

Phytosome / phospholipid complexes

Here the polyphenol is complexed with a phospholipid (typically phosphatidylcholine) at a defined stoichiometry. The phospholipid acts as a chemical chaperone, raising the active's affinity for the lipid-rich enterocyte membrane and improving its passage across the gut wall. The format is a stable, dry-blendable powder, which is an advantage for capsules and tablets. Trade-offs: the phospholipid carrier dilutes the active per gram, the complex must be properly characterised (complexation ratio, phospholipid content), and the lift is active-specific.

Self-emulsifying systems (SEDDS / SMEDDS)

Self-emulsifying drug delivery systems (SEDDS) and their self-microemulsifying cousins (SMEDDS) are isotropic blends of oil, surfactant and co-solvent that spontaneously form a fine emulsion or microemulsion on contact with gut fluid, presenting the lipophilic active already dissolved in dispersed droplets. This sidesteps the dissolution bottleneck for Class II/IV actives and can recruit lipid-absorption pathways. They are among the more effective approaches for lipophilic compounds. Trade-offs: high surfactant loads, formulation complexity, a tendency to suit liquid or softgel formats rather than dry powders, and the risk of the active precipitating if the system is diluted or designed poorly.

Cyclodextrin inclusion complexes

Cyclodextrins are ring-shaped oligosaccharides with a hydrophobic interior cavity that can host a lipophilic guest molecule, presenting a water-soluble inclusion complex to the gut. This raises apparent solubility for suitably sized actives. Trade-offs: only molecules that fit the cavity benefit, the cyclodextrin adds considerable mass per unit of active, and complexation efficiency must be demonstrated rather than assumed.

Solid dispersions and amorphous forms

Many actives are far more soluble in their disordered amorphous state than as a crystal, because no crystal-lattice energy must be overcome to dissolve. Amorphous solid dispersions — the active molecularly dispersed in a polymer matrix — exploit this to generate supersaturation and faster dissolution. Trade-offs: amorphous forms are thermodynamically unstable and can recrystallise over shelf life (the polymer's job is to suppress this), so physical-stability data over time is essential, and processing (e.g. spray-drying, hot-melt extrusion) is more demanding.

Lipid-based delivery

Beyond SEDDS, a broader class of lipid formulations — simple oil solutions, emulsions, solid lipid nanoparticles — keeps a lipophilic active in a dissolved or finely dispersed lipid environment and can promote lymphatic uptake, partly bypassing first-pass hepatic metabolism. Trade-offs: oxidative stability of the lipids, format constraints (often liquid/softgel), and lift that depends on the lipid type and dose.

Co-administered bioenhancers (e.g. piperine)

Rather than modifying the active, a bioenhancer is co-administered to change how the body handles it. Piperine, from black pepper, is the best-known example. Its proposed mechanisms are pharmacokinetic: inhibition of Phase II conjugating enzymes (notably UDP-glucuronosyltransferases) in the gut and liver, slowing the conjugation that otherwise clears compounds such as curcumin, and modulation of efflux transport. By reducing first-pass loss, more parent compound survives to circulate. Trade-offs are important to state plainly: a bioenhancer that inhibits metabolising enzymes and transporters can, in principle, alter the disposition of other co-administered substances, so the mechanism that helps is the same mechanism that warrants caution about interactions. The effect is also active-specific and dose-specific.

Salt and cocrystal forms

For ionisable actives, forming a salt can sharply increase aqueous solubility and dissolution rate; for those that do not ionise usefully, a cocrystal (the active co-crystallised with a benign partner molecule) can achieve a similar improvement by engineering a more soluble solid form. Trade-offs: not every molecule has a tractable ionisable group or a viable coformer, the new solid form must be characterised and shown to be stable, and solubility gains do not help a compound that is permeability-limited rather than solubility-limited.

Evaluating bioavailability evidence critically

Enhancement claims are supported by data at very different levels of rigour, and the level matters enormously. The hierarchy runs from cheap, abstract in vitro measurements to expensive, definitive in vivo pharmacokinetics, and a claim is only as strong as the highest tier of evidence behind it.

In vitro solubility and dissolution

The first tier measures whether and how fast the active dissolves in simulated gastrointestinal media. These tests are quick and informative for solubility-limited compounds — a formulation that demonstrably dissolves faster has cleared a real barrier. But solubility is necessary, not sufficient: dissolving better says nothing about permeating, surviving metabolism, or escaping efflux. A solubility result alone cannot substantiate a bioavailability claim.

Cell-based permeability (Caco-2)

The Caco-2 monolayer — a sheet of human intestinal-type cells — models permeation across the gut wall and, because these cells express transporters including P-gp, can flag efflux and estimate apparent permeability. It is a valuable mechanistic screen that adds the permeability dimension solubility tests lack. Its limits: it is a simplified static model with no microbiota, no realistic luminal dynamics, and incomplete metabolic enzyme expression, so it predicts directionally rather than absolutely.

In vivo pharmacokinetics

The definitive tier is an in vivo PK study that measures the active (and ideally its relevant metabolites) in plasma over time, yielding the parameters that actually describe exposure: Cmax (the peak plasma concentration), AUC (area under the concentration–time curve, the integral of total exposure), and Tmax (the time to peak). Relative bioavailability compares the AUC of the enhanced formulation against a fair unformulated comparator at the same dose. A credible “X times more bioavailable” claim should resolve to a relative-AUC figure from a controlled study — not an in vitro number dressed up as an absorption result.

The recurring trap is to let a lower tier of evidence stand in for a higher one — to quote a dramatic solubility improvement or a Caco-2 flux as though it were proven systemic exposure. In vitro data is genuinely useful for mechanism and screening, but it cannot by itself establish that more active reaches the bloodstream. Only PK can do that.

Absorption challenges by compound class

Naming the barrier for a few well-studied classes makes the framework concrete. These are descriptions of absorption behaviour, not statements about effects in the body.

Curcuminoids

The textbook poorly absorbed active. Curcuminoids combine very low aqueous solubility (a dissolution-limited Class II profile), chemical instability at intestinal pH, rapid and extensive Phase II conjugation, and brisk elimination. The result is low and variable plasma exposure of the parent compounds from an unformulated extract. This is precisely why curcumin is the most common target for the enhancement technologies above — phospholipid complexes, lipid and self-emulsifying systems, particle-size reduction, and piperine co-administration are all routinely applied to it.

Boswellic acids

The boswellic acids of Boswella are large, lipophilic pentacyclic triterpenoids with poor aqueous solubility, and the individual acids differ markedly in how readily they appear in plasma — some are detected at far higher concentrations than others after oral dosing. The practical consequence for a formulator is that the standardised marker may not be the species that achieves the best exposure, so the marker on the COA and the absorbed profile can diverge.

Quercetin

Quercetin illustrates the glycoside-and-metabolism story. It occurs in plants largely as glycosides whose absorption depends on deglycosylation by gut enzymes and microbiota, the rate varying with the specific sugar attached. Once absorbed, the aglycone is extensively conjugated, so circulating material is mostly metabolites rather than free quercetin — which means PK studies must specify whether they measured free aglycone or total (deconjugated) quercetin, or the numbers are not comparable.

Resveratrol

Resveratrol is reasonably permeable but is metabolised so rapidly and extensively — glucuronidated and sulfated in the gut wall and liver — that the parent compound's systemic exposure is low and short-lived despite decent absorption. It is the classic case where the barrier is first-pass metabolism rather than solubility, so the appropriate levers are metabolic and efflux strategies rather than pure solubility enhancement.

What to ask when a supplier claims 'enhanced bioavailability'

“Enhanced bioavailability” is among the most over-used and under-substantiated phrases in the ingredient trade. Work through this sequence before accepting it; the order moves from defining the claim to verifying it.

1. 1Enhanced relative to what? Identify the comparator formulation and confirm it is a fair one — a standard unformulated extract at the same active dose, not a deliberately weak control.
2. 2Which barrier does the technology address — solubility, permeability, first-pass metabolism, or efflux — and does that match the known limitation of this active?
3. 3What is the evidence tier: in vitro solubility/dissolution, Caco-2 permeability, or in vivo PK? Insist on seeing the highest tier claimed.
4. 4For any in vivo claim, what are the actual PK parameters — relative AUC, Cmax, Tmax — the dose, the number of subjects, and the study design?
5. 5What analyte was measured — parent compound, specific metabolites, or total (deconjugated)? A multiple based on total exposure is not comparable to one based on parent compound.
6. 6Does the enhanced material carry the same standardised marker and method as the conventional grade, so you are comparing like with like on content as well as exposure?
7. 7How does the technology affect the format and dose — carrier dilution, mass per serving, capsule vs softgel vs liquid — and does that fit your finished dosage form?
8. 8What is the physical stability of the enhanced form over shelf life (e.g. recrystallisation of an amorphous dispersion, particle growth of a nanocrystal, oxidation of a lipid system), with data to support it?
9. 9If a bioenhancer is included, what is its mechanism and dose, and have interaction and labelling implications been considered for the destination market?
10. 10Can the supplier provide the primary study, not a marketing summary — so you can read the methods, the comparator and the statistics yourself?