Measuring intestinal permeability: methods, markers and interpretation

Intestinal permeability is often discussed as though it were a single biological quantity that can be measured and compared across studies.

It is not.

A permeability result is produced by a particular method, using a particular probe or marker, from a particular biological compartment, over a particular period of time and in a particular population.

Those details are not technical background.

They determine what the result means.

A urinary sugar ratio, transepithelial electrical resistance, serum intestinal fatty acid-binding protein, circulating lipopolysaccharide-binding protein and a commercial “zonulin” measurement may all appear in studies concerned with intestinal-barrier function. They do not measure the same biological process.^¹–⁵

This is one of the central interpretative problems in permeability research.

The question should therefore not begin with:

Was intestinal permeability increased?

It should begin with:

What was measured — and what aspect of intestinal-barrier biology can that measurement actually represent?

Permeability is a functional property

Strictly understood, permeability concerns the passage of substances across a barrier.

For the intestinal epithelium, this may involve transcellular or paracellular routes, different molecular sizes and charges, and different anatomical regions of the gastrointestinal tract.^¹,²

A functional permeability test therefore asks whether a defined molecule crosses the intestinal barrier differently under specified conditions.

This is conceptually different from measuring epithelial injury, tight-junction-associated proteins, microbial products in the circulation or an inflammatory response.

Those processes may be related to permeability.

They are not permeability itself.

This distinction sounds simple, but much of the literature becomes difficult to interpret precisely because the terminology is broader than the measurement.

Sugar probes provide functional information

Orally administered sugar probes remain among the best-established approaches for assessing intestinal permeability in humans.

The underlying principle is straightforward.

A defined quantity of one or more molecules is ingested. After traversing the gastrointestinal tract, the amount appearing in urine or plasma is quantified. The recovery of the probe provides information about its passage across the gastrointestinal mucosa.^¹,⁶

Dual-sugar tests commonly combine a larger disaccharide, such as lactulose, with a smaller monosaccharide such as mannitol or L-rhamnose.

The ratio between the two is intended to reduce the influence of factors that affect both probes, including dilution, gastric emptying, intestinal transit, distribution and renal handling.^¹,⁶

But the ratio is not a direct image of a tight junction.

It is an integrated physiological measurement influenced by the properties of both probes and by events occurring between ingestion and sample collection.

That distinction is essential.

The denominator matters

A permeability ratio can change because the numerator changes, because the denominator changes, or because both change.

This means that an elevated lactulose-to-mannitol ratio cannot automatically be interpreted as increased lactulose passage through an abnormally permeable epithelium.

Reduced absorption or recovery of the smaller probe may also increase the ratio.

Mucosal surface area, transit, renal clearance and other physiological factors can influence probe recovery.^¹,⁶

The apparently simple ratio therefore contains several biological processes.

Interpretation requires examining the individual probe recoveries as well as the ratio itself whenever the data permit.

Dose can influence what is being measured

The test solution is also part of the experiment.

High concentrations of poorly absorbed sugars can alter luminal osmolarity and potentially influence the physiology being measured. This has contributed to efforts to reduce probe loads and improve analytical sensitivity.

The development of LC–MS-based multi-sugar assays allowed lower doses of lactulose to be used while maintaining analytical detection and simultaneously incorporating additional probes.^⁷–⁹

In a controlled crossover study, van Wijck and colleagues demonstrated that a multi-sugar approach could detect indomethacin-associated changes while providing region-specific information across the gastrointestinal tract.^⁹

The methodological lesson is broader than the particular assay.

A measurement method can perturb the system it is intended to measure.

The composition and dose of the probe solution therefore belong to the interpretation, not merely to the laboratory protocol.

The intestine is not one anatomical compartment

Another limitation of the phrase “intestinal permeability” is anatomical.

The gastroduodenal region, small intestine and colon differ substantially in epithelial physiology, luminal chemistry, microbial density and transit.

A permeability result should therefore be interpreted in relation to the region the method can reasonably assess.

Multi-sugar protocols have attempted to address this by using probes with different absorption and degradation characteristics together with timed sample collection.^⁷–⁹

Sucrose can contribute information about gastroduodenal permeability because it is normally rapidly degraded after leaving the proximal gastrointestinal tract.

Lactulose paired with a monosaccharide such as rhamnose is commonly used for small-intestinal assessment.

Sucralose has been used in approaches intended to capture more distal permeability because of its greater resistance to bacterial degradation.^⁷–¹⁰

Recent systematic methodological work continues to emphasise that probe selection and collection windows are critical to claims of regional specificity.^¹⁰

A result reflecting predominantly small-intestinal permeability should not quietly become a claim about “the gut barrier” as a whole.

Timing changes the biological question

Sample timing is equally important.

Early urinary recovery reflects a different period of gastrointestinal exposure from a 24-hour collection.

Transit varies between individuals and can change with diet, medication, illness and physiological state. A probe intended to represent one intestinal region may contribute information from another if transit and collection windows are not adequately considered.^¹,¹⁰

The time at which permeability is measured relative to an exposure also matters.

A transient increase after exercise, medication or another physiological stressor is not equivalent to a persistent alteration observed at baseline.

Measurement is therefore spatial and temporal.

The same numerical value can have different biological meaning depending on where and when it was generated.

Plasma and urine are not interchangeable specimens

Probe recovery can be measured in urine or blood.

Urine collection integrates probe excretion over time and has historically been widely used. Plasma-based measurements can reduce some practical problems associated with prolonged urine collection and may allow shorter assessment windows.^⁷,⁹

But changing specimen changes the measurement context.

Renal filtration, distribution volume, timing and analytical sensitivity remain relevant.

The specimen is not merely a convenient container for the result.

It is part of the method.

Ex vivo measurements answer another question

Human intestinal biopsies can be studied outside the body using systems such as Ussing chambers.

These approaches preserve native tissue architecture and allow electrical properties and molecular flux to be measured directly across excised mucosa.^¹,¹¹

This provides information that an oral probe test cannot.

The tissue can be exposed experimentally to defined conditions, and transepithelial transport can be examined without many of the systemic variables affecting whole-body probe recovery.

But excision removes the tissue from its normal physiological environment.

Blood flow, neural signalling, endocrine inputs and systemic immune regulation are altered or absent.

An ex vivo permeability result is therefore closer to the tissue but further from the intact organism.

Neither measurement is intrinsically superior.

They answer different questions.

Electrical resistance is not synonymous with permeability

In experimental systems, transepithelial electrical resistance is frequently used as an indicator of epithelial-barrier properties.

TEER is valuable because it provides a functional measure of ionic conductance and can be monitored repeatedly or continuously.

But electrical resistance and molecular permeability are not identical.

Small ion-selective pores and pathways permitting larger molecular flux can be regulated differently. Changes in TEER may therefore occur without proportional changes in macromolecular passage, and vice versa.^²

The distinction becomes particularly important when findings from cell or organoid models are translated into statements about human intestinal permeability.

A decrease in TEER is evidence about electrical resistance across that experimental epithelial system.

It should not automatically become a clinical claim about “leaky gut”.

Biomarkers broaden the field — and the terminology

The difficulty of direct functional testing has encouraged increasing use of blood and faecal biomarkers.

Recent literature reflects how broad this field has become.

A 2025 review of body-fluid approaches to intestinal-permeability assessment includes sugar probes alongside endotoxin, D-lactate, intestinal fatty acid-binding protein, diamine oxidase, citrulline, zonulin and lipopolysaccharide-binding protein.^⁴

The practical attraction is clear.

Blood or stool samples can be easier to obtain than prolonged functional testing, and biomarkers may provide information about biologically important processes related to the intestinal barrier.

The interpretative danger is equally clear.

Placing several markers under the heading “intestinal permeability biomarkers” does not make them measures of the same phenomenon.

I-FABP reflects epithelial injury

Intestinal fatty acid-binding protein is a small cytosolic protein expressed predominantly in mature intestinal enterocytes.

When enterocytes are injured, I-FABP can be released into the circulation. It has therefore been used as a marker of intestinal epithelial injury in settings including ischaemia, inflammatory disease and acute physiological stress.^⁴,⁵,¹²

This can be highly informative.

But epithelial injury is not identical to permeability.

Injury may contribute to altered barrier function and increased molecular passage. A rise in I-FABP may therefore sit within the same biological process as increased permeability.

It remains a different measurement.

Calling I-FABP a direct permeability marker collapses that distinction.

Citrulline reflects epithelial mass rather than molecular passage

Circulating citrulline provides another example.

Citrulline is produced substantially by small-intestinal enterocytes and has been used as an indicator of functional enterocyte mass, particularly in conditions involving extensive mucosal loss or intestinal failure.^⁴,⁵

A low citrulline concentration may therefore provide clinically meaningful information about epithelial capacity.

It does not directly measure paracellular permeability.

Again, the marker is relevant to barrier biology without being equivalent to the property of permeability itself.

D-lactate and microbial products represent another level

D-lactate is generated largely through bacterial metabolism and has been investigated as a marker associated with intestinal-barrier disturbance and microbial translocation.^⁴

Endotoxin or lipopolysaccharide measurements move the inference still further towards microbial exposure.

If bacterial products appear systemically, one possible explanation is increased passage across a compromised intestinal barrier.

But systemic concentrations are also influenced by microbial production, hepatic clearance, immune binding, sampling, assay methodology and other biological processes.

The measurement therefore lies downstream of the epithelial barrier.

It can support a biological interpretation.

It does not directly quantify permeability.

LBP and soluble CD14 reflect host response to microbial exposure

Lipopolysaccharide-binding protein and soluble CD14 are frequently discussed in relation to microbial translocation.

These molecules participate in host responses to bacterial products and can provide indirect information about exposure to microbial components.^⁵

A major 2026 review of blood-based gut-barrier biomarkers makes the broader issue particularly clear: I-FABP, citrulline, zonulin, LBP and soluble CD14 represent different aspects of epithelial damage, epithelial mass, tight-junction-related biology, microbial exposure and host response.^⁵

No single one captures intestinal-barrier function in full.

This is precisely why a panel may eventually prove more informative than a single marker.

But a panel is only useful if the biological meaning of its components remains visible.

Combining five indirect markers does not turn them into a direct permeability measurement.

Diamine oxidase requires similar caution

Diamine oxidase is highly expressed in intestinal mucosal cells and circulating DAO has been investigated as an indicator of mucosal integrity or injury.^⁴

As with other indirect markers, its interpretation depends on the biological context, assay performance and the processes influencing circulating concentrations.

The methodological principle remains the same:

association with intestinal-barrier disturbance does not establish equivalence with intestinal permeability.

Zonulin is a particularly important example

Zonulin-related measurements deserve separate attention because the term is used at several different levels.

There is a biological pathway associated with regulation of tight junctions and paracellular permeability.^¹³

There is a molecular identity historically linked to pre-haptoglobin 2.

There are commercial assays marketed as measuring “zonulin”.^¹⁴

And there are human studies in which a reported serum or faecal concentration is interpreted as a measure of intestinal permeability.

These levels cannot simply be collapsed.

Ajamian and colleagues demonstrated that commercial assays used as serum zonulin measurements did not necessarily detect pre-haptoglobin 2, reinforcing concerns about treating an assay result as a direct measurement of the proposed biological molecule or pathway.^¹⁵

This does not invalidate zonulin-related barrier biology.

It changes what can be inferred from a particular measurement.

Because the issue is sufficiently important, zonulin as pathway, biomarker and measurement is considered separately in the next BIRNA Research article.

My own systematic review exposed the same problem

The distinction between biological concept and measurement was central to my systematic review and meta-analysis of zonulin-dependent intestinal permeability in children diagnosed with mental disorders.^¹⁶

The included literature used zonulin-related measurements to investigate a possible relationship between intestinal permeability and psychiatric or neurodevelopmental conditions.

The review identified a signal across parts of the literature, but it also exposed substantial heterogeneity in populations, study designs and methodology.^¹⁶

That heterogeneity matters.

If the underlying assay does not measure the biological process with the specificity assumed by the interpretation, pooling results cannot repair that problem.

Meta-analysis can increase statistical precision.

It cannot create measurement validity.

This is one reason why permeability research must remain method-aware even when the evidence is synthesised quantitatively.

Population and context influence the result

Permeability is not measured in an abstract human.

Age, diet, medication, inflammatory state, exercise, infection, disease activity and other exposures can influence intestinal physiology.^¹

Non-steroidal anti-inflammatory drugs provide a useful example because they can increase gastrointestinal permeability even in otherwise healthy individuals.^¹,⁹

Intense endurance exercise can also produce transient alterations in intestinal injury and permeability under conditions involving heat stress and altered splanchnic perfusion.^¹

These observations demonstrate why a permeability difference between two groups is not automatically evidence of a stable intrinsic barrier defect.

Context matters.

So does the timing of the measurement relative to that context.

Analytical performance is part of biological interpretation

Every measurement also depends on an analytical method.

Chromatography, mass spectrometry, immunoassays and other detection platforms have different sensitivity, specificity and susceptibility to interference.

The evolution from older sugar-detection techniques towards more sensitive LC–MS methods made it possible to reduce probe doses and quantify multiple sugars simultaneously.^⁷–⁹

Similarly, advances in blood-based biomarker detection may improve sensitivity and permit increasingly complex marker panels.^⁴,⁵

But analytical sensitivity is not biological specificity.

A highly reproducible assay can precisely measure the wrong molecular target.

A low detection limit does not establish that the analyte is a valid surrogate for the biological property of interest.

The analytical question is therefore:

Can we measure this accurately?

The biological question is:

Does this measurement represent what we say it represents?

Both must be answered.

Correlation between methods does not make them identical

Studies sometimes compare a biomarker with a functional permeability test and report a correlation.

Such comparisons are useful.

But even a strong correlation does not establish equivalence.

Two measurements may respond to the same underlying biological process while capturing different aspects of it.

Conversely, weak correlation may not necessarily mean that one method is invalid. The measurements may operate at different biological levels, anatomical regions or time scales.

Method comparison therefore requires a mechanistic understanding of what each measurement is expected to reflect.

Without that, agreement and disagreement are both difficult to interpret.

The absence of a universal gold standard matters

A recurring problem in intestinal-permeability research is the absence of one universally accepted reference method against which every other measure can be validated.^¹⁷

A recent systematic review of tools for assessing intestinal epithelial-barrier integrity reflects this methodological diversity, spanning non-invasive and invasive in vivo approaches together with ex vivo and in vitro methods.^¹⁰

That diversity is not merely a sign that the field has failed to choose the correct test.

It reflects the fact that “intestinal-barrier function” contains several biological dimensions.

A functional sugar-permeability test, histological assessment, TEER, molecular flux, epithelial-injury marker and microbial-translocation marker cannot all be reduced to one latent variable without losing biological information.

The absence of a universal gold standard should therefore make interpretation more precise, not more permissive.

A measurement should be named before it is interpreted

A simple discipline can improve the reading of permeability studies.

Before interpreting a result, name the measurement.

Not:

intestinal permeability was increased.

But:

urinary lactulose-to-rhamnose recovery was increased during a defined collection window.

Or:

circulating I-FABP was elevated.

Or:

TEER decreased in an epithelial monolayer.

Or:

a commercial assay reported a higher serum “zonulin” concentration.

Only then should the biological interpretation begin.

This language may seem less elegant.

It is scientifically more informative.

It preserves the distance between observation and explanation.

From measurement to meaning

The methods used to study intestinal permeability are not interchangeable windows onto the same object.

They reveal different parts of a living system.

Functional probes tell us about molecular passage.

Electrical measurements tell us about conductance in a defined epithelial preparation.

Tissue studies preserve architecture while removing systemic physiology.

I-FABP can inform us about epithelial injury.

Citrulline can reflect enterocyte mass.

LBP and soluble CD14 can reflect host responses associated with microbial exposure.

Zonulin-related assays introduce a separate set of questions about pathway, molecular identity and analytical specificity.

Each can contribute evidence.

None should silently inherit the meaning of another.

For that reason, the most important step in interpreting an intestinal-permeability study often occurs before looking at the result.

Ask what was measured.

Then ask where, when, in whom and by which method.

Only after those questions have been answered can the result be given biological meaning.