Table 1. Advantages and limitations of evaluation methods for intestinal barrier function.
| Advantages | Limitations | |
|---|---|---|
| Intestinal permeability | ||
| In vivo | ||
| 51Cr-EDTA | Whole intestine, easy detection, not naturally present | Limited use in humans being radio-active, single probe |
| PEGs | Whole intestine | Laborious detection with HPLC, GC-MS or LC-MS |
| Sugars | Combination in multi-sugar tests, widely used | Baseline food contamination, laborious detection with HPLC, GC-MS or LC-MS |
| Sucrose | Specific for stomach | Degraded by sucrase in the duodenum |
| Lactulose | Specific for small intestine | Degraded by intestinal bacteria |
| Mannitol, Rhamnose | Specific for small intestine | Degraded by intestinal bacteria |
| Sucralose | Resistive to bacterial degradation | Long collection time |
| Macromolecules | Correlation with food-related antigens | Limited use in humans |
| Ovalbumin, HRP, dextrans,… | Specific when used in intestinal loops, differently sized probes | Invasive when used in intestinal loops |
| Ex vivo | ||
| Ussing chambers | Region-specific, wide range of molecules, combination with cell lines possible | Need for fresh tissue, limited viability (2 hours), laborious, operator-dependent |
| In vitro | ||
| Intestinal cell lines | Long follow-up times, wide range of molecules, different test conditions | Less representative to the in vivo condition, cell culture variability |
| In situ intestinal barrier defects | ||
| Mucus & tight junctions | Mechanistic view, co-staining with bacteria possible | Does not represent functional characteristics of the barrier |
| Biomarkers | ||
| Urine | ||
| Claudin proteins | Rapid detection of tight junction loss without tissue sections/test molecules | Non-specific for gut (e.g. release from kidney epithelia) |
| FABP | Region-specific dependent on isoform, detectable in urine and blood | Only useful for acute damage |
| α-GST | Detectable in urine and blood | Non-specific, possibly only useful for acute damage |
| Blood | ||
| FABP | Region-specific dependent on isoform, detectable in urine and blood | Only useful for acute damage |
| α-GST | Detectable in urine and blood | Non-specific, possibly only useful for acute damage |
| Citrulline | Specific for enterocytes, not present in exogenous sources | Laborious detection |
| Zonulin | Specific for the small intestine, correlation with IP | Low specificity for detection with ELISA |
| Bacteria and bacterial products | Representation of effective bacterial translocation | Not all bacteria can be cultured, high false positive rate for LPS detection |
| EndoCab | Indirect representation of effective bacterial translocation | Indirect marker for bacterial translocation, correlation with IP conflicting |
| D-lactate | Easy detection | Limited data in humans |
| Faeces | ||
| Calprotectin | Stable in faeces for up to 7 days at room temperature, easily detectable | Indirect marker dependent on presence of intestinal inflammation |
51Cr-EDTA, Crohmium-labelled EDTA; PEG, polyethylene glycol; HRP, horseradish peroxidase; FABP, fatty acid-binding proteins; GST, glutathione S-transferase; EndoCab, endotoxin core antibodies; HPLC, high performance liquid chromatography; GC-MS, gas chromatography mass spectrometry; LC-MS, liquid chromatography mass spectrometry; LPS, lipopolysaccharide; IP, intestinal permeability