This article aims to educate about effective ways to evaluate for increased intestinal permeability. Moreover, we will discuss various diagnostic markers currently available and the pathophysiology of leaky gut.
The small intestinal lining is a single-celled mucosal layer that is the body’s largest and most important mucous membrane surface. GI inflammation and damage to the brush border allow molecules to “leak” directly into the gut-associated lymphoid tissue (GALT) and bloodstream, which activates the immune system. So damage to the brush border can result in poor nutrient absorption, bacterial dysbiosis, and inflammation. All of the above can be contributing factors in many chronic disease processes, including:
- All autoimmune diseases (e.g., celiac disease, rheumatoid arthritis, lupus, type 1 diabetes)
- Endocrine diseases (e.g., polycystic ovary syndrome, type 2 diabetes)
- Neurological disorders (e.g., Parkinson’s disease, multiple sclerosis, schizophrenia)
- Cardiovascular disease
- Allergies and asthma
- All GI disorders (e.g., IBS, Crohn’s disease, ulcerative colitis, celiac disease)
- Neoplastic disease
- Inflammatory diseases (e.g., arthritis and other causes of joint pain)
- Chronic infections
INTESTINAL PERMEABILITY – THE SCIENCE
The intestinal barrier acts to absorb nutrients and water via two distinct mechanisms: Intra/transcellular absorption and paracellular absorption. These mechanisms rely on a system of complex cellular proteins. When functioning correctly, the intracellular transport system and tight paracellular junctions ensure that only appropriate molecules, such as water and nutrients, can pass into the GALT and the bloodstream. The mechanism of transcellular absorption relies on either gradient-based transportation or membrane-driven active transport to bring molecules in and out of the intestinal cells. Paracellular transport uses the actin-myosin cytoskeletal system and three paracellular structures called desmosomes, adherens junctions, and tight junctions, to prevent or allow passage of molecules between the intestinal cells.1 Some believe that smaller molecules pass through the intestinal barrier via active transport intracellularly, while larger molecules will pass paracellularly.
Much research has focused on the particular importance of the paracellular transport mechanisms and their role in increased intestinal permeability. The paracellular proteins were initially thought to act like cement, tightly adhering the intestinal cells together at all times to create a snug intestinal barrier. However, we now know that the tight junction is a dynamic space that can adapt to the body’s absorption needs. The paracellular junctions between the intestinal cells dramatically change in size in response to the modulator, zonulin, with the tight junctions being the most sensitive to this protein.2
Zonulin regulates these tight junctions in the small intestine by acting on the cytoskeleton structure, or “actomyosin.” It changes in size to create paracellular space to accommodate macromolecules a result of physiological or pathological stimulation. Pathogenic bacteria, gliadin, and celiac disease all induce zonulin secretion. Intestinal permeability rapidly increases in response to zonulin, potentially causing an influx of large macromolecules past the barrier. This action is quick and reversible.3 On the other side of the intestinal barrier is the GALT, ready to respond to molecules that require action by the immune system. In a healthy small intestine, the GALT assists in promoting tolerance rather than immune activation. In a pathological situation, the GALT may instead mount an immune response against antigens. A blood test can measure Zonulin. Elevated levels of zonulin indicate increased intestinal permeability and a compromised brush border.2
Lipopolysaccharide (LPS) is one molecule that may access the GALT when zonulin is elevated. LPS is a membrane component of gram-negative enteric bacteria, which can increase pro-inflammatory cytokines when at high amounts in the blood.4 An intact brush-border prevents LPS from interacting with the GALT, thereby reducing the risk of systemic inflammation. LPS antibodies found in the blood reflect GALT interaction with LPS and thus can serve as a useful marker in identifying leaky gut and its severity.5
When the tight junctions are compromised, the actomyosin component becomes exposed. Recall that actomyosin holds the tight junctions together. When the tight junctions begin leaking, exposed actomyosin may trigger antibody production to the degrading actomyosin. You can measure these antibodies to actomyosin in the blood. They may be a valuable marker for detecting mucosal damage that can lead to increased intestinal permeability.6
An older test for intestinal permeability is the lactulose and mannitol (or rhamnose) urinary excretion test.7 Rather than directly measure intestinal permeability, this test reflects the amount and ratio of non-metabolized large (lactulose) and small (mannitol or rhamnose) sugars that are passively absorbed in the intestines and excreted in the urine. Lactulose typically reflects paracellular absorption, whereas mannitol or rhamnose reflects mostly transcellular absorption.8 Higher absorption of these sugars is commonly found in individuals with Crohn’ s/IBD or celiac disease, and in users of NSAIDs, which tend to increase GI permeability.9 The lactulose/mannitol test has been considered the standard test for assessing GI permeability over at least the past decade; however, rates of excretion can vary greatly depending on sugar dosage, the timing of collection, individual patterns of excretion, and NSAID intake.9 Due to this variability, other test markers for intestinal permeability may be more accurate.
ANTI-CDTB & ANTI-VINCULIN ABS
Anti-CdtB (cytolethal distending toxin B) and anti-vinculin antibody testing help diagnose IBS-D (diarrhea-predominant IBS) by distinguishing it from IBD. These antibody tests may be useful for evaluating intestinal permeability as well, given the frequent association between IBS and increased intestinal permeability. CdtB is a toxin from many pathogenic bacteria that cause acute gastroenteritis.10 Vinculin is a cytoplasmic protein that is important in cell signaling and adhesion in many tissues, including the small intestine.11
Frequently, after an acute GI illness, the body can develop antibodies to CdtB that may cross-react with the host cell adhesion protein, vinculin, though molecular mimicry.12 While the antibodies to CdtB may clear the toxin, they remain in circulation and cross-react with vinculin in the intestinal lining; this can continue to cause damage long after the aggravating factor is gone. High levels of anti-CdtB may indicate the presence of IBS-D; these markers may also indicate concomitant increased intestinal permeability.12 Furthermore, levels of circulating antibodies to CdtB and vinculin were shown in a rat model to correlate with a diagnosis of SIBO. 10 While anti-vinculin and anti-CtdB have not been evaluated for diagnosing leaky gut, to this author’s knowledge, such studies could be informative. Similarly, anti-CtdB and anti-vinculin levels appear to be useful in assessing both severity and causative factors of increased intestinal permeability.
In short, using specific lab markers to evaluate increased intestinal permeability can be very useful in an individual who has digestive upset, diarrhea, or chronic constipation. The markers mentioned above help to differentiate IBS from IBD, celiac disease, and other digestive problems. They also offer insight into chronic disease etiologies. Many sufferers of GI illnesses feel a sense of relief when they hear the underlying cause of their chronic ailment is leaky gut, because they may test negative for organic causes of GI disease. The same relief may happen when patients hear that leaky gut is the cause of their GI symptoms, such as constipation, diarrhea, or abdominal pain. Together the above tests can successfully diagnose increased intestinal permeability and thereby point a clinician toward appropriate treatment.
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Sequeira IR, Lentle RG, Kruger MC, Hurst RD. Standardizing the Lactulose Mannitol Test of Gut Permeability to Minimize Error and Promote Comparability. PLoS One. 2014;9(6):e99256. Available at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0099256. Accessed October 10, 2017.
Pimentel M, Morales W, Pokkunuri V, et al. Autoimmunity Links Vinculin to the Pathophysiology of Chronic Functional Bowel Changes following Campylobacter jejuni Infection in a Rat Model. Dig Dis Sci. 2015;60(5):1195-1205.
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Pimentel M, Morales W, Rezaie A, et al. Development and validation of a biomarker for diarrhea-predominant irritable bowel syndrome in human subjects. PLoS One. 2015;10(5):e0126438.