Intestinal permeability (IP) is an underlying pathophysiologic finding to varying degrees in most, if not all, individuals with chronic inflammatory conditions (CICs). As Functional Medicine practitioners, we’re routinely assuming and addressing IP, even though we might not have tested for it directly. Yet, there’s a world of non-invasive biomarkers out there to guide treatment, several of which you’re probably already ordering.
CICs together form a pretty large bucket, which includes:
- Cardio-metabolic diseases
- Autoimmune diseases, including IBD, MS, rheumatoid arthritis, celiac, type I diabetes
- Allergic and atopic diseases including food allergy and asthma
- GI disorders like SIBO and IBS
- Neurological conditions such as autism, schizophrenia, Alzheimer’s and Parkinson’s
Start with an assessment of inflammation:
In Functional Medicine, when someone presents with CIC we generally cast a wide net with laboratory biomarkers, understanding that the drivers of inflammation can be varied and unique to the individual, regardless of clinical diagnosis. Off the top of my head, here are some tests I’ll consider depending on what my patient is presenting with:
ESR, hsCRP, cytokines (such as IL-6, IL-17, TNF-alpha, INF-gamma, etc), complement proteins, CBC with manual differential, PLA2, MPO, urine quinolinate, food or inhalant allergies, food sensitivities, immunoglobulin levels, autoantibodies (including ANA, RF, CCP, thyroid antibodies, celiac antibodies, and on), gut microbiome, gut inflammatory markers (like calprotectin or secretory IgA), infectious causes of inflammation (such as Lyme and co-infections, EBV, mycotoxins, etc), arachidonic acid, and omega 3 fatty acids.
It’s a big, if incomplete, list. There are quite a few ways we can get a reasonably solid snapshot of what’s happening with systemic inflammation in our CIC patients. You’ll see that a number of these labs, such as calprotectin, GI microbiome and so forth, also reflect the state of GI health.
What chronic inflammatory conditions don’t have IP as an underlying pathophysiologic process?
For those with CICs, I assume IP is present to some degree (whether I have laboratory data or not) which is yet another reason why, in Functional Medicine, we can’t go wrong if we start with the gut. That said, given the importance of gut health in CICs, SOME assessment is wise here.
What about a reliable evaluation of intestinal permeability (IP)?
The physical aspect of the mucosal barrier includes the cellular component comprised of the vascular endothelium, the epithelial cell lining and the mucus layer. Next to the physical is the chemical barrier, including digestive secretions, immune molecules, antimicrobial peptides, nutrients. Of course, the microbiota is actively in the mix too, playing a major role in barrier wellness.
Laboratory options for the assessment of mucosal barrier functionality
In the clinical setting, reliable assays for the diagnosis of IP evaluation are scant. Old stand-by tests like the lactulose/mannitol ratio, considered to be the gold standard laboratory assessment, are considered difficult to accurately conduct outside of the research setting, and even if done correctly, the L/M ratio may actually miss diagnosing certain types of IP.
But, understanding that the mucosal barrier is remarkably complex, there are plenty of surrogate biomarkers we can look at. Indeed, if you’re practicing FxMed, you’re probably already looking at a number of these in the normal course of a work-up. Here’s a list of analytes that while not necessarily validated for diagnosing intestinal permeability, can certainly offer clues to barrier functionality and provide direction for individualized treatment.
Lactulose/mannitol test: As mentioned above, while wearing the lofty title of “gold standard”, there is reason to believe that this test probably isn’t quite as golden as we’d hoped. That said, there is more research published on it than any other IP laboratory biomarker. If you use the L/M ratio, consider coupling it with some other barrier assessment(s) for confirmation.
Serum zonulin. Zonulin is considered a “master regulator” of intestinal permeability, the “only physiologic modulator of intercellular tight junctions discovered so far…” Protracted elevation of zonulin is the problem: If the gate is open too long, damaging GI inflammatory debris enters into circulation. And importantly, zonulin may elevate long before an autoimmune disease is present, providing a window of treatment to prevent disease expression or progression. A direct measurement of zonulin in serum, an ELISA test developed by Alessio Fasano—the scientist who first discovered zonulin and its role in IP—is now more widely available and may prove to be the most useful of all laboratory biomarkers for diagnosing IP.
Fecal zonulin has just been released by a couple of labs, although research on it is very limited at this point. I am curious about how this marker will play out. Since zonulin is released locally at the mucosal barrier, if the protein remains intact during transit, a fecal measurement could prove really useful, sensitive and specific. Here’s one clinical study showing a reduction of FZ levels in athletes using a probiotic blend.
Zonulin/occluding/actinomycin antibodies: There is at least one case report involving these antibodies, but no clinical research to my knowledge, nor are these validated for diagnosing intestinal permeability. Interestingly, Fasano podcasted with Chris Kresser in 2012, where he expressed his skepticism over the utility of these tests as general markers of IP, but I personally would say the jury is still out. I would love to see a clinical trial and a comparison with serum zonulin.
Gut microbiome: Dysbiosis can trigger release of zonulin, pro-inflammatory cytokines and loss of tolerance, ushering in the development of chronic inflammatory conditions of all stripes. The role of dysbiosis cannot be understated in the development of CICs. Indeed, Fasano himself points out multiple cases in his practice where the development of full-blown celiac disease in highly genetically susceptible individuals didn’t occur until adulthood (and two specific reports of celiac onset delayed until the 7th decade!) despite regular gluten consumption. In an individual who has the genetic predisposition and is exposed to the environmental trigger, Fasano believes it’s a healthy, resilient gut microbiome that separates those without the disease from those who develop it.
IgE allergies: The IgE sensitization process to foods (the first step in the development of IgE food allergies) requires intestinal permeability—even a single food allergy requires barrier permeability. Thus, when we see IgE food allergies in our patients, it’s absolutely logical to consider IP, especially if they are still consuming the problem foods. Further, zonulin elevation is found in the development and propagation of allergic diseases. Here’s a great paper on IgE food allergies and barrier permeability.
IgG foods sensitivities: Akin to the permeability requirement for IgE allergy development, In the FxMed world, multiple IgG food sensitivities are suggestive of IP. However, this interpretation has not yet been demonstrated. Perhaps now, as serum zonulin is more widely available, an industrious clinician will undertake this investigation! Here’s a 2017 paper positing the hypothesis that IgG food hypersensitivity plus IP are involved in pathogenesis of depressive disorders.
IgG food sensitivity testing is available through multiple labs, including standard reference labs.
**Note that the corollary barrier protein skin, nasopharynx and lungs is called filaggrin. Mutations to filaggrin are associated with increased permeability in these areas and the development of allergic disease (including, in very sensitive individuals, to foods). We do indeed sometimes test for filaggrin mutations in our clinic.
Butyrate: This highly-prized four-carbon saturated fatty acid found in butter (butyrate is from the Greek word βούτυρο meaning butter) is an essential nutrient for mucosal barrier maintenance. Look for a healthy microbiome, filled with butyrate-producing bugs like Faecalibacterium prausnitzii, Akkermansia muciniphila, genus Roseburia, Clostridia species and loads of others. Keep these butyrate-producing bugs thriving and doing their job with a prebiotic-rich diet of foods known to increase butyrate.
Butyrate is readily assessed in most comprehensive stool tests. These same tests will include at least a few butyrate-producing bacterial species.
Lipopolysaccharide (LPS, endotoxin): From gram-negative bacterial cell wall components, translocated LPS (LPS in circulation) is clearly indicative of a barrier breach. Further, circulating LPS elicits a strong immune response, including, but not limited to, binding TLR-4 receptors which potently upregulate NFKb. LPS may be tested for in serum, although my original go-to lab doesn’t seem to offer it right now. I’m tracking down a new laboratory that does a good job with LPS. I will keep you posted.
D-Lactate. Like LPS, elevated circulating D-lactate may suggest mucosal barrier damage. D-lactate is a product of GI bacterial fermentation by many bugs, including most famously Lactobacillus acidophilus. Under certain conditions, (best researched is short bowel syndrome), D-lactate can rise to dangerously high levels, risking D-lactic acidosis. However, less emergent circulating levels of D-lactate suggest GI dysbiosis and mucosal barrier damage, with the potential for extra-intestinal symptomatology. Because of the higher quantity, D-lactate measurement in urine may be preferable to plasma. Urine D-lactate is readily available through a number of specialty labs and Mayo Clinic’s laboratory.
Diamine oxidase: DAO oxidizes diamines including histamine, putrescine and cadaverine in the gut. DAO in serum (primarily sourced to intestinal mucosa, although we do make DAO elsewhere to a lesser extent) may inversely correlate with IP of the small intestine. Indeed, damage to the apical end of the high-activity enterocyte villi results in lower DAO output, and may therefore be reflected as lower serum DAO. Honzawa et al demonstrated in a small study that DAO is always decreased in individuals with IBD, regardless of disease activity. Thus, low serum DAO may suggest not only small intestinal permeability, but also an impaired abilityto digest or tolerate histamine-containing foods.
Fecal calprotectin: Calprotectin is a calcium- and zinc-binding protein complex with antimicrobial properties found primarily in the cytosol of neutrophils, but also in macrophages and monocytes. Activated neutrophils, recruited to the site of intestinal injury, release calprotectin–which resists enzymatic degradation– into the feces. An elevated fecal calprotectin indicates marked intestinal inflammation, and may require Gastroenterology referral. Classically, a markedly elevated calprotectin has been associated with IBD and possibly a number of GI cancers. However, a newer study demonstrates that a subset of IBS patients may have slightly elevated calprotectin, too.
Secretory IgA (sIgA): Humans secrete about 3 grams of sIgA into the GI tract daily with a robust amount found in the mucus layer of the mucosal barrier where it neutralizes the microbial threats before they reach epithelial cells. Microbial-specific IgA limit penetration of GI bacteria into circulation, whereas IgA deficiency can lead to breach of intestinal barrier via increased penetration of GI microbes.
Fecal and salivary sIgA are readily available. Fecal sIgA is included on most comprehensive stool tests.