A single layer of contiguous cells separates the inside of the body from the external environment, protecting it from potentially harmful microbes and antigens. This intestinal barrier is composed of a lining of epithelial cells connected by tight junctions that seal the paracellular spaces between enterocytes. The tight junctions, in turn, are made up of transmembrane proteins, including claudins, occludin and junctional adhesion molecules, and zonula occludens proteins that link the transmembrane proteins to the cell cytoskeleton. Originally considered static, tight junctions are now known to be dynamic gatekeepers, opening and closing in response to dietary factors, neuronal signals and pathogens.
The importance of the intestinal barrier became apparent in the 1990s when researchers injected extracts from luminal bacteria directly into the colonic wall of rats. This triggered both systemic inflammation and an inflammatory disorder in the bowel resembling Crohn's disease (CD). Since then, loss of intestinal barrier integrity has been shown to affect immune homeostasis and play a key role in the development of a variety of conditions, including inflammatory bowel disease, irritable bowel syndrome (IBS), celiac disease, Clostridium difficile infection, graft-versus-host disease, type 1 diabetes mellitus, rheumatoid arthritis and other autoimmune disorders.
The current thinking is that the pathogenesis of autoimmune disease depends on three factors:
- Genetic susceptibility
- Exposure to a triggering non-self antigen
- Loss of the protective function of the epithelial barrier
Celiac disease is a classic example of autoimmunity because of its close association with HLA environmental genes, an autoimmune response against tissue transglutaminase and the presence of a known environmental trigger, gluten. It is thought that exposure to gluten (gliadin) upregulates zonulin, a novel protein known to open tight junctions. This allows more antigens to enter the gut submucosa, triggering a gluten-specific autoimmune response in genetically predisposed people.
Despite the immense interest in gut permeability and its pathogenesis — an interest that has generated thousands of research papers in the last decade — much remains unknown. Maria I. Vazquez Roque, M.D., a gastroenterologist at Mayo Clinic's campus in Jacksonville, Florida, who has studied the effect of gluten-free and gluten-containing diets on intestinal permeability, says even the timing of the barrier defect remains somewhat in doubt.
"Does intestinal permeability occur before clinical onset of disease or after it as a consequence of mucosal inflammation? Some data in Crohn's disease suggest that increased intestinal permeability is a very early event in disease pathogenesis and precedes the development of symptoms. Persistent increased intestinal permeability has also been shown to be a risk factor for relapse in CD. Still, the timing of the barrier defect — even in celiac disease — is not well-understood," she says.
Dr. Vazquez Roque was lead author of a study published in Gastroenterology in 2013 that compared gluten-free and gluten-containing diets in patients with diarrhea-predominant irritable bowel syndrome (IBS-D). Results of the four-week, randomized controlled trial showed that patients on the gluten-containing diet had more bowel movements a day and greater small bowel permeability than the gluten-free group did. The gluten group also showed significant decreases of zonula occludens 1, claudin-1 and occludin in the rectosigmoid mucosa. These effects were markedly greater in patients who were HLA-DQ2/DQ8 positive.
She points out the challenges of studying the effect of dietary factors on intestinal permeability as well as the difficulty in evaluating existing research.
"The methodology for studying barrier function is very heterogeneous; depending on the disease, you may study the large or small intestine. There is also great variability in how intestinal permeability is measured, including differences between in vitro and in vivo studies," she says. The difficulties multiply when studies look beyond known environmental triggers such as gluten and gut microbes. Studying the barrier function in people with suspected food sensitivities is particularly challenging, Dr. Vazquez Roque says.
"The first challenge is determining whether a person is truly sensitive to a particular food. Diagnosing a food sensitivity is already difficult because there are no good biomarkers. Other factors that affect gut permeability, such as nonsteroidal anti-inflammatories and smoking, have to be ruled out at the time of measuring permeability. And then there are alterations in the gut microbiome that are known to affect intestinal permeability. If a patient with a food sensitivity could be properly diagnosed and all these other factors were taken into account, then it would be possible to have better assessments of the barrier function when it relates to food."
German researchers published such a study in Gastroenterology in 2014. They described investigating suspected food intolerances in 36 patients with IBS using confocal laser endomicroscopy (CLE) for real-time visualization of structural changes in the intestinal epithelium after food challenge.
During the study, diluted food antigens were administered directly to the small bowel mucosa through an endoscope. Twenty-two of 36 patients showed a response to the antigens; within five minutes of exposure, epithelial gaps formed and intervillous spaces widened in the intestinal mucosa. There was no response in the remaining 14 patients or the 10 controls. Patients who showed a positive response were then placed on an exclusion diet. All experienced a more than 50 percent improvement in symptoms after four weeks and a 74 percent improvement at 12 months.
Intriguing as the study is, Dr. Vazquez Roque observes that CLE is not a well-recognized methodology for studying the intestinal barrier and that some of the commercial assays for gut permeability have not been validated at the research level for use in clinical practice.
At Mayo Clinic's campus in Minnesota, a differential sugar absorption test is used clinically. The test measures the ability of two nonmetabolized sugar molecules, lactulose and mannitol, to permeate the intestinal mucosa. The test is being used for research purposes at Mayo Clinic campuses in Florida and Arizona.
Dr. Vazquez Roque is currently conducting a study involving patients with self-reported gluten sensitivity who are on a gluten-free diet. Their barrier function is assessed at baseline and at the conclusion of a 30-day gluten challenge using both confocal laser endomicroscopy and the sugar absorption test. "It's a fascinating area, and definitely needs more research to better understand the interactions between diet and gut permeability," she says.
For more information
Vazquez Roque MI, et al. A controlled trial of gluten-free diet in patients with irritable bowel syndrome-diarrhea: Effects on bowel frequency and intestinal function. Gastroenterology. 2013;144:903.
Fritscher-Ravens A, et al. Confocal endomicroscopy shows food-associated changes in the intestinal mucosa of patients with irritable bowel syndrome. Gastroenterology. 2014;147:1012.