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Keeping an Eye on Gut Health

Gut Health Gut Health
EuroMedica
 
Longevity By Nature

Most health practitioners are familiar with the gut’s connection to many other organ systems in the body—the immune system, the brain, the skin and the cardiovascular system. Recent research has elucidated another surprising, yet important connection that extends from the digestive system to the windows of the soul—the eyes. This connection is known as the gut-eye axis.

Though this area of investigation is still in its infancy, vision research has shed light on how the gut microbiome can impact eye health. From imbalances in gut bacteria to increased intestinal permeability, the gut has been associated with a wide range of ocular conditions.

Uveitis

When it comes to gut health and the eye, the most studied ocular condition is uveitis, a group of conditions characterized by inflammation of the eye. Uveitis can be divided into various categories, depending on which part of the eye is involved—the anterior, intermediate or posterior segments, and also whether the inflammation is infectious or non-infectious. In non-infectious uveitis, there is an imbalance in the retina in the ratio of effector T cells—including both T helper (Th1) and Th17 cells—to regulatory T cells (Tregs). Effector T cells are involved in inciting the inflammatory cascade, while regulatory T cells normally prevent autoimmune diseases from occurring.

Based on animal models of experimental autoimmune uveitis, known as EAU, certain strains of bacteria have been shown to promote pro-inflammatory effector T cell types, like Th17. This differential T cell induction, based on changes in the intestinal microbiome, may result in either protection from or worsened autoimmune uveitis.

Researchers at the National Eye Institute have shown that T cells could be activated in the gut to respond to retinal antigens through a process referred to as molecular mimicry. It is hypothesized that these activated effector T cells travel from the gut to the eye via the bloodstream, cross the blood-retinal barrier (which normally protects the retina from exposure to immune cells and inflammation) and trigger uveitis. However, it is not yet known which bacterial species or molecules from bacteria are involved in this pro-inflammatory process in the gut that then negatively impacts the eye.

Uveitis has also been linked to increased intestinal permeability, commonly known as “leaky gut syndrome.” Researchers in Oregon have found that changes in intestinal permeability can be associated with increased severity and perhaps even the pathogenesis of ocular inflammation in uveitis.

Other research linking the gut to ocular inflammation showed that mice with EAU who were given a dose of short-chain fatty acids, naturally occurring metabolites of intestinal bacteria fer­mentation of dietary fiber, experienced decreased severity of uveitis. The short-chain fatty acids also promoted intestinal bacterial changes that increased regulatory T cells in the gut.

Another study showed decreased severity of EAU in mice given oral antibiotics, suggesting a link between the eradication of certain bacterial species in the gut with reduced ocular inflammation. Again, it is not fully understood which exact bacterial species are associated with these changes.

Aside from these animal studies, human studies have revealed differences in the gut microbiome between uveitis patients and healthy controls. One study from the National Eye Institute (NEI) found that uveitis patients had less diversity in microbiota and fewer anti-inflam­matory microbes.

One particular form of uveitis, known as Vogt-Koyanagi-Harada (VKH) disease, has clearly been linked to gut dysbiosis. This research from China led to a VKH classifier, based on 37 differentially depleted or enriched microbes. The classifier distinguishes patients with good prognoses from those unlikely to respond to im­munosuppressive therapy. For example, enrichment with Prevotella species and de­pletion of Clostridium species helped to shift the gut microbiome from dysbiosis toward normal.

Dry Eye Syndrome

Dry eye syndrome is a common condition in which the ocular surface dries out. It affects more than 50 million people, both adults and children. Dry eye is also more common in women, suggesting a hormonal component.

In dry eye, the tear film that normally coats the cornea and conjunctiva evaporates quickly, leaving the ocular surface dry and irregular. Dysfunction of tiny glands in the eyelids, known as meibomian glands, that secrete oils for the tear film, is a common underlying root cause of dry eye. Inflammation of these glands and the ocular surface plays a key role in the pathogenesis of dry eye.

Research suggests that an imbalance in gut bacteria, or gut dysbiosis, may contribute to the development of dry eye syndrome. An imbalance can lead to an inflammatory response affecting the eyes, potentially contributing to the discomfort and dryness characteristic of dry eye syndrome.

An animal study of dry eye showed that germ-free mice without bacteria in their gut have a more severe dry eye phenotype with corneal stain­ing and surface inflammation. When these germ-free mice were put in a cage with healthy mice with normal gut microbiomes, the mice ate each other’s stool. The guts of the germ-free mice were thus recolonized with bacteria, with improvement of their dry eye.

Researchers from Miami, FL performed a study looking at the potential benefit of a fecal microbiota transplant for patients with immune-mediated dry eye. The study participants were given two enemas with fecal microbial transplants; while the symptoms of some patients improved, others remained stable. The researchers are now looking into whether the gut microbiome was altered with two fecal transplants, or whether more treatments are necessary.

Age-related Macular Degeneration

Age-related macular degeneration (AMD), is a leading cause of irreversible blindness, estimated to affect at least 200 million people worldwide. AMD tends to affect older individuals above 50 years, with the prevalence rising with age. In AMD, the central retina, called the macula, is affected and central vision may be lost.

One root cause of AMD is inflammation, with complement activation implicated. Researchers are looking at whether gut dysbiosis may activate complement, that then may result in ocular disease.

Another study found changes in certain metabolic pathways, represented by intestinal bacteria, in AMD patients compared to controls.

Glaucoma

Glaucoma is a common eye disease that can cause irreversible vision loss and even blindness. In glaucoma, there is characteristic damage to the optic nerve, which connects the eye to the brain, with resultant loss of peripheral vision.

The association between oral and gut microbiome and the develop­ment of glaucoma is another emerging area of investigation. Some patients with primary open-angle glaucoma, the most common form of glaucoma, have an abundance of certain bacterial species in their oral cavity, specifically streptococci. Though this finding suggests an association, it is far from proving causation and a direct link to developing glaucoma. It has been hypothesized that gut dysbiosis triggers changes in cytokine signaling and complement activation in the immune system.

Tips for Maintaining Gut and Eye Health

Much of the research on the gut-eye axis is still in its infancy, and we have much to learn about the exact mechanisms linking gut health to ocular disease. However, one thing is clear—not just for vision health, but for overall health—it is important to maintain a healthy gut microbiome and to reduce intestinal permeability.

Here are some simple, yet effective ways by which to support gut health using nutrition and principles of functional medicine:

1. A High-fiber Diet: A high-fiber diet will promote certain bacteria to be more pre­dominant in the gut, and these bacteria produce short-chain fatty acids that promote regulatory T cell differentiation and reduce the propensity to develop ocular inflammation.

2. Probiotic and Prebiotic Foods: Certain strains of commensal bacteria found in the gut tend to be anti-inflammatory. A diet rich in live probiotics, foods such as kimchi, yogurt, sauerkraut and fermented vegetables, can help maintain gut diversity. Also, prebiotic foods such as those that provide resistant starches, inulin, soluble fiber and beta glucan can help support a healthy gut microbiome.

3. Supplementation: Taking probiotic, prebiotic and even postbiotic supplements can be beneficial. It is important to choose a probiotic with a high colony forming unit count and a diversity of bacterial strains. Consider a probiotic formulation curated for eye health called Balance. Balance is part of the eye health supplement line, Ageless by Dr. Rani. It provides 10 bacterial strains most backed by research.

Postbiotics are short chain fatty acids that are by-products of intestinal bacteria and are believed to help to decrease inflammatory pathways. Supplementation with omega-3 fatty acids that are anti-inflammatory, DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), also supports eye health on multiple levels.

4. Hydration: Staying well-hydrated is essential for both gut and eye health. Proper hydration can contribute to healthy elimination of waste products by the gut, as well as promote tear production and maintain the mucous membranes of the eyes.

5. Stress Management: Chronic stress can negatively impact the gut microbiome and contribute to inflammation. Adoption of stress management techniques like mindfulness, meditation, or yoga can be helpful.

6. Regular Exercise: Physical activity has been linked to a healthier gut microbiome and can contribute to overall well-being, including eye health. Additional strategies on the horizon to support the gut-eye axis include:

1. Antibiotics: Changing intestinal bacteria with a targeted antibiotic has been shown to dramatically reduce the severity of experimental autoimmune uveitis based on animal models. This is believed to occur through bacterial changes in the gut that promote regulatory T cell differentiation. Targeted antibiotics may prove to be a useful strategy to heal gut dysbiosis along with ocular inflammation.

2. Fecal Microbial Transplants: A healthy person’s stool can be harvested and transplanted via colonoscopy, nasogastric tube or an enema into the gastrointestinal tract of a person with disease. It is well established that in cases of C. difficile infection, a fecal transplant is generally successful in eradicating infection, especially in the setting of ulcerative colitis. Once commensal and pathogenic bacterial strains for ocular disease can be reliably identified, as well as healthy donors, fecal transplant for ocular disease management may be a treatment strategy to keep in mind for the future.

Summary

As the intricacies of the gut-eye connection continue to be unraveled, it becomes increasingly clear that the state of the gut microbiome, often influenced by dietary and lifestyle choices, can have far-reaching effects on the health of the eyes. By nurturing the gut, it may be possible to unlock new possibilities for protecting and preserving vision. As research advances, so too does our understanding of how healthy vision can be supported using integrative strategies.

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Dr. Rani Banik is a board-certified ophthalmologist and fellowship-trained neuro-ophthalmologist with additional certification in integrative and functional medicine. Dr. Banik focuses on the root cause of eye diseases, and uses integrative strategies for conditions such as thyroid eye disease, macular degeneration, cataract, dry eye, glaucoma and other autoimmune diseases of the visual system. She runs a private practice based in New York, NY and is also associate professor of Mount Sinai in New York City where she serves as an educator and researcher. Dr. Banik is frequently featured as an expert in the media and has been interviewed on Good Morning America, CBS, NBC, ABC, The New York Times, The Washington Post and Fox, amongst many others. Dr. Banik’s first book, Beyond Carrots – Best Foods For Eye Health A to Z focuses on the 30-plus nutrients and 40 foods that best provide complete nutrition for your eyes. Its companion cookbook, The Beyond Carrots Cookbook, includes more than 160 delicious and nutritious recipes.