Eyeworld

EW GLAUCOMA 136 April 2016 by Tony Realini, MD, MPH Recent studies have begun to identify nutrients that may be associated with glaucoma risk, leading people to ask: Is there a diet to help fend off glaucoma? O ur glaucoma patients often ask if there is any- thing they can do—in addition to taking their medications and keeping their follow-up appointments—to reduce their risk of vision loss from this disease. Is there anything they should—or shouldn't—eat? What about vitamins? Exercise? Smoking? The short answer is no. But the real answer is getting more com- plex—and more interesting. Recent epidemiological studies have begun to identify various nutrients that might be associated with glaucoma risk. While the strength of these associations—and the clinical signif- icance of these findings—remains unresolved, these preliminary ob- servations open the door to a whole new array of potential therapeutic targets in glaucoma. Medicines from nature Some people scoff at the notion of deriving health benefits from nutrients or compounds found in nature, writing them off as homeo- pathic or "natural" treatments that lack the scientific credibility. Their skepticism is not unreasonable. Lack of credible data from well-designed trials plays a significant role in the limited acceptance of many of these nutrients as legitimate therapies. Most vitamins and nutrients are in the public domain, so patent- ability is limited, and thus little profit motive for industry to fund the studies necessary to prove their value, explained Robert Ritch, MD, New York. However, for most of history, medicines have been derived from plants, and with the reawakening of this concept, interest in herbal and plant-derived medications is now showing greatly renewed interest, particularly in compounds used in traditional native medical systems and in Chinese and Ayurvedic medicine. Aspirin from the willow tree, digitalis from the foxglove plant, and paclitaxel from the Pacific yew tree are but a few examples. Most recently, the rosy periwinkle has been shown to be an important source of vinblastine and vincristine. More than 6,000 plants and their effects are known in southeast Asia alone, and as both forested areas and indigenous people disappear, a great effort is being made by ethnologists and pharmaceutical companies to discover potential sources of drugs before they are gone. Vitamins also play a vital role in health maintenance. Vitamin C prevents scurvy, and vitamin D prevents rickets. A more mod- ern example is the role of vitamin supplementation in preventing the progression of dry to wet age-related macular degeneration, as proven in the Age-Related Eye Disease Study (AREDS). The latter is a notable exception that has been held to the high standard of randomized clinical trial evaluation. Glaucoma diet? Lacking well-designed and ade- quately powered studies establishing the relevance to glaucoma, is there lower-tier evidence that what we eat may affect our glaucoma risk? Yes, there is. Several compounds found in our diet have physiological effects that may play a role in the glaucoma disease process. Ginkgo biloba, for example, is a tree dating back to the Permian pe- riod 250 million years ago. "Ginkgo contains 60 bioactive compounds," Dr. Ritch said, "about 30 of which are found nowhere else in nature." Ginkgo biloba extract has many beneficial effects in physiology and pathophysiology. "It is a power- ful antioxidant, protects against lipid peroxidation and free radical damage, inhibits glutamate toxic- ity, stabilizes mitochondrial aging changes, preserves mitochondrial function, and is neuroprotective in many models of ischemia-reperfu- sion injury," Dr. Ritch explained. "Further, it improves peripheral, cerebral and ocular blood flow," he added, "and improves symptoms of intermittent claudication and Raynaud's phenomenon, which is strongly associated with normal tension glaucoma." In a small ret- rospective study of several hundred patients in Korea, he added, patients with normal tension glaucoma exhibited "marked slowing of visual field progression after the initiation of ginkgo biloba extract supplemen- tation." One problem with over-the- counter supplements sold in phar- macies and on the Internet is the lack of standardization and quality control, with counterfeit products being common. In addition to ginkgo, curcum- in, found in the spice turmeric, may play an important role in ocular health. There are now more than 100 trials of curcumin listed on clinicaltrials.gov, about half of them dealing with cancer. Curcumin is a broad spectrum anti-inflam- matory agent and inhibits many transcription factors, cytokines, and inflammatory mediators, such as iNOS, IL-6, COX-2, and NF-kappaB. It has shown benefit in rheumatoid arthritis, psoriasis, ulcerative colitis, depression, and cognitive disorders. "Beneficial effects of curcum- in have been reported in uveitis, diabetic retinopathy, central serous chorioretinopathy, and other ocular disease states," Dr. Ritch said. "This compound may be beneficial as a broad-spectrum agent against ischemia, oxidative damage, low- grade inflammation, mitochondrial dysfunction and excitotoxicity in glaucoma and protect retinal gangli- on cells from neurodegeneration." Perhaps our mothers were right to extoll the virtues of eating our vegetables, according to Louis Pasquale, MD, Boston. "Our recent work has shown that a diet rich in vegetables with a high nitrate content is associated with a reduced risk of primary open-angle glaucoma presenting with early paracentral visual loss," he said. High-nitrate vegetables include beets, carrots, celery, spinach, and cabbage, among others. There is a biologically plausible mechanism by which nitrates may be beneficial in glaucoma. "Exoge- nous nitrates are converted to nitric oxide inside cells and may help to correct the impaired nitric oxide signaling that is believed to contrib- ute to primary open-angle glaucoma presenting with paracentral visual loss," Dr. Pasquale said. Coffee connection In addition to nutrients that may be beneficial for glaucoma patients, there are some that perhaps should be minimized. "There is evidence that high coffee consumption—more than 3 cups per day—is associated with an increased risk of exfoliation glauco- ma," Dr. Pasquale said. "High coffee consumption can lead to elevated homocysteine levels, a serum biomarker that is strongly associated with exfoliation glaucoma," he explained. "Discov- ering how elevated homocysteine levels might contribute to the de- velopment of exfoliation glaucoma represents one of the holy grails of glaucoma research." Practical implications While the data is mounting that diet may play a role in glaucoma risk, recommending dietary changes for our glaucoma patients is not yet justifiable. "I would not say the evidence is compelling," said Dr. Pasquale, who pointed out the need for additional studies. The studies to date have been epidemiologic and cohort-based, he said. "These studies were not randomized clinical trials and should not be construed as grounds for counseling patients to make dietary changes. With that said," he added, "no harm is done by increasing dietary vegetable in- take or moderating coffee consump- tion, but that is not a substitute for conventional therapy as recom- mended by an eyecare provider." EW Editors' note: The physicians have no financial interests related to their comments. Contact information Pasquale: louis_pasquale@meei.harvard.edu Ritch: ritchmd@earthlink.net Glaucoma diet: Can nutritional choices affect glaucoma risk?

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