October 2020 Issue
Eye Health: Nutrition and Glaucoma Risk
By KC Wright, MS, RDN
Vol. 22, No. 8, P. 18
While traditionally thought to have limited or no role in glaucoma prevention and treatment, dietary interventions for the disease are garnering interest and propelling new research.
Glaucoma is a progressive optic neuropathy characterized by degeneration of the retinal neurons that receive visual information from photoreceptors. Affecting more than 3 million Americans, it’s a leading cause of irreversible blindness.1
Glaucoma occurs when the eye can’t drain intraocular fluid effectively, resulting in elevated intraocular pressure (IOP), the most significant risk factor for the disease.2 Because glaucoma develops slowly, there typically are no detectable symptoms in its early stages. Thus, an estimated one-half of Americans with glaucoma are unaware of their condition.1
Reducing IOP has been the standard treatment for glaucoma and usually is done so through medications, laser therapy, or surgery. Previously, lifestyle interventions, including nutrition, appeared to have little influence on glaucoma management. However, complementary and alternative medicine targeting IOP recently has gained attention among both the medical community and consumers. RDs should be prepared to meet with glaucoma patients who seek information about any dietary influence on disease progression.
This research update investigates the potential for nutrition to affect IOP and the incidence and progression of glaucoma.
As oxidative stress is one reported factor in the pathophysiology of glaucoma, much of the research on diet and glaucoma has focused on nutrients with antioxidant activity.3
Fruits and vegetables contain a plethora of antioxidant nutrients hypothesized to decrease risk of developing glaucoma. Identifying a single protective source can be difficult due to a wide and varied nutrient composition of different plant foods. Yet, in one study, women who consumed a diet rich in vitamin A, carotenes, and vitamin C were found to have a decreased risk of glaucoma.2 High dietary nitrate intake, primarily from green leafy vegetables, also has been associated with a lower risk of developing the disease.4
But there’s far from a consensus. The Nurses’ Health Study and the Health Professionals Follow-up Study found no significant associations between antioxidant nutrients and glaucoma.3 In other research, lower systemic antioxidant capacity was associated with more severe visual damage in glaucoma, explained in part by its roles in elevating IOP.5
A recent cross-sectional study investigated possible associations of nutrients on glaucoma. Diet intake was collected using a food-frequency questionnaire from almost 600 Japanese Americans living in Los Angeles, who also were screened for optic discs indicative of glaucoma. Results demonstrated that low intakes of vitamin A and vegetable fat (omega-6 fats) and a high intake of iron were significantly associated with glaucoma risk.6 In other research, high iron and calcium intakes were associated with a greater risk of developing glaucoma.7
Magnesium is thought to have a potential role in the treatment of glaucoma. The mineral acts as a natural physiologic calcium channel blocker, improving ocular blood flow and reducing oxidative stress.6 It’s demonstrated improvement in ocular blood flow and prevention of retinal cell loss; more research may help to determine any possible efficacy for glaucoma management.8
In both healthy individuals and patients with glaucoma, alcohol consumption has been shown to lower IOP in the very short term, possibly due to alcohol’s hyperosmotic effect (the same effect as hyperosmotic agents, which are medications often prescribed to reduce IOP). In addition, alcohol has been shown to increase blood flow to the optic nerve head, which provides a protective effect from glaucoma.2 The precise amount of alcohol intake for a protective factor isn’t yet determined, but limiting alcohol to one drink per day for women and two drinks per day for men is important for overall health.
Coffee, a rich source of biologically active caffeine, has been associated with an increase in IOP.2 The association is assumed to be secondary to a caffeine-induced increase in homocysteine levels, which are thought to trigger stimulation of basement membrane material (ie, the layer attached to the epithelium, the cornea’s most outermost layer).9
While tea generally contains much less caffeine than coffee, it’s a rich source of flavonoids—major polyphenols in tea—shown to have a protective effect against glaucoma by reducing oxidative stress and improving blood flow.2 Recently, a cross-sectional report from the National Health and Nutrition Examination Survey on the association between glaucoma and commonly consumed beverages demonstrated that people who consume at least one cup of hot tea daily are less likely to have glaucoma compared with non–tea drinkers. While coffee also contains polyphenols, there was no significant association between coffee and glaucoma risk in this study.10
Dark chocolate is another rich source of flavonoid polyphenols that’s suggested to be beneficial in patients with certain CVDs, though it hasn’t been shown to decrease oxidative stress in glaucoma.2
Calories and Weight
Greater BMI, a major anthropometric indicator of obesity, has been associated with higher IOP in observational research, although the mechanism of action is unclear.11,12 In contrast, epidemiologic studies have demonstrated mixed results.13 Because BMI doesn’t differentiate between fat and muscle composition, the influence of other obesity markers (eg, waist circumference, total body fat mass, percent body fat) were considered and found to have a positive linear relationship with IOP.11,14
Metabolic syndrome—the cluster of hypertension, hyperglycemia, and hyperlipidemia—also has been linked to increased IOP and glaucoma.15
The ketogenic diet, which has become substantially popular for weight loss in the past few years, may have some utility in glaucoma. After feeding mice a fat-rich (90% fat) diet for eight weeks, researchers found the mice to have higher energy and increased antioxidant levels, and showed less deterioration in their optic nerves. Of course, these results may not be reproducible in humans. Questions also remain as to the safety and sustainability of long-term use of the ketogenic diet.16
Diets that focus on plant foods, specifically those that contain vitamins A and C and carotenes, along with nitrate-containing green leafy vegetables, can support adequate polyphenol and antioxidant intake to help prevent glaucoma. Plant-based and Mediterranean-type diets would provide these nutrients. These diets also are in line with consuming adequate omega-6 vegetable oils balanced with omega-3 fats. Omega-3 fatty acids are beneficial for glaucoma patients, as they decrease IOP.17
The benefit of dietary supplements is well established for patients with age-related macular degeneration, but there doesn’t appear to be much evidence to support their use in glaucoma. Currently, there’s no convincing data that supplementation can help prevent or treat glaucoma.18 People may choose to take a standard multivitamin, but those touted as specific “eye” supplements aren’t recommended.2
Ginkgo biloba extract (GBE) has a variety of pharmacologic properties considered in the pathogenesis of glaucoma. Current evidence suggests GBE increases ocular blood flow and protects against oxidative stress.19 More research is needed to confirm these results. In addition, it’s imperative to identify therapeutic purity and dosage of GBE or any supplement, as there’s no regulatory standardization for supplements in the United States.
Most of the current body of research on diet and glaucoma demonstrates the influence of nutrition on reducing risk of the disease rather than having a significant therapeutic effect. Although there still appears to be much to learn about the diet-glaucoma relationship, good nutrition may help complement glaucoma therapies. It makes practice sense to encourage clients and patients, with or without established glaucoma, to consume fruits and vegetables high in antioxidants and moderate their intake of coffee and alcohol. This is especially warranted because one-half of all those with glaucoma don’t know they have it.
Nutrition therapies should continue to include adequate intake of omega-6 vegetable oils in a healthful balance with omega-3 fats for their association in reducing glaucoma risk. Therefore, a Mediterranean diet high in plant foods, which has proven health benefits for those at risk of or being treated for CVD, diabetes, and certain cancers, is recommended.
The Bright Focus Foundation, which funds innovative scientific research for mind and eye health, offers a Healthy Living Disease Toolkit for Glaucoma with specific food recommendations, such as dark green, yellow, and orange fruits and vegetables, that RDs can provide to clients and patients.18 Finally, in addition to eating a diet high in antioxidants, maintaining a healthy body weight and partaking in regular physical activity also are essential for managing risk and treatment of most diseases, including glaucoma.
— KC Wright, MS, RDN, is a research dietitian at Dartmouth-Hitchcock Health in New Hampshire, and advocates for sustainable food at WildberryCommunications.com.
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14. Zhao D, Kim MH, Pastor-Barriuso R, et al. A longitudinal study of association between adiposity markers and intraocular pressure: the Kangbuk Samsung Health Study. PLoS One. 2016;11(1):e0146057.
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16. Walczyk T, Wick JY. The ketogenic diet: making a comeback. Consult Pharm. 2017;32(7):388-396.
17. Huang WB, Fan Q, Zhang XL. Cod liver oil: a potential protective supplement for human glaucoma. Int J Ophthalmol. 2011;4(6):648-651.
18. Disease toolkit for glaucoma. Bright Focus Foundation website. https://www.brightfocus.org/glaucoma/diet-and-exercise. Accessed March 29, 2020.
19. Kang JM, Lin S. Ginkgo biloba and its potential role in glaucoma. Curr Opin Ophthalmol. 2018;29(2):116-120.