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Diets high in antioxidant vitamins, nutrients may prevent onset of cataracts
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CHICAGO - Amidst a multitude of studies that have investigated a possible connection between diet and cataract prevention, the strongest evidence suggests antioxidants such as vitamins C and E help protect ocular tissue against damage caused by free radicals, unstable oxygen that is increased by pollution, stress and disease.
"Cataract prevention can begin at any age, and a nutritional `cure' has been in the literature for decades," said Stuart Richer, OD, PhD, FAAO, chief optometrist at the Veterans Administration Medical Center in north Chicago.
While nutrition can be a valuable defense against cataracts, optometrists should first determine the cause of the cataract before beginning dietary therapy, Dr. Richer said. "Clinically, optometrists have a medical, legal and ethical responsibility to rule out systemic causes of noncongenital cataracts be fore considering nutritional or environmental intervention," he said.
Glutathione most important
The most important antioxidant for ocular health is glutathione, which reacts directly with oxidants and free radicals to help protect the adenosinetriphosphatase pump in the anterior epithelium.
"Glutathione, a simple tripeptide (gamma glutamyl-cysteinyl-glycine) is reduced in every known experimental and clinical model for cataracts, so it really is the Achilles' heel of the lens," Dr. Richer said.
Glutathione can be found naturally in fruits and vegetables such as watermelon, onions, garlic, asparagus and dark green leafy vegetables and, to a lesser degree, in meats. Because only a small percentage of glutathione crosses the duodenal barrier into the bloodstream, glutathione pre-cursors, which help the body produce the nutrient, are also important. Among the most common is N-acetylcysteine, which has been used as a mucus thinner and to prevent respiratory distress syndrome since the 1960s, Dr. Richer said. The compound is deacetylated to cysteine, the rate-limiting amino acid in glutathione synthesis.
Other pre-cursors include selenium, a co-factor for glutathione peroxidase used to detoxify hydrogen peroxide and riboflavin, (vitamin B2), a co-factor for recycling glutathione between the oxidized and useful reduced form. Glutathione is also an anticarcinogen and a liver detoxifier and works synergistically with vitamin C in the body to promote both ocular and overall health.
Dr. Richer questions the value of glutathione eye drops because it has not been proven whether glutathione, a water-soluble molecule, passes through the lipid components of the cornea, such as the corneal epithelium, to reach the anterior chamber.
A problem with any eye drop formulated to reduce the onset and progression of cataracts is that it would require reinstillation during the day, which has a poor record of compliance, Dr. Richer said.
Vitamin C can reduce risk
A study published in the American Journal of Clinical Nutrition indicated that long-term supplemental use of vitamin C could substantially reduce the risk of developing cataracts.
The study followed 247 Boston-area women, ages 56 to 71, who had not previously been diagnosed with cataracts during a 10- to 12-year span. Re searchers found that women who used vitamin C for more than 10 years had a 77% lower incidence of cataract formation and an 83% lower prevalence of moderate lens opacities than women who did not take a vitamin C supplement. Women who took supplements for less than 10 years did not achieve the same benefit.
Paul Jacques, ScD, and Allen Taylor, PhD, researchers from the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston, said their study does not establish a minimal amount of vitamin C to attain the maximum benefit, but their previous work suggests a daily intake between 200 mg and 300 mg.
"It seems that many of the eye tissues, not necessarily the lens, saturate or approach saturation with vitamin C at intake levels between 200 and 300 mg per day," said Dr. Taylor, director of the Laboratory for Nutrition and Vision Research at Tufts. "We think we are seeing some protection in terms of risk of cataract at that level, too. The data don't allow us to say that clearly just yet."
Dr. Taylor also cautioned that while 200 to 300 mg of vitamin C is easily attainable through normal diet, this study only examined the value of dietary supplements. Follow-up studies are needed to establish the importance of vitamin C, he said.
Vitamin C protects ocular tissue
As an antioxidant, vitamin C acts to protect ocular tissue from damage caused by free radicals, said Dr. Jacques, acting chief of the epidemiology program at Tufts. "Evidence suggests that damage that ultimately results in cata racts is a consequence of oxidative damage to the lens proteins and other constituents of the lens itself," he said.
Vitamin C has also been shown to mitigate the damaging affects of ultraviolet radiation, a strong etiologic agent in damaging epithelial cell DNA, which can result in cataracts, Dr. Richer said.
Vitamin E, another antioxidant, may play a role in preventing cataracts. The risk of cataract was reduced by approximately half in regular users of vitamin E supplements and people with high plasma levels of the vitamin in the Longitudinal Study of Cataract, published in 1998 and authored by M. Cristina Leske, MD, MPH, chair of the Department of Preventive Medicine at the University Medical Center at Stony Brook in New York.
Nutritional and ophthalmic data, including assessment of dietary intake, use of vitamin supplements and plasma levels of vitamin E, were collected from 764 participants who were followed during a 5-year period. In addition to the benefits for users of vitamin E supplements, the study also found that the risk of nuclear cataract was reduced by one-third in regular users of multivitamin supplements. The study does not allow researchers to recommend the amount of vitamin E necessary for the optimum benefit or to conclude that vitamin E prevents cataracts, Dr. Leske said.
She cautioned that in observational studies such as this, the evidence is suggestive, but not conclusive. Data from randomized clinical trials, such as the Age-Related Eye Diseases Study being conducted by the National Eye Institute, are needed before scientifically valid conclusions can be drawn on the role of nutrition in the onset and progression of cataracts.
Vitamin E is a lipid-soluble, chain-breaking antioxidant located in the lens fiber membrane. It prevents photoperoxidation of lens fiber lipids and stabilizes lens cell membranes, Dr. Richer said.
Other nutrients, carotenoids
| Dr. Sude's recommendations for nutritional supplements | |
|---|---|
| Dunaliella salina (form of beta-carotene) | 2,500 IU |
| Vitamin C | 150 mg |
| Vitamin E (alpha, beta, gamma, delta natural tocopherols) | 75 IU |
| Zinc | 2.5 mg |
| Selenium (L/selenomethionine) | 35 µg |
| N-acetylcysteine | 100 mg |
| Taurine | 100 mg |
| Tumeric extract (minimum 95% curcumin) |
50 mg |
| Gingko biloba extract (flavonglycosides, minimum 6% terpene lactones) | 30 mg |
| Alpha lipoic acid | 10 mg |
| Lutein (from marigold flower extract) | 2.5 mg |
Lutein and zeaxanthin, carotenoids found in the macula, are emerging as more important than beta-carotene in affording protection against cataracts and macular degeneration, Dr. Richer said. Both lutein and zeaxanthin are lipid-soluble substances and may play a role in maintaining membrane integrity and absorption of actinic radiation. They can be found in leafy green vegetables, pumpkin, red pepper and egg yolks.
Other vitamins associated with a decreased risk of cataracts include vitamin B6, which helps convert methionine to cysteine, and vitamin B3 (or niacin) though its role in preventing cataracts is unknown, Dr. Richer said. Zinc and copper support an antioxidant enzyme called superoxide dismutase, which neutralizes the free radical superoxide.
An emerging group of compounds are the bioflavonoids, which include grape seed extract and bilberry extract, Dr. Richer said. As a group, these compounds, found in the peels of fruits and vegetables and in teas and herbs, have tended, in varying degrees, to be antiviral, antibacterial, immune-enhancing and very effective anticataract agents.
While the most effective way to see benefits from nutrients may be to include them in a normal diet, the belief that it is impossible to secure the necessary nutrients by eating normally is be coming more widely accepted, said Jerome Sude, OD, in private practice in Tallmadge, Ohio. The University of California at Berkeley Wellness Letter has acknowledged that people can no longer achieve optimal health by eating normally, and many of the biggest critics of supplementation are now taking neutral positions, he said.
"In the medical industry there's a paradigm shift from a curative to a preventive modality, and that's very healthy," Dr. Sude said.
Investigate features of supplements
When considering dietary supplements, optometrists should investigate certain features, Dr. Sude said. Supplements should be readily absorb able, and one indicator is that the product is chelated, with an amino wrap that maximizes systemic absorption. Another feature to consider is that gel caps are more bioavailable than pills, which have binders and fillers that do not absorb well, he said.
One mistake some patients make is purchasing different vitamins and nutrients separately, Dr. Sude said.
"What's equally important is the synergism between the products - having the right formulation - because certain products can inhibit the results of another or certain ingredients can accelerate the result of another," he said. "It's important to deal with a company where they do everything based on good data, and that you have access to that data."
Work with a patient's GP
Dr. Sude added that he takes a conservative approach with dietary supplements in his practice. He makes recommendations but does not prescribe supplements. He also asks the patient's general practitioner to review his recommendations before the patient begins taking any supplement.
One problem with recommending specific doses of different nutrients is that patients' needs can vary widely because of factors such as weight, gender and whether the product is fat- or water-soluble, Dr. Richer said.
Another confounding factor is the need for greater levels of antioxidants as a person's physiological age increases. The physiological need of an individual is determined by factors such as disease, stress, drug use, nutritional biochemical status, physical activity and genes, said Dr. Richer.
While it is unknown if diet alone can provide sufficient protection from cataract, spinach has been shown to have a multitude of nutrients that are effective in preventing the condition, Dr. Richer added. Spinach is an excellent source of glutathione, cysteine, vitamins B6 and C, selenium, zinc, lutein and zeaxanthin, although it contains iron, which can cause other problems, he said.
Diabetes-related cataracts
In addition to an increased need for antioxidants due to a higher susceptibility to free radical damage, diabetic patients are at risk for posterior subcapsular cataracts. These are associated with increased triglycerides, in creased hyperglycemia and obesity, Dr. Richer said.
Nutrition therapy for diabetic patients should include reducing refined carbohydrates and increasing intake of nutrients that improve insulin and glucose handling as well as decrease the risk of diabetic complications. Important nutrients include magnesium, omega 3 fatty acids, chromium and vitamins C and E, which improve insulin resistance at the cell level, said Dr. Richer.
A nutrient that is usually not considered in the United States is alpha lipoic acid, a prominent source of sulfur. It has been shown to be effective in treating diabetic retinopathy, diabetic peripheral neuropathy and diabetic cardiomyopathy, Dr. Richer said. Several studies are examining it as an anticataractogenic agent be cause it is a good source of sulfur, which would tend to enhance glutathione production.
Alpha lipoic acid also appears to reduce oxidative stress suffered by diabetics and regenerates three important antioxidants: glutathione and vitamins C and E.
For Your Information:
- Paul Jacques, ScD, acting chief of the epidemiology program, and Allen Taylor, PhD, director of the Laboratory for Nutrition and Vision Research, can be reached at Tufts University, 711 Washington St., Boston, MA 02111; (617) 556-3000; fax: (617) 556-3156.
- M. Cristina Leske, MD, MPH, distinguished service professor of Preventive Medicine and Ophthalmology and chair of the Department of Preventive Medicine at the University Medical Center at Stony Brook, can be reached at L3 101 Health Sciences Center, Stony Brook, NY 11794-8036; (516) 444-1290; fax: (516) 444-7525; e-mail: cleske@prevmed.som.sunysb.edu.
- Stuart Richer, OD, PhD, FAAO, is chief optometrist at the Department of Veterans Affairs Medical Center, 3001 Green Bay Rd. 112 Eye, Chicago, IL 60064-3095; (847) 688-1900; fax: (847) 578-6924; e-mail: stuart.richer@med.va.gov.
- Jerome Sude, OD, can be reached at 518 West Ave., Tallmadge, OH 44278; (330) 630-9699; fax: (330) 630-2173.
- Drs. Jacques, Sude, Leske and Richer have no direct financial interest in any products mentioned in this article, nor are they paid consultants for any companies mentioned. Dr. Taylor did not disclose if he has a direct financial interest in any products or if he is a paid consultant for any companies mentioned.