For generations, mothers have advised their children to eat their carrots for the sake of their eyes. Indeed, carrots do contain compounds that are vital to vision. But today’s moms and others wanting to eat for eye health should know that eating for better vision is not just about carrots anymore.
Researchers have been homing in on evidence that certain dietary habits may help stave off two common degenerative eye diseases: age-related macular degeneration (AMD), the most common cause of legal blindness and vision impairment in older Americans, and cataract, a condition affecting more than 20 million Americans. What you eat can also help to control blood glucose levels, which is important to reducing the risk of developing diabetic retinopathy, a common eye complication of diabetes.
-- Keep an eye on your vision! Learn about preventive steps and treatments for diabetic retinopathy from retinal specialist Dr. Charles Wykoff. >>
An in-depth look at nutritional strategies for healthy eyes follows, but first it’s helpful to understand a little more about these common eye diseases.
Age-related macular degeneration
According to the National Eye Institute, more than 1.7 million older Americans have advanced-stage age-related macular degeneration. Because the risk of developing AMD rises with age – the condition is rare in people under 60 – the number of people diagnosed with the disease is expected to increase sharply as the baby-boom generation ages.
The macula of the eye is a tiny region at the center of the retina, the thin tissue that lines the eyeball. Light-sensitive cells in the macula are responsible for sharp vision, the kind needed for everyday activities such as reading, driving, and watching television. Scientists are not sure exactly how the most common form of macular degeneration starts, but over time these cells can break down. In most cases, this breakdown happens slowly, gradually leaving less of the macula to handle light properly. With increasing degeneration, vision can blur and a blind spot can develop in central vision. There are no proven treatments to stop this progression, although new research indicates that a specially formulated high-dose supplement may be helpful (see below). Smokers, those with a family history of AMD, and women are at an increased risk for AMD. Whites are more likely to lose their vision from AMD than African-Americans. High cholesterol may also increase one’s risk for a rarer and more rapidly damaging form of AMD.
The second common eye disorder, cataract, affects more than half of all Americans age 65 and over. A cataract is a cloudy spot in the lens of the eye. In the healthy eye, the lens, composed of water and protein, is transparent. For reasons that are not fully understood, this protein can clump together, forming a cloudy spot that causes light to be distorted as it passes through the lens.
Most cataracts are related to aging, and more than half of all Americans age 65 and over have a cataract. In addition to age, diabetes can also cause proteins in the lens to clump. People with diabetes, therefore, are more likely than the general public to form cataracts and often develop cataracts at a younger age. Symptoms of cataracts include blurry vision, reduced night vision, fading of colors, and problems with glare.
Besides diabetes and advancing age, smoking and long-term exposure to the sun’s ultraviolet rays are believed to increase the risk for developing a cataract. Medicines such as steroids can also cause cataracts.
Attention has also been focused on the biological process of oxidation as one of the factors that may lead to both AMD and cataract. This in turn has raised interest in the potential of antioxidants, compounds that appear to mitigate some of the damage caused by oxidation.
Another area of research has concentrated on diet, specifically diets high in green leafy vegetables.
Some of the best news regarding eating for eye health involves consuming leafy green vegetables like spinach, kale, collards, and broccoli. For a number of years, evidence has been accumulating that people whose diets are relatively high in leafy green vegetables are less likely to develop AMD.
Scientists believe two substances, lutein (LOO-tee-in) and zeaxanthin (zee-ah-ZAN-thin), may be responsible for this effect. Both lutein and zeaxanthin are carotenoids (plant pigments), similar to the better-known beta-carotene.
Lutein and zeaxanthin are of particular interest because the macula contains higher concentrations of these two substances than does any other spot in the body. In fact, the macula is yellow due to its rich stores of lutein and zeaxanthin, which are yellow pigments. (This yellow color is masked by the green color of chlorophyll in many plant foods.)
Researchers have found that people with advanced AMD tend to have lower blood concentrations of lutein and zeaxanthin than people without the disease. Studies comparing the diets of those with AMD to those free of the disease have also indicated that people with low dietary intakes of foods containing lutein and zeaxanthin are more prone to the disease. In one study, researchers found that people who typically ate lutein- and zeaxanthin-rich greens two to four times a week were half as likely to have AMD as people who ate those foods less than once a month.
Authors of a review that examined the results of years of research on this subject also concluded that “generous” consumption of lutein and zeaxanthin from foods such as broccoli and spinach was associated with up to a 40% reduction in risk for AMD. The same level of intake of these carotenoids was also linked to a 20% reduction in the risk of cataract.
Lutein and zeaxanthin may confer their protection by absorbing the potentially damaging blue light found in sunlight before it causes harm to the retina. The two compounds may also act as antioxidants, neutralizing damage to cells caused by “free radicals,” a type of damaging oxygen.
There is not yet enough information to make a specific recommendation for including leafy dark greens in the diet for the sole purpose of benefiting the eyes. However, leafy dark greens are among the most nutritionally dense foods nature provides. They are low in calories and contain a wealth of nutrients such as vitamin A in the form of beta-carotene, vitamin C, vitamin K, fiber, folate, iron, and magnesium. Because some of these greens can be unfamiliar, a cooking guide for greens is here.
Leafy greens are not the only source of lutein and zeaxanthin. Other vegetables and some fruits contain either one or both of the compounds. In a study analyzing the lutein and zeaxanthin content of a number of fruits and vegetables, researchers found that corn, kiwifruit, red grapes, spinach, zucchini, yellow squash, orange peppers, and orange juice were among the foods that contained substantial amounts of one or both of the compounds.
Egg yolks, too, are an excellent source of both lutein and zeaxanthin, because of the plant-based diet of the chickens producing the eggs. For people with diabetes, however, and others at high risk for heart disease, dietary guidelines from the National Cholesterol Education Program suggest limiting total daily cholesterol intake to 200 milligrams (mg) or less. Given that a large egg yolk contains 212—220 mg of cholesterol, fruits and vegetables are generally more healthful sources of lutein and zeaxanthin.
C for sight
As discussed earlier, a cataract occurs when protein in the eye’s lens clumps together, possibly in part due to the process of oxidation. Antioxidants such as vitamin C, vitamin E, and beta-carotene may help counteract this damage.
Vitamin C may be particularly important because this nutrient is concentrated in the lens of the eye much as lutein and zeaxanthin are concentrated in the macula. A number of studies have shown a link between relatively high intakes of vitamin C and a lower risk of developing cataracts. For example, researchers looking at a subset of women participating in the ongoing Nurses’ Health Study found that those women with intakes of vitamin C averaging about 350 mg a day were more than half as likely to develop two different types of cataracts compared to those with intakes below 140 mg a day. Women who took vitamin C supplements for 10 years or more also saw their risk of developing cataracts dip significantly compared to those who took supplements for a shorter period of time or not at all.
Again, knowledge is still evolving and it is too early for definitive recommendations. For example, both the timing and duration of vitamin C intake may be important. Cataracts usually develop over years and for vitamin C to be effective, intake may need to be high for a prolonged period.
However, there is no harm in consuming plenty of vitamin-C rich fruits and vegetables at any stage in life. While an intake of vitamin C of 350 mg such as that seen in the studies cited above is well above the current recommended dietary allowance for vitamin C of 75 mg per day for women and 90 mg per day for men, it is well below the tolerable upper intake level (the highest amount one can take before experiencing adverse health effects) of 2,000 mg per day. With careful choices, people who consume the recommended five to nine servings of fruits and vegetables a day can take in this amount through diet alone. Four ounces of orange juice, a cup of strawberries, a large kiwifruit, a cup of broccoli and a salad that includes a cup of romaine lettuce and 1/4 cup of red bell pepper, for example, provides more than 350 mg of vitamin C.
Vitamin C from supplements can also be included. Most multivitamin supplements contain at least 60 mg of vitamin C.
Fat and eye health
Scientists have also been scrutinizing the relationship between dietary fat and eye disease. Indeed, some scientists believe that the process of AMD is similar to that of cardiovascular disease; the same type of problem-causing plaques that form in the arteries leading to the heart may also form in the blood vessels in the eyes.
In one large survey of more than 110,000 volunteers, researchers found that people who consumed a high-fat diet were more likely to have AMD than those consuming a lower-fat diet. More than total dietary fat, what may be important is the type of fat and the ratio of one type of fat to another. Omega-3 fat, the kind found in fish, walnuts, almonds, and flaxseed, may decrease the risk of AMD. An increased risk of AMD may be linked to high intakes of certain fats found in some animal foods such as beef, pork, and lamb, and of the types of fat found in packaged baked goods and other commercially processed snack foods.
Scientists are still teasing out the nature of the fat—eye health relationship. Still, an overall low-fat diet that includes several servings of fatty fish each week and limits commercially processed baked goods and snack foods is likely to provide benefits. Even if the eyes aren’t helped, the heart certainly will be.
The Age-Related Eye Disease Study
Scientists have long been interested in whether supplemental doses of certain vitamins and minerals could affect the onset and progression of AMD and cataracts. Late in 2001, researchers released results from a large-scale, long-term study sponsored by the National Institutes of Health and known as the Age-Related Eye Disease Study (AREDS), which was designed to probe this relationship. Their evidence was compelling enough to cause the study authors to recommend a supplement formulation for people at high risk of developing advanced AMD.
The AREDS involved about 4,700 men and women aged 55 to 80. About 3,600 of them were diagnosed with borderline or early, intermediate, or advanced AMD in at least one eye and were followed for about six years for the AMD portion of the study. Those participants who took a supplement containing antioxidants and zinc were about 25% less likely to develop further AMD, compared to those who took a sugar pill. The supplements did not seem to have much effect on those who began the study with borderline or mild AMD, and did not affect the risk of cataract.
The seeming ability of supplements to stave off further progression of AMD is particularly noteworthy because, up to this point, there has been no treatment for most cases of AMD. While supplements did not reverse damage or restore vision already lost from AMD, they did appear to help stop or delay progression of the disease.
The supplement that showed promise contained 500 mg of vitamin C, 400 International Units of vitamin E, 15 mg of beta-carotene, 80 mg of zinc, and 2 mg of copper. Copper must be added to the supplement to prevent copper deficiency, which can occur with high zinc intake.
Note that this is a high-dose formulation whose long-term effects, if any, are unknown. High doses of beta-carotene, such as those found in this mix, may be harmful to smokers.
The people in this study who benefited were those who already had AMD (the study was not designed to test whether supplements can prevent AMD) and whose disease had already progressed to a certain point. In other words, these supplements are only appropriate for certain people, and anyone considering their use should discuss it with their doctor first.
There is no evidence of benefits from other supplements, such as a pill containing lutein and zeaxanthin. (Lutein and zeaxanthin were not included in the AREDS study.) The jury is still out as to whether substances such as lutein and zeaxanthin are truly helpful in reducing the risk of eye disease. All that is known right now is that there is an association between eating foods high in these compounds and reduced risk. Because most studies of lutein and zeaxanthin have involved food sources of these nutrients, it is difficult to say if a supplement of these carotenoids alone would be beneficial; other nutrients in these carotenoid-rich foods might also be necessary to have an effect.
What you can do
Safeguarding vision, then, can come down to some sensible – and familiar – behaviors. These include the following:
- Wear sunglasses that protect the eyes from both ultraviolet A and ultraviolet B rays.
- Keep blood glucose under control.
- Stop smoking or cut back as much as possible if you smoke.
- Eat five to nine servings of produce a day. Because carotenoids add color to produce, choose brightly and deeply colored fruits and vegetables as often as possible. Include leafy green vegetables in your meals several times a week.
- Include plenty of vitamin C in your diet by choosing several daily servings of foods such as cantaloupe, strawberries, citrus fruits, kiwifruit, mango, papaya, broccoli, Brussels sprouts, sweet peppers, tomatoes, cauliflower, potatoes, and, again, leafy greens.
- Eat fatty fish two or three times a week. Limit total fat and commercially processed baked goods and snack foods.
- Have an annual eye exam (unless your doctor advises a different schedule) that includes dilating the pupils.
The good news is that there is no need to wait for the latest research to begin eating for healthy eyes. Eating dark leafy greens or foods high in vitamin C as part of a diet rich in fruits and vegetables is advice virtually every health professional can back.