We all grew up knowing that carrots are good for our eyes. Carrots, among other deeply colored vegetables and fruits, are rich sources of beta carotene, the plant-based building block for vitamin A, which is required to form the visual pigment rhodopsin that allows people to see in the dark.
One eventual consequence of a chronic vitamin A deficiency is night blindness, which remains a scourge among children in many poor countries and sometimes among poorly nourished children in this country.
But now medical science is beginning to appreciate a lot more about the influence of diet on vision. And, it turns out, it is not just carrots and vitamin A. Other potentially important nutrients include two related carotenoids, lutein and zeaxanthin; vitamins C and E; docosahexanoic acid, or DHA, found primarily in fish oil; and possibly zinc.
While evidence for the effects of these various nutrients on both visual development and prevention of sight-robbing eye diseases is not yet conclusive, there are enough strong hints from recent research to justify a number of dietary improvements. Even if some of those changes turn out to be only minimally helpful to eyes, they have myriad other health benefits.
DHA for Developing Vision
Evidence for the vital role DHA plays in visual development comes from premature babies. Infants born eight or more weeks early arrive almost fat-free, lacking both a reserve of stored energy and the fatty acid DHA that is essential for normal visual (and brain) development. DHA normally accounts for more than one-third of the fatty acids in the retina of the eye as well as in the brain's gray matter. The retina develops rapidly in the final months of pregnancy and in the first six months of infancy.
Fetuses begin to acquire large amounts of DHA only in the last three months of pregnancy. Unless a premature baby is fed breast milk (which naturally contains DHA) or formula fortified with DHA, the child's visual acuity is likely to be compromised. Deficits in visual inputs early in life can permanently alter the development and organization of the brain's visual cortex.
Nearly a decade ago, for example, Dr. Susan E. Carlson and colleagues at the University of Tennessee at Memphis described the benefits of DHA supplementation in a study of 67 infants born about two months early. When compared with premature babies given standard preterm formula, those who received formula fortified with DHA showed gains in visual acuity for about four months.
In a study by Dr. Eileen E. Birch and colleagues at the University of Texas Southwestern Medical Center in Dallas, preterm babies fed formula fortified with fish oil developed visual acuity similar to that of term infants and preterm infants fed breast milk. But preterm infants fed formula without DHA had notably poorer visual acuity.
And in a study published last year in The American Journal of Clinical Nutrition by Cathy Williams, now at the Bristol Eye Hospital in England, full-term infants were also seen to benefit visually from DHA. Children who were breast-fed for four months were more likely to achieve a high degree of stereoscopic vision at the age of 3 1/2 than were children who were not breast-fed. And children whose mothers ate fatty fish during pregnancy developed better stereopsis than those whose mothers did not.
For breast-fed babies, the amount of DHA in breast milk depends on the mother's diet. The breast milk of many Europeans is twice as rich in DHA as that of Americans, and that of Japanese women contains about three times as much DHA.
The best dietary sources of DHA are cold-water fish like mackerel, Atlantic salmon and cod, bluefish, halibut, herring, sockeye salmon, striped bass, tuna and flounder. Two to three servings of such fish per week can maintain adequate levels of DHA in an adult. Pregnant women who do not eat fish may take a daily DHA capsule, sold as a dietary supplement in health food stores and pharmacies.
Age-Related Eye Disorders
Two common sight-robbing disorders, cataracts and age-related macular degeneration, have been linked in several large studies to dietary deficits of certain nutrients. While it is not yet proved that maintaining an adequate intake of these nutrients can prevent the eye disorders, it certainly cannot hurt. Last summer, the newsletter Nutrition Action, published by the Center for Science in the Public Interest in Washington, described recent findings on the role diet might play in preventing these disorders.
Nearly everyone who lives long enough in this society develops cataracts, a clouding of the eye's lens that progressively reduces the ability to see clearly. The main cause of this cloudiness is believed to be oxidative damage by so-called free radicals caused by exposure to sunlight. Thus, antioxidants, which act as scavengers for free radicals, are believed to protect against this damage.
One antioxidant nutrient, vitamin E, occurs naturally in the lens, and in animal studies, supplements of vitamin E have slowed the rate at which cataracts form. In several studies in people, notably the Baltimore Longitudinal Study of Aging, the Beaver Dam (Wisconsin) Eye Study and the Lens Opacities Case-Control Study, higher rates of cataracts occurred among people whose intake of vitamin E from their diets or supplements was low.
Vitamin C, another antioxidant found in the lens, has also prevented cataracts in experimental animals. In a study of 247 nurses, those who took vitamin C supplements for at least 10 years had a prevalence of beginning cataracts that was 77 percent lower than that found among the nurses who took little or no vitamin C.
But for both vitamins E and C, it is not possible to say for sure that the vitamins themselves were protective and not simply a reflection of a healthier way of life that protects eyes from age-related damage.
The strongest evidence for eye benefits from dietary ingredients involves the two carotenoids lutein and zeaxanthin, both found in large amounts in the lens and retina. Lutein may help prevent cataracts through its ability to absorb damaging ultraviolet light, blocking oxidative damage.
Perhaps even more important, these carotenoids may prevent and even partly reverse otherwise untreatable damage to the macula, the area in the center of the retina that allows people to see whatever is in the center of their visual fields. People with advanced macular degeneration cannot see faces, read, watch television or drive; they are legally blind and can only see the edges of their visual field.
Lutein and zeaxanthin are most prominent in dark green leafy vegetables, especially kale, collard greens, spinach and turnip greens. Some experts on low vision now recommend eating half a cup of one of these cooked vegetables daily to prevent development or progression of macular degeneration.
Zinc, too, is concentrated in the retina and is part of enzymes that defend against oxidative stress. But thus far, evidence of its effectiveness in people is inconsistent. One study found that taking 100 milligrams of zinc a day reduced visual loss in people with advanced macular degeneration, but another study found no benefit.
Chart: ''Diet and Vision'' A recent study showed that the consumption of 14,000 micrograms of lutein and zeaxanthin each day may help protect against cataracts. FOOD: Cooked, cup Kale LUTEIN, ZEAXANTHIN -- Micrograms: 10,270 Collard greens LUTEIN, ZEAXANTHIN -- Micrograms: 7,690 Spinach LUTEIN, ZEAXANTHIN -- Micrograms: 6,340 Turnip greens LUTEIN, ZEAXANTHIN -- Micrograms: 6,080 Broccoli LUTEIN, ZEAXANTHIN -- Micrograms: 1,740 Corn LUTEIN, ZEAXANTHIN -- Micrograms: 1,480 Brussels sprouts LUTEIN, ZEAXANTHIN -- Micrograms: 1,010 Green beans LUTEIN, ZEAXANTHIN -- Micrograms: 440 Okra LUTEIN, ZEAXANTHIN -- Micrograms: 310 Raw Spinach, 1 cup LUTEIN, ZEAXANTHIN -- Micrograms: 3,580 Zucchini, cup LUTEIN, ZEAXANTHIN -- Micrograms: 1,320 Squash, cup LUTEIN, ZEAXANTHIN -- Micrograms: 190 Tomato, cup LUTEIN, ZEAXANTHIN -- Micrograms: 80 Cabbage, cup LUTEIN, ZEAXANTHIN -- Micrograms: 50 Other Romaine lettuce, 1 cup LUTEIN, ZEAXANTHIN -- Micrograms: 1,480 Peas, cup LUTEIN, ZEAXANTHIN -- Micrograms: 1,150 Orange juice, 1 cup LUTEIN, ZEAXANTHIN -- Micrograms: 340 Baby carrots, 8 LUTEIN, ZEAXANTHIN -- Micrograms: 290 Orange, 1 LUTEIN, ZEAXANTHIN -- Micrograms: 240 Iceberg lettuce, 1 cup LUTEIN, ZEAXANTHIN -- Micrograms: 190 (Sources: U.S. Department of Agriculture; Center for Science in the Public Interest)