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Nutrition and Eye Health: Vitamin A and the Carotenoids
Together with vitamins C and E, vitamin A and the carotenoids play an important role in nutrition and eye health, especially in preventing cataracts. A cataract clouds the lens of the eye, causing painless yet insidious loss of vision. Most cataracts are associated with aging. Age-related cataracts are the leading cause of loss of vision in most of the world, and in the United States alone, over 1 million operations a year are performed to remove them. This is nearly 500,000 more surgeries than the next nine most frequent operations combined. A key concern in nutrition and eye health is preventing cataracts is supplying the eye with the nutrients needed to manufacture the antioxidant glutathione. When concentrations of this antioxidant fall below a critical level in the lens, it loses its ability to regulate the flow of sodium and potassium in and out of its cells. The interference with the sodium channels makes the cells less able to use oxygen and less able to detoxify free radicals of hydrogen peroxide. The lens is left vulnerable to inflammation and swelling. Also with the depletion of glutathione in the lens, its proteins are no longer protected against UV radiation. Over a period of years, the proteins form cross-linkages in a star pattern. As more and more proteins in the lens are damaged by UV light, the result is a cataract. Nutritional research has identified vitamins C and E as the key antioxidants needed by the lens to maintain healthy levels of glutathione. One group of researchers found that taking 400 IU of vitamin E (alpha-tocopherol) daily reduces the risk of developing age-related cataracts by 56 per cent. Taking that 400 IU of vitamin E with as little as 100 mg of vitamin C reduces the risk by 70 per cent. Vitamins C and E and key elements in nutrition and eye health, but they are more useful at preventing certain kinds of cataracts than others. The Vitamin E and Cataract Prevention Study (VECAT) has found that in people aged 55 to 80, vitamins C and E are useful for preventing cortical cataracts (the cataracts that cause trouble driving at night and distortions in color perception) but not nuclear cataracts (cataracts resulting in nearsightedness). That is why it is not enough to get vitamins C and E. Vitamin A and the carotenoids are also essential. Much of the evidence for the usefulness of vitamin A and the carotenoids that the body can transform into vitamin A comes from studies of diet and cataracts. An Italian study found that people who ate vitamin A and beta-carotene rich melon, peppers, citrus, tomatoes, spinach, and cruciferous vegetables (cabbage, kale, turnips, rutabaga, Brussels spouts) along with modest amounts of meat and cheese that help the body absorb the nutrients were the least likely candidates for cataract surgery. In the United States, the Nurses' Health Study found that regular consumption of Kale and spinach, also rich sources of carotenoids, was protective against cataracts in women. The Health Professionals Follow-Up Study found that carotenoids-rich broccoli and spinach reduced risk cataracts in men. Two lesser-known carotenoids are also important in nutrition and eye health. Lutein and zeaxanthin defend the lens against the degenerative damaged induced by years of exposure to ultraviolet light. The Nurses' Health Study found that women who consumed the greatest amounts of lutein and zeaxanthin were 22 per cent less likely to have cataract surgery compared to women who consumed the least. The Health Professionals Follow-Up Study found 19 per cent fewer cataract surgeries with the highest intake of lutein and zeaxanthin in men. |