That yellow stuff in the eye

Some 1.7 million people in the United States are affected by AMD. It is the leading cause of blindness among persons over age 65, although fewer than 10 percent of people with the disease become totally blind. Most instead develop what are known as macular holes—patches of their visual field that go permanently blank.

The problem afflicts the central portion of the eye’s retina, which reacts to light and relays visual signals to the brain. Rods and cones, the light-sensing elements in this region, begin to fail. In the beginning, some areas of the visual field may get blurry. Eventually, a blind spot can develop (see diagram). In the worst type of this disease, known as wet AMD, full and total blindness becomes a serious risk. This form accounts for about 20 percent of all AMD.

The retina’s macula—where AMD strikes—is yellow. Ten years ago, Frederik J.G.M. van Kuijk and his colleagues discovered that the macula’s coloring reflects this tissue's role as a rich store of carotenoids—pigments derived from plants in the diet.

The importance of the finding was driven home 6 years later, when the national study indicated that people with advanced AMD tend to have far lower blood concentrations of lutein and zeaxanthin—both yellow carotenoids—than do persons without the disease (SN: 11/12/94, p. 310). These two carotenoids are selectively deposited in the macula.

In the Nov. 9, 1994 Journal of the American Medical Association, Johanna M. Seddon of the Massachusetts Eye and Ear Infirmary, and her colleagues, concluded "that increasing dietary intake of foods that are rich in antioxidants, particularly certain carotenoids, may reduce the risk of developing advanced AMD." Indeed, they argued, it "seems prudent" that people boost dietary consumption of vegetables—especially "dark green, leafy vegetables that are rich in lutein and zeaxanthin."

Van Kuijk knew from his own work that at least a few other foods contain substantial amounts of these pigments. However, most published assays lumped lutein and zeaxanthin together.

Recognizing that a food with a high concentration of the pair might represent an abundance of just one—and that good eye health may require getting adequate amounts of both—he set about assaying these yellow pigments individually in a host of foods.

 

Put a rainbow in your salads

In the August British Journal of Ophthalmology, van Kuijk, now at the University of Texas Medical Branch in Galveston, and his international team report their findings.

Carotenoids not only color most plant-derived foods but also various tissues in the animals that eat them. That explains how egg yolk got to the top of van Kuijk's list. Almost 90 percent of its carotenoids come from lutein and zeaxanthin, present in a roughly 60:40 ratio. Yellow corn proved almost as rich a source, with its lutein and zeaxanthin occurring in about a 70:30 ratio. In a few other foods, such as orange juice and honeydew melon, the pair also appear in a balanced ratio—though together they make up only about one-third of the carotenoids coloring these fruits.

Prefer to mix and match? Lutein provides some 30 to 50 percent of the carotenoids coloring kiwi, red grapes, zucchini squash, pumpkin, spinach, yellow squash, cucumber, peas, butternut squash, and celery. It accounts for 12 to 27 percent of the carotenoid pigments in Brussels sprouts, green grapes, scallions, green beans, broccoli, red delicious apples, green lettuce, and yellow peppers.

You can then balance these by making sure you take in some oranges or mango—which derive about 15 percent of their coloring from zeaxanthin. Better yet, reach for an orange pepper, which gets more than a third of its color from the carotenoid. Minor sources of zeaxanthin include nectarines, peaches, and grapes.

From their appearance, you might expect carrots, cantaloupe, and dried apricots to figure prominently in one category or the other. In fact, van Kuijk’s group finds, each of these contains only negligible traces of lutein and no zeaxanthin. Most of their bright orange coloring instead traces to their high concentrations of the better known beta- and alpha-carotenes.

 

Why we need them

While lutein and zeaxanthin color the macula, no one is quite certain what their function is, van Kuijk acknowledges. One theory is that they provide on-site antioxidant defense for the eyes’ vulnerable photoreceptors.

Several studies have suggested that AMD may be caused by—or at least aggravated by—oxidizing free radicals, such those spawned by cigarette smoke. Indeed, cigarette smoking, known to deplete the body’s antioxidant loading, is an established risk factor for AMD.

However, van Kuijk observes, the macular carotenoids also have the "unique" ability to filter out wavelengths of blue light that fall just below the threshold of detection by the eyes’ photoreceptors. Since this light doesn’t contribute to vision and carries a lot of energy, he notes, "we think it’s harmful." Certainly, he told Science News Online, "we know from animal experiments that [these blue wavelengths] can cause degeneration of the retina more easily than visible light can."

Scientists remain uncertain about not only the pigments' function, but also how their concentrations relate to AMD, van Kuijk points out. Does the inexorable antioxidant assault in the body—which tends to build over time, as natural defense systems wear out—deplete the macula’s carotenoids faster than the diet can replenish them? Or does a lifetime of insufficient quantities of these compounds in the diet merely leave the retina woefully unprotected?

As yet, van Kuijk concedes, there are no answers. But it certainly can’t hurt to supply the body with a heaping helping of these pigments each day, he says. Moreover, he insists, this dietary practice should not be reserved for gray-haired diners. Like heart disease, macular degeneration probably constitutes the symptomatic end of some chronic—indeed, lifelong—assault on retinal tissue. So the earlier the carotenoids are in place and the longer they remain in full strength, he says, the more likely an individual will ward off this debilitating condition.

This week, Henkel Nutrition and Health Group, a global producer of ingredients used in vitamins and nutrition supplements, unveiled a new product to be marketed to dietary-supplement manufacturers. The company says that it contains lutein and zeaxanthin in the same chemical form present in most fruits and vegetables, although the lutein is actually extracted from marigold flowers grown in Ecuador.

However, van Kuijk would prefer that consumers reach into the garden or the refrigerator to get their carotenoids. "I think you want to make a multi-colored salad. That way you’re sure to get a nice mix of all the carotenoids"—not to mention all the other vitamins, minerals, and fiber that they bring to the table.

  This week's Food for Thought has been prepared by Janet Raloff, senior editor of Science News.

copyright 1998 ScienceService