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Misty L. Trepke

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Scientists Link Nutrition & Eye Health

 

 

About 16 million people in the United States over age 45 report some

vision loss. This group may find hope in a growing body of evidence

that diet can influence eye health. ARS has several scientists

studying the possibility of reducing-by way of dietary modification-

the risk of two common sight-robbing disorders: cataract formation

and age-related macular degeneration.

 

About 20 years ago, scientists were hard-pressed to find published

research studies on correlations between nutrition and risk of eye

disease. But steady efforts by government and academic researchers

over the years have led to a clearly established discipline of

ophthalmologic nutrition and epidemiology.

 

A Cloudy Matter

For baby boomers reaching an age at which steady vision can no

longer be taken for granted, many are wishing they'd worn sunglasses

when young. Sunlight is somewhat of a natural enemy to the eye's

lens.

 

" Lens cells make a specific, predominant set of proteins called

crystalline, " says bio-organic chemist Allen Taylor. He is chief of

the Laboratory for Nutrition and Vision Research at the Jean Mayer

USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts

University in Boston, Massachusetts. " Those proteins act like fiber

optics, allowing light to pass through the lens and onto the

retina, " he says. They must function over decades with little

opportunity for repair.

 

Red, blue, green, yellow, and ultraviolet (UV) wavelengths penetrate

the transparent lens. But UV light appears to be particularly

damaging to the lens, and blue light appears to damage the retina-a

complex, sensory membrane that lines the eye and receives the images

formed by the lens. Normal byproducts of metabolism, called oxygen

free radicals, also cause damage. If not neutralized by an

antioxidant, over time such oxidation damages the lipids, proteins,

and other components of the lens. The result is a clouding of the

lens in a gradual slide from transparent to opaque. These opacities

are called cataracts.

 

Antioxidants are compounds in foods that help maintain healthy cells

and tissues in the eye and other organs. Inside the lens are high

levels of vitamins C and E as well as some lutein and zeaxanthin.

The latter two fall within a class of phytochemicals called

carotenoids, and they are concentrated in the retina.

" As damaged proteins gather, they result in lens opacities, " says

Taylor. His research suggests that protective, antioxidant-rich

nutrition could be the least costly and most practical means to

delay cataracts. " The accumulation of oxidized or modified proteins

we've observed is consistent with the failure of protective systems

to keep pace with the insults that damage lens proteins, " he says.

The protective systems include protein-digesting enzymes, which may

seek out and destroy damaged proteins, as well as antioxidants,

which can lessen initial damage and may keep protective enzymes

functioning longer.

 

In economic and human terms, damage to lens proteins is costly.

About half of those over 75 in the United States will experience a

visually significant cataract. The costs of cataract-related

disability and cataract surgery now total $6 billion annually

worldwide.

 

Various Causes

Three distinguishable areas of the lens can be affected by

cataracts: the nuclear, cortical, and posterior subcapsular (PSC)

areas. The nuclear and cortical areas are associated with age-

related cataracts, while the PSC area is associated with diabetes-

related cataracts.

 

Nuclear lens opacity has been the most widely studied of the three

lens areas. Paul F. Jacques, Taylor, and colleagues reported in 2001

that antioxidant nutrients play a role in the prevention of nuclear

cataracts. Jacques is chief of HNRCA's Nutritional Epidemiology

Program.

 

The scientists looked at 478 nondiabetic women from Boston, aged 53

to 73 years and not previously diagnosed with cataracts. These women

were sampled from the Nutrition and Vision Project (NVP), a substudy

of the federally funded Nurses' Health Study. Researchers conducted

eye exams to study the relationship between newly diagnosed nuclear

opacities and nutrient intake over time. Food intake was assessed

from multiple food frequency questionnaires completed over 13 to 15

years.

 

The study showed that women with the highest intakes of vitamins C

and E, riboflavin, folate, beta carotene, lutein, and zeaxanthin had

a lower prevalence of nuclear opacification than did those with the

lowest intakes of those nutrients. Moreover, those who used vitamin

C supplements for 10 or more years were 64 percent less likely to

have nuclear opacification than those who never used vitamin C

supplements.

 

Taylor, Jacques, and colleagues reported similar findings in 2002

when they looked for cataracts in the cortical and PSC regions of

the lens in some NVP participants. Those findings support a role for

vitamin C in reducing the risk of cortical cataracts in women

younger than 60. The data also indicated that women who consumed

higher amounts of carotenoids had a lower risk of PSC cataracts if

they had never smoked.

 

In the same NVP population, women who regularly took vitamin E had

less progression of eye lens damage, as Taylor reported during last

year's Association for Research in Vision and Ophthalmology

proceedings in Fort Lauderdale, Florida. " The increase in nuclear

opacification-over 5 years of follow-up after their initial

examination-was 30 percent lower among women who used vitamin E

supplements for at least 10 years than among those who had never

used vitamin E supplements, " says Jacques.

 

Another recently completed study explored the relationship between

body mass index, waist circumference, diabetes, and the presence of

age-related cataracts in women. The study supports other findings

that diabetes is a strong risk factor for PSC opacities and that

abdominal fat and obesity may also be associated with PSC.

Several variables complicate a comprehensive evaluation of the

existing evidence linking nutrition and age-related vision

loss. " Definitions of cataract may differ from one study to another,

and the various methods for assessing the intake or status of

nutrients, such as antioxidants, certainly complicate matters, " says

Jacques. " There are several questions that still need to be

resolved. "

 

At this point, what scientists do know is that oxidative damage

within the eye is harmful to several eye tissues.

 

The Yellow Spot

Among Americans who are 55 or older, age-related macular

degeneration (AMD) is reported to be a leading cause of blindness

and vision impairment. According to the National Eye Institute

(NEI), more than 1.6 million Americans in that age group have

advanced AMD. Some experts estimate up to 7 million more may be at

the intermediate stage. They see fine now, but they are at high risk

for developing the advanced form, which causes vision loss.

Among the causes of AMD, scientists describe a breakdown of light-

sensitive cells within the retina. The focus is on a 3-millimeter-

wide yellow spot, called the macula lutea, toward the back and

center of the eye. The macula plays a key role in the central part

of visual images. But as the eye ages, oxidized proteins, or debris

called drusen, begin to pile up and cause trouble. Taylor and other

scientists are seeking to unravel the mystery of why this process

happens.

 

Scientists have long known that the yellow color, or pigment, inside

the macula comes from the carotenoids lutein and zeaxanthin. Many

scientists believe that these plant chemicals help protect the eye

by absorbing blue light and neutralizing free radicals. But as the

body ages, the importance of carotenoids in the macula may increase

because of the lifelong exposure to damaging light.

 

These two carotenoids circulate in the food supply and in blood

plasma at a ratio of about one part zeaxanthin to about six or seven

parts lutein. As blood passes by the macula through retinal blood

vessels, these pigments pass through the macula's outer layer to

rest in high concentrations inside its center. Perhaps most

interesting is that people with macular degeneration have been found

to have lower levels of zeaxanthin and lutein in the macula than

people without-which supports the premise that these antioxidants

provide some protection.

 

Absorbing Research

Nutritional biochemist Elizabeth J. Johnson, who is with HNRCA's

Carotenoids and Health Laboratory, is now leading a study aimed at

determining differences in the body's absorption and use-known as

bio-availability-of lutein from eggs, spinach, and supplements.

After study volunteers-healthy adult men-consumed cooked spinach,

eggs, and lutein supplements, Johnson measured levels of lutein and

triglyceride-rich lipoproteins in their blood serum. The study,

which is supported by the Egg Nutrition Center, in Washington, D.C.,

used eggs from chickens that had been fed marigold petals, which are

high in lutein. Consumption of these eggs considerably increased the

lutein in volunteers' blood.

 

" After volunteers ate eggs as a source of lutein, their blood serum

level of lutein was two to three times greater than it was after

they ate the same amount of lutein from other sources, " says

Johnson. These preliminary results provide compelling evidence that

eggs can be a more bioavailable source of lutein than more

conventional sources, such as spinach and supplements. " We don't

know why the lutein in egg yolks is more bioavailable, but we think

it's due to other components in the yolks, such as lecithins. "

The " designer " eggs used in the study had about six times-about 1.5

milligrams-the lutein of standard eggs. Still, spinach has about 11

milligrams per 2-ounce serving. " Even though the lutein in the eggs

is a comparatively tiny amount, it goes right into the bloodstream, "

says Johnson.

 

She has also studied and will soon report the effects of lutein and

zeaxanthin supplementation on carotenoid levels in the blood,

adipose (fat) tissue, and macula of monkeys. That research has led

to new findings about the source of an important form of zeaxanthin,

called meso zeaxanthin. Curiously, that form is found in the macula,

but not in food or blood. It may be better than lutein at reducing

damage from light entering the eye. Johnson believes meso zeaxanthin

could actually be formed from lutein once it's inside the macula

itself.

 

Diminishing Risk

In 2001, NEI researchers reported results from the 7-year Age-

Related Eye Disease Study, or AREDS. Results showed that people

lowered their risk of developing advanced AMD by about 25 percent

when they took a high-dose combination of vitamins C and E,

betacarotene, and zinc for more than 6 years.

 

NEI defines high risk as having intermediate or advanced AMD in one

eye. In those with advanced AMD, the nutrients reduced their risk of

further vision loss by about 19 percent. NEI concluded that while

the nutrients will not restore vision already lost from the disease,

they may play a key role in helping high-risk people keep their

remaining vision.

 

Lutein supplements were not available at the study's inception, but

NEI is starting to study it now. " Lutein is compelling because of

evidence that it neutralizes free radicals, " says Johnson. " Since

it's in the macula, it's right where it needs to be to protect

against damage. " In the meantime, during regular examinations, eye

doctors can see the telltale signs of early, intermediate, and

advanced AMD.

 

The AREDS supplements had no significant effect on cataract

development or progression. But intervention was only for about 6

years, and some people in the control group had already taken, or

continued to take, antioxidant supplements.

Cataract surgery is the most expensive outpatient surgery covered by

Medicare. While some see surgery as a stopgap intervention for

cataracts, there is as yet no known surgical remedy for AMD, making

optimal nutrition all the more attractive.

Researchers focusing on eye health today agree that for some,

nutrition will play an important role in lessening the risk of

developing these sight-robbing eye disorders.

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