Dietitian Helen Rasmussen
weighs exact amounts of
lutein-enriched eggs and
other ingredients for an
egg frittata that participants
will eat during testing to see
whether food type affects
lutein absorption into the
"Lens cells make a specific, predominant set of proteins called
crystallins," 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 retinaa 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.
Nutritional biochemist Elizabeth
Johnson extracts lipids from
blood plasma to test for
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 opacificationover
5 years of follow-up after their initial examinationwas 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
Chemist Allen Taylor views
films that show that oxidative
stress damages eye lens
| 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 withoutwhich supports the
premise that these antioxidants provide some protection.
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 useknown as bioavailabilityof
lutein from eggs, spinach, and supplements.
After study volunteershealthy adult menconsumed 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 timesabout
1.5 milligramsthe 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,"
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.
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, beta-carotene, 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
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,
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.By Rosalie Marion Bliss,
Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National Program
(#107) described on the World Wide Web at www.nps.ars.usda.gov.
To reach scientists mentioned in this article, contact Rosalie
Bliss, USDA-ARS Information
Staff, 5601 Sunnyside Ave., Beltsville, MD 20705; phone (301) 504-4318;
fax (301) 504-1641.
"Scientists Link Nutrition & Eye Health" was published
in the August
2003 issue of Agricultural Research magazine.