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Aging Pigment Eyed as Cause of Macular Degeneration
Major cause of
blindness in senior citizens may come from thinning pigment
By
Toni Baker
March 31, 2006 - Whether a tiny yellow pigment is
the main thing standing between many older people and macular
degeneration is under study at the Medical College of Georgia.
 Researchers are measuring this macular pigment that
sits on the retina at the fovea, the point of highest vision acuity and
best color vision, to better understand what a healthy, normal pigment
looks like, says Dr. John Nolan, vision scientist and Fulbright
postdoctoral fellow in the MCG Department of Ophthalmology.
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The idea is that thinning of this pigment –
comprised of yellow antioxidants that come from food or dietary
supplements – opens the door for retinal cell destruction, the hallmark
of macular degeneration.
The hope is that measuring this protective pigment
will one day be part of routine vision screening and macular
degeneration will lose its distinction as the leading cause of blindness
in people over age 60, says Dr. Nolan.
“This pigment is thought to protect the retina from
damage by light and excess oxidation, which over a lifetime can
accumulate and contribute to the process of macular degeneration,” says
Dr. Max Snodderly, MCG vision scientist and Dr. Nolan’s sponsor.
The retina receiving too much high-energy blue
light – the same light that gives the sky its color – seems to be a
major cause of oxidation and cell death.
“So you are kind of rusting as you get old,” Dr.
Snodderly says. A macular pigment that’s thin because of genetics or
gets that way because of a poor diet or smoking, enables such rusting.
Dr. Snodderly’s studies of monkeys raised on a
synthetic diet lacking components of the pigment – lutein and zeaxanthin
– showed they also lacked the pigment until the components were added to
the diet. “The retina was able to take it up, even though it had never
seen it before,” he says of the natural pigments found in dark green
leafy vegetables such as spinach, turnips and collards, as well as
colored fruit and egg yolk.
Studies of more than 800 people age 20-60 with good
vision back in Dr. Nolan’s homeland of Ireland have parallel findings:
that the protective macular pigment can be increased with dietary change
and/or dietary supplements of lutein and zeaxanthin.
His studies, based at the Waterford Institute of
Technology, included a subset of 200 people with healthy vision whose
parents had macular degeneration.
“This group with perfect vision had significantly
lower levels of macular pigment than the control group. This reinforces
the macular pigment story,” says Dr. Nolan, whose work has indicated a
thinning of the macular pigment occurs in most aging adults although
other scientists, including Dr. Snodderly, have not seen that
consistently.
“It may be that once you go beyond 60, which is the
age when macular degeneration typically starts developing, the pigment
is depleted for several reasons, including increased oxidative stress
and a poor diet, both associated with an increase in age,” Dr. Nolan
says.
“We found that smokers have significantly less
macular pigment, both because they tend to have a poorer diet and
smoking causes increased oxidative stress.
“It makes biological sense that if you are really
deficient in macular pigment that you will get macular degeneration,”
Dr. Nolan says of mounting evidence.
But proving it is another matter and is one of the
things that brought him to MCG for a year to work with Dr. Snodderly,
whose contributions in the field include helping identify components of
the pigment and helping invent a way to measure it.
Dr. Nolan’s latest study, sponsored by his
Fulbright scholarship and Dr. Snodderly’s lab, is looking again at 50
people age 20-60 with no major visual problems to precisely measure
macular pigment and surrounding anatomy.
Measurements can be taken quickly and painlessly,
without even dilating the eyes. Optical coherence tomography or OCT,
already used by some eye doctors, provides three-dimensional images and
measures of the fovea. The densitometer, which Dr. Snodderly helped
develop and largely remains a research tool, enables measurement of the
macular pigment.
“The long-term goal is that we can screen for
people who are deficient in this pigment, and these are the people who
should be targeted,” says Dr. Nolan. “What we are trying to find out in
the meantime is, ‘Who are those people’? (And) what is a significantly
lower level of macular pigment? We know what the average is, but what is
a critically low level and is it different between individuals?”
He notes the solution may be as simple as an
improved diet. Longitudinal studies by the National Eye Institute have
shown a 25 percent reduction in progression to macular degeneration
among those who took antioxidants.
Participants in Dr. Nolan’s study also are asked
questions about diet, smoking and other lifestyle habits. The process
takes about one hour. Interested volunteers can reach Dr. Nolan at
706-721-6382 or
jnolan@mcg.edu.
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