|
by Philip Filner, Ph. D.
Some normal metabolic processes produce reactive byproducts which can do damage if not kept under control. Among these reactive byproducts are
three forms of oxygen: molecular oxygen, peroxide and superoxide . They all react by taking electrons from other molecules, a type of reaction called oxidation.
The body
prevents damaging oxidations by providing compounds with which the oxidants can react harmlessly. These compounds are called antioxidants.
Researchers have found that, as we
get older, our bodies' systems for controlling potentially harmful oxidants by reacting them with antioxidants, become less effective. The enzymes which catalyze the reactions may not
be as abundant, or the antioxidants consumed in the reactions may not be as abundant, as they are in younger bodies.
One class of compounds thought to possibly be antioxidants
in the body are xanthophylls, a subclass of carotenoids. The most famous of carotenoids is beta carotene, the compound which colors carrots yellow-orange. Synthetic beta-carotene is
used as food coloring, notably to color margarine yellow.
Xanthophylls are also yellow. They are made by plants, and occur in chloroplasts, the complex structures in green
leaves which perform photosynthesis. Antioxidant xanthophylls contain an -OH group. The reactivity of the -OH group of these xanthophylls allows them to function as antioxidants,
which means they can give up electrons to oxidants. In doing so, they convert oxygen-containing oxidants to something much less reactive: water.
The most common xanthophylls
are lutein and zeaxanthin. They enter the diet primarily as constituents of leafy green vegetables. They are present in blood. The concentrations in blood decline in the elderly.
Attention was first focussed on a correlation between high serum carotenoids and lower risk of macular degeneration in a report published in 1993 (1). However, because serum
carotenoids decline with age, and likelihood of macular degeneration increases with age, these two correlates of aging could conceivably not have a cause-effect relationship with each
other, but rather be independent consequences of aging.
In one report published in 1994 by Seddon et al (2) , a retrospective study of people and their diets, it was found
that people with diets high in leafy green vegetables developed macular degeneration at a lower frequency than people with other types of diets. The authors of that report speculated
that maybe the xanthophylls in leafy green vegetables are responsible for the lower risk of macular degeneration. There are at least two serious weaknesses with this interpretation.
1: Diets high in leafy green vegetables provide many other compounds besides xanthophylls, which are not found in abundance in other diets; and
2: People whose diets
are high in leafy green vegetables are likely to have life styles with substantial differences from the life styles of people with other types of diets.
Consequently, there are
many other potential reasons besides xanthophylls for the lower risk of macular degeneration among people whose diets are high in leafy green vegetables. It remains to be determined
if increasing lutein or any other xanthophyll in the diet has any value as a treatment for macular degeneration.
Recently, carotenoids have been extracted from the maculae of
human eyes donated for research. Lutein and zeaxanthin were found to be the only carotenoids there (4).
In living people, there is a "psychophysical" technique for
estimating the amount of macular pigment (xanthophylls) in the fovea/macula region. Light at a wavelength absorbed by xanthophylls is shone on a very small portion of the fovea or
macula, and then the wavelength is switched to one not absorbed by xanthophylls, then back, for many cycles, which results in the indivdual sensing a flicker. By increasing the
intensity at the absorbed wavelength until no flicker is observed, the amount of light absorption can be estimated, and from that the amount of absorbing compound can be estimated.
Diets high in vegetables rich in xanthophylls, or a diet supplement containing purified lutein, can increase macular pigment an average of about 25% in most patients, while
others show no increase (4,5). It remains to be determined if such a modest increase has an impact on macular degeneration.
Some questions in need of answers based on
well-controlled studies:
1. If someone who has not had a diet high in leafy green vegetables, or has not been taking xanthophyll-containing diet supplements, switches to such
a diet, will it reduce that person's risk of developing macular degeneration ?
2. If someone who has not been on a diet high in leafy green vegetables or not been using
xanthophyll-containing diet supplements, has macular degeneration, and switches to a diet high in leafy green vegetables or begins taking xanthophyll-containing diet supplements, will
the progress of macular degeneration slow, stop or even reverse ?
REFERENCES
1. EDCC Study Group, Antioxidant Status and neovascular-Related macular
Degeneration,(1993), Arch. Ophthalmol. 111:104 -109.
2. Johanna M. Seddon, MD, Umed A. Ajari, MBBS, Robert D. Sperduto, MD, rita Hiller, MS, Norma Blair, MD, Thomas C. Burton,
MD, Marylyn D. Farber, Ph. D., Evenagelos S. Gragoudas, MD Julia Haller, MD, Dayton T. Miller, Ph. D., Lawrence A. Yannuzzi, MD ; Walter Willett, MD, for the Eye Disease Case-Control
Study Group, Dietary Carotenoids, Vitamins A, C, and E, and Advanced Age-Related Macular Degeneration, (1994), JAMA 372:1413-1420
3. J. T. Landrum, Richard A. Bone, Mark D.
Kilburn, The Macular Pigment: A Possible Role in Protection from Age-Related Macular Degeneration, (1997) Adv. in Pharmacol. 34:537-555.
4. Billy R. Hammond, Jr., Elizabeth J.
Johnson, Robert M. Russell, Norman J. Krinsky, Kyung-Jin Yeum, Ross B. Edwards and D. Max Snodderly, Dietary Modification of Human Macular Pigment Density, (1997) Investigative
Ophthalmology & Visual Science 38:1795-1801.
Posted August 29, 1997
Revised December 24, 1997 |
|
