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Omega-3
Fatty Acids and Prevention of
Age-Related Eye Diseases
By Morten Bryhn,
MD, PhD
9/1/2003
Age-related
macular degeneration is the most common disease leading
to permanent blindness. The number of seniors getting this
disabling disease will increase, as more people are reaching
high age. The risk factors are the same as for getting cardio-vascular
disease but it seems that high intake of omega-3 fatty acids
can prevent development of macular degeneration.
Diseases
of the eye leading to blindness are almost exclusively a function
of ageing. But certain risk factors make us age faster. There
are genetic dispositions for developing eye diseases (1),
and in these cases control of possible risk factors is extremely
important. Today age-related macular degeneration (ARMD) is
the most common disease leading to permanent blindness. The
number of seniors getting this disabling disease will increase,
as more people are reaching high age. The disease is located
to the retina, which is the thin transparent layer of nerve
tissue that forms the innermost coat of the eye. (Fig. 1)
The
retina is continuous with the optical nerve leading electrical
impulses to the posterior part of the brain. In the retina
light energy is transformed into electrical impulses by
conformational changes of the light-sensitive protein rhodopsin.
The omega-3 fatty acid docosahexaenoic acid (DHA) plays
an important part in the generation of these impulses, which
are later decoded in the visual cortex of the brain making
us perceive what we are casting our eyes on.
The
cause of ARMD is unknown but smoking, high intake of saturated
fat and cholesterol as well as other cardio-vascular risk
factors like diabetes and hypertension have shown statistically
associations with development of the disease (Fig2A and
2B).
Frequent
sunlight exposure throughout life is another risk factor
(2). It has been claimed that chronic use of pharmaceuticals
could increase the risk of getting ARMD but this could not
be demonstrated in the large Beaver Dam Eye Study (3). Interestingly
populations with a high intake of fish, like in Japan and
Island have low incidence of ARMD. DHA, which is highly
enriched in the retina, is one of the most abundant omega-3
fatty acids in fish.
Smith
and co-workers (4) examined 3654 individuals older than
49 years in order to reveal clinical signs of ARMD. At the
same time dietary questionnaires were collected to correlate
the intake of certain nutrients with the occurrence of the
disease. 229 of the participants were found to have clinical
signs of ARMD in different stages of development. Correlation
with the different dietary intake of nutrients among the
participants in the study clearly showed that a regular
intake of fish (1-3 fish meals per month) prevented development
of ARMD while a high intake of saturated fat and cholesterol
predisposed to a higher risk of contracting the disease.
In another
study, the large Nurses' Health Study from Boston including
42 743 women and 29 746 men followed for almost 10 years,
567 developed ARMD (5). Fat intake in the study was assessed
by a food-frequency questionnaire. As in the study by Smith
total fat intake positively correlated with increased risk
of getting the disease while a high intake of fish was associated
with a 35% lower risk compared to those who only seldom
had fishmeals.
Obviously
DHA has important functions in the retina. Premature born
children not supplemented with DHA have slower development
of visual acuity compared to those given this essential
fatty acid (6). Experiments on rat retina have demonstrated
that DHA prevents damages of oxidative stress, an important
factor related to aging (7). Another experiment on monkeys
has shown that animals given omega-3 fatty acids may improve
nutritional utilisation in the retina and at the same time
reduce negative effects of oxidative damage (8). Several
studies have been presented using antioxidants to prevent
oxidative damage. While vitamin E alone has not demonstrated
any positive effects, the combination of antioxidants and
zinc in a large controlled study slowed the progression
of ARMD (9). A pharmaceutical product based on DHA and a
synthetic antioxidant was recently launched in Switzerland.
Earlier
this year a scientific meeting was arranged in Paris discussing
the scientific basis for using DHA-enriched omega-3 supplements
for prevention and treatment of ARMD (10). There was a general
agreement that the scientific basis for using DHA was present
and preparations are ongoing for a large controlled clinical
study in France using a recently launched DHA based nutritional
supplement.
The
development of age related degenerative diseases such as
cardiovascular disease, Alzheimer's disease and maculopathy
starts probably decades prior to the presence of symptoms.
In fact early stages of cardiovascular disease have been
documented already in young adults. It is therefore important
for the individual and for the population in general to
induce measures to prevent these disabling diseases. Conclusions
from epidemiological studies and animal studies should be
treated cautiously and positive results should be tested
in controlled clinical trials. However, based on the two
large epidemiological studies presented above there are
firm evidence for the positive effects on increasing the
intake of fish for prevention of ARMD. The results of controlled
clinical studies will provide more information on effects
of marine omega-3 fatty acids, especially DHA.
While
waiting for these results increased intake of fatty fish
should be encouraged or people should start taking omega-3
nutritional supplements. The principle of prophylactic treatment
with omega-3 fatty acids should be introduced early in life
to prevent the development of degenerative diseases, not
least ARMD, which could shorten life expectancy and severely
reduce quality-of-life in the autumn of life.
REFERENCES.
1) Musarella
MA. Molecular genetics of macular degeneration. Doc Ophtal
Advances in ophthalmology 2001;102:165-177
2) Cruickshanks
KJ, et al. Sunlight and the 5-years incidence of early age-related
maculopathy: the Beaver Dam eye study. Arch Ophtalmol 2001;119:246-250
3) Klein
R, et al. Medication use and the 5-year incidense of early
age-related maculopathy. Arch Ophtalmol 2001;119:1354-1359
4) Smith
W et al. Dietary fat and fish intake and age-related maculopathy
Arch Ophtalmol 2000;118:401-404
5) Cho
E et al. Prospective study of dietary fat and the risk of
age-related macular degeneration
Am J Clin Nutr 2001;73:209-218
6) Neuringer
M. Infant vision and retinal function in studies of dietary
long-chained polyunsaturated fatty acids. Am J Clin Nutr
2000;71:256-267
7) Rotstein
N, et al. Protective effect of docosahexaenoic acid on oxidative
stress-induced apoptosis of retina photoreceptors. Invest
Ophtalmol & Visual Sci 2003;44:2252-2259
8) Elner
VM. Retinal pigment epithelial acid lipase activity and
lipoprotein receptors: effects of dietary omega-3 fatty
acids. Transactions Am Ophtalmol Soc 2002;100:301-308
9) Frennesson
C, et al. Age-related macular degeneration-new possibilities
for prophylactic measures. Lakartidningen 2002;99:3194-3197
10)
Micronutrition en ophtalmologie: Lipides et DMLA. Paris13
juin, 2003. Chauvin/Bausch & Lomb
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