Can
your diet contribute to blindness or macular
degenaration (MD)?
What is
MD?The macula is in the centre of
the retina at the back of the eye and
is responsible for detailed vision. When
one has macular degenation or MD this
means that parts of the retina are dying
for vascular reasons - not enough oxygen
is getting to cells of the retina which
need oxygen to flourish. This can lead
to blurred vision and a dark empty area
in the centre of vision. People at risk
of developing MD include the elderly,
smokers, people with heart disease, diabetes,
high blood pressure, high blood fats,
or people who are oberweight/obese or
inactive or who have low intakes of antioxidants
from foods. The number of people with
MD is increasing in developed countries
and this is partly due to our aging populations
- about 25% of people aged over 75 have
some signs of MD.
Why has
MD suddenly hit the media? The reasons
why MD has suddenly become topical is
because one prominent ophthalmologist
in Sydney has promoted the message in
the media (http://seven.com.au/todaytonight/story/macular)
that margarines and vegetable oils in
general cause MD, except extra virgin
olive oil, and may increase the progression
of the disease. Butter has been promoted
as a safe fat. Confused?
The studies:
There have been some Australian eye studies
exploring the impact of diet on MD but
they have not really found any conclusive
association. In contrast, most of the
studies showing an adverse effect of dietary
fats on MD have come from the US. A prospective
cohort study conducted by Seddon et al
published in the Archives of Opthalmology
in 2003
and 2004
on subjects with MD found a significant
link between increased intakes of vegetable
fats, animal fat, polyunsaturated, monounsaturated
and trans fats - but the stronger more
significant association was found to be
with trans fats. Trans fats are usually
associated with vegetable fats in margarines
or in hardened vegetable fats used in
some processed foods and fatty fast foods.
In other studies high intakes of animal
fats/saturated fats have also been linked
to MD. Butter is high in both animal and
saturated fat. So the suggestion that
butter is a preferable fat choice is not
consistent with this evidence. Furthermore,
nuts have been found to be protective
against MD despite their high content
of vegetable fat - but they are free of
trans fats. Fish has also been linked
to better eye health.
Link with
trans fatty acids? Trans fatty acids
can occur naturally in dairy foods and
in meat fats. However, in margarines they
are created during processing. In Australia,
trans fats were removed from many margarines
about five to six years ago, because they
were found to behave like saturated fatty
acids. This means they can put up your
cholesterol and can increase your risk
of heart disease. So many margarine manufacturers
in Australia have changed their processing
so that trans fatty acids do not occur.
However, that is not true of America where
a lot of this data for MD has come from.
So is the Australian food supply free
of trans fats? Australian
margarines may be low in trans fats but
the food supply can still contain trans
fats via certain processed food products
containing 'vegetable fat or vegetable
shortening or hydrogenated vegetable fat
(read the label) such as some baked goods.
They can also be found in fatty fast food
(like hot chips, fried battered fish)
cooked in hydrogenated vegetable fat -
often outlets using this fat will claim
that they use 'cholesterol free' vegetable
oil (all vegetable oils are cholesterol
free). Unfortunately the oil used e.g
canola oil or cottonseed is in a hardened
'hydrogenated' form which will be high
in trans fats and saturated fat. Hard
oils are often preferred to liquid oils
by the food industry because they are
more stable at high temperatures and take
longer to go rancid. More
info on trans fats
Verdict?More
studies are needed to shed more 'light'
on this interesting link between diet
and MD. In theory, vegetable fat without
trans fats, should be beneficial whereas
vegetable fat which contains trans fats
is not going to be good for your heart
and probably your eyes. Keeping animal
fat/saturated fat intake low may also
help. Eating more nuts and fish and more
plant foods (high in antioxidants) may
also help your eyes.
HEC nutrition experts recommend you use
a variety of margarines (low in trans
fats <1% - read the label ) and oils,
especially ones high in monounsaturated
fat, omega 3 fats and antioxidants - such
as canola, peanut and extra virgin olive
oils. However, it is preferable to consume
fat from ‘natural (unrefined) ’
sources, such as nuts, seeds/grains (in
bread, tahini), soy/tofu, avocado, oils
produced with minimal chemical processing
(such as cold pressed oils) or fatty fish
because you will also get some nutrients,
less trans fats and beneficial phytochemicals
along with the fat.
What about butter versus margarine?
Butter is much higher in saturated
fat so it will raise your blood
cholesterol and it will make your platelets
more sticky. Margarine will lower your
cholesterol, and intervention studies
have shown that eating margarine, particularly
a canola oil based margarine, reduces
the rate of heart disease, which has not
been shown to be the case for butter.
Having said that - both are similarly
high in calories.
How much
fat should we eat?Click
here to find out from HEC nutrition
experts
More
information about MD:
1. Australian Radio (ABC) Health Report
- http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s1232820.htm
2. Australian Dietitians Association Position
Statement
3. CSIRO Commonwealth Serum Laboratores
Research Organisation Media
Release
