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Controversy surrounding
food additive MSG (monosodium glutamate-a common flavor enhancer)
was re-ignited when a Japanese researcher suggested that consuming
too much of the ingredient could make you go blind.
Researchers at
Hirosaki University in Japan have found that rats fed diets
high in MSG suffer vision loss and have thinner retinas than
those in the control group. Glutamate, a group of chemicals
that includes MSG, is an amino acid that acts as a neurotransmitter.
It has already been shown to cause nerve damage in experiments
where it is injected directly into the eye.
MSG, a sodium salt
of the amino acid glutamic acid and a form of glutamate, is
used as a flavor enhancer in a variety of foods prepared in
restaurants and by food processors. While technically MSG
is only one of several forms of free glutamate used in foods,
consumers frequently use the term MSG to mean all free glutamate.
Its use has become
controversial in the past 30 years because of reports of adverse
reactions in people who have eaten foods that contain MSG.
According to the U.S. Food and Drug Administration (FDA),
research on the role of glutamate in the nervous system has
also raised questions about the chemical's safety.
According to lead
researcher Hiroshi Ohguro, his is the first study to show
that eye damage can be caused by eating food that contains
MSG. A report in the New Scientist this week explains that
in the study, rats were fed three different diets for six
months, containing either high or moderate amounts of MSG,
or none. In rats on the high-MSG diet, some retinal nerve
layers thinned by as much as 75 percent, and tests that measured
retinal response to light showed they could not see as well.
Rats on the moderate MSG diet also had damage, to a lesser
extent.
The researchers
found high concentrations of MSG in the vitreous fluid, which
bathes the retina. MSG binds to receptors on retinal cells,
destroying them and causing secondary reactions that reduce
the ability of the remaining cells to relay electrical signals.
Ohguro acknowledged
that large amounts of MSG were used-20 percent of the total
diet in the highest group. "Lesser amounts should be
OK," he said, "but the precise borderline amount
is still unknown."
He said the findings
might explain why in eastern Asia, there is a high rate of
normal-tension glaucoma, a form of the eye disease that leads
to blindness without the usual increase in pressure inside
the eyeball. The higher rate, however, could also be due to
genetics.
The New Scientist
report continues that Peng Tee Khaw, a glaucoma specialist
at Moorfields Eye Hospital in London, said the amounts of
MSG in the highest diet are "a lot, lot higher than you'd
eat. But if you're a sodium glutamate junky, then you could
potentially run into problems with your retina."
And while the amount
of glutamate in the rats' diets was extremely high, lower
dietary intakes could produce the same effects over several
decades.
With such persuasive
evidence it is clear that food processors must brace themselves
for another wave of anti-MSG activity from global consumer
associations. In a bid to reassure these same campaigners,
food scientists must continue to investigate the impact of
MSG, and free glutamates, on human health, as well as looking
into alternatives.
One step toward
finding alternatives was reported by FoodNavigator.com in
April of this year. New research into the use of human taste
and smell receptors in functional assays to screen for novel
receptor activators and blockers was reported. Senomyx, the
U.S. company that carried out the research, is hoping that
their research will lead them to discover alternatives to
MSG, as well as enhancers of sweet and umami (the taste conveyed
by several substances naturally occurring in foods, including
glutamate) tastes.
Experimental
Eye Research September, 2002 75: 307
Food
Navigator.com October 23, 2002
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