October 25, 2002.
By Susan Schwartz,
South China Morning Post.
Eating too much monosodium glutamate (MSG), the flavour enhancer widely used in Hong Kong cooking and in snack foods, could cause blindness.
Rats fed on diets high in MSG suffered vision loss and a thinning of the retina, the layer of light-sensitive cells on the rear wall of the eyeball, in a Japanese study at Hirosaki University published this week in British weekly New Scientist.
However, Chinese University optometry professor Dennis Lam Shun-chiu said it was too early to jump to conclusions about the research findings. "The dosage used in these experiments is a lot higher than normally used in real life," Professor Lam said.
"We still do not know what the cumulative affects are from eating MSG, although it has been shown previously that when you have a moderate to high degree of MSG there will be damage to cells."
Previous studies have shown that when MSG is injected directly into the eye, it can cause nerve damage. But this study is the first to show that it can have the same effect when ingested through food.
The high-dose MSG diet fed to the rats comprised 20 per cent of the flavour booster, a proportion that lead researcher Hiroshi Ohguro acknowledged was large.
"Lesser amounts should be okay, but the precise borderline amount is still unknown," he said.
Rats were fed three different diets for six months that contained either high levels of MSG, moderate levels, or none.
In rats on the high-MSG diet, some retinal nerve layers thinned by as much as 75 per cent.
Test tests that measured retinal response to light showed they could not see as well as normal.
Rats on the moderate diet also had damage, to a lesser extent.
Glutamate is an amino acid that acts as neurotransmitter - a chemical that helps send signals among brain cells.
The findings might go some way to explaining why there is such a high rate of normal-tension glaucoma in eastern Asia, the researchers suggest.
The condition leads to blindness without the increase in pressure inside the eyeball normally associated with glaucoma.
Lower dietary intakes of MSG could produce the same effects over decades, which may be why people tend not to develop normal-tension glaucoma until they are in their forties.
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