Eating too much monosodium glutamate - the flavour enhancer common in oriental and processed foods - could make you go blind.

Researchers at Hirosaki University in Japan have found that rats fed on diets high in MSG suffer vision loss and have thinner retinas. Glutamate 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.

But according to lead researcher Hiroshi Ohguro, his is the first study to show that eye damage can be caused by eating food containing MSG.

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 per cent. And tests that measured retinal response to light showed they could not see as well. Rats on the moderate diet also had damage, to a lesser extent.

Borderline unknown

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 acknowledges that large amounts of MSG were used, 20 per cent of the total diet in the highest group. "Lesser amounts should be OK," he says. "But the precise borderline amount is still unknown."

He says 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.

Peng Tee Khaw, a glaucoma specialist at Moorfields Eye Hospital in London, says 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. This may be why people tend not to develop normal-tension glaucoma until they are in their forties.

Journal reference: Experimental Eye Research (vol 75, p 307)