Controversy surrounding food additive E621, otherwise known as monosodium glutamate - a common flavour enhancer - was re-ignited this week 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 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.
Monosodium glutamate (MSG), a sodium salt of the amino acid glutamic acid and a form of glutamate, is used as a flavour 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 US Food and Drug Administration (FDA), research on the role of glutamate - a group of chemicals that includes MSG - 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 containing 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 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.
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 per cent 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.
On such alternative was reported by FoodNavigator.com in April this year. We reported on new research into the use of human taste and smell receptors in functional assays to screen for novel receptor activators and blockers. Senomyx, the US company that carried out the research, are hoping that their research will lead them to discover alternatives to MSG, as well as enhancers of sweet and umami tastes.
Full findings of the Japanese study are published in the latest issue of Experimental Eye Research (vol 75, p 307). The International Glutamate Information Service meanwhile said that the allegations about glutamate ignore the wealth of scientific evidence which demonstrates the safety of glutamate.