Monosodium glutamate has been blamed for a huge range of non life threatening but disturbing effects, from dizziness, light headedness, mental confusion and panic attacks, to hyperactivity and behavioural problems in children. Now, some experts are becoming concerned that MSG is even more hazardous than was first thought.
Recent studies have shown glutamate to be one of a small group of brain transmitter substances called excitatory amino acids. A leading researcher in this very specialised field is Professor J Timothy Greenamyer, a neuroscientist at Emory University, Illinois. "Glutamate is now thought to be the neurotransmitter of several clinically important pathways, including cortical association fibers, corticofugal pathways, such as the pyramidal tract, and hippocampal, cerebellar, and spinal cord pathways," he was suggesting over a decade ago (Arch Neurol, 1986; 43: 1058-63). However, because glutamate is so potent, the brain has mechanisms to keep levels under control.
"Glutamate has neurotoxic properties and can produce 'excitotoxic' lesions reminiscent of human neurodenegerative disorders," Greenamyre now warns. "Abnormally enhanced glutamatergic neurotransmission may cause excitotoxic cell damage and lead to neuronal damage and death in a variety of acute and chronic neurological disorders like olivopontocerebellar atrophy (degeneration of the cerebellum of the brain) and Huntington's disease (degeneration of the cerebral cortex)". He also sees it as potentially responsible for the onset of senile dementia. "Anatomical and biochemical evidence suggests that there is both pre and post synaptic disruption of EAA pathways in Alzheimer's disease. Dysfunction of EAA pathways could play a role in the clinical manifestations of Alzheimer's disease, such as memory loss and signs of cortical disconnections" (Cerebrovasc Brain Metab Rev, 1993, Summer: 5: 61-94 and Neurobiol Aging, 1989; 10: 593-602).
In a recent book, Excitotoxins, The Taste that Kills, US neurophysician Dr Russel Blaylock argues that adding MSG to our diet artificially raises blood glutamate concentrations to potentially neurotoxic levels. He believes MSG may also be responsible for nervous system damage in the very young. "It appears that an imbalance of these excitotoxins during critical periods of foetal brain development can result in an abnormal formation of brain pathways, ie, a 'miswiring' of the brain. This may lead to serious behavioural problems, such as hyperactivity, aggression, attention deficit disorders and poor learning ability."
Dr Blaylock accuses the "defenders of MSG safety" of academic chicanery. "First, they denied that brain lesions could result from any dose of glutamate. Then, when the evidence became overwhelming, they claimed that these lesions only occurred when MSG was injected and not ingested. When this was disproved, they denied that human blood levels could reach concentrations that would be toxic to the brain. When this was shown not to be true, even by one of their own defenders, they simply stated, 'So what? It still cannot enter the brain because of the blood brain barrier' the body's own 'gatekeeper' which normally excludes toxic chemicals from entering the brain from the blood." But, as Dr Blaylock points out, the brain has several vital areas that have no barrier, such as the hypothalamus. And studies have shown that glutamate can pass into protected areas of the brain by seeping through the unprotected areas.
Also, there are many medical conditions that cause the barrier to fail, such as hypertension, diabetes, brain tumours, brain trauma, heat stroke, vascular stroke, multiple sclerosis, neuro degenerate diseases and certain drug regimes. "All people affected by these would be at great risk," he says.
In the US, a campaign for stricter labelling of MSG has begun, with the aim of compelling the industry to declare all "free glutamic acid" contents, not just the addition of the white MSG powder. The campaigners argue that free glutamate is produced as a byproduct of many food manufacturing processes, and is found in hydrolysed proteins, plant protein extracts, sodium and calcium caseinate, yeast extract, autolysed yeast and textured protein.
Canadian workers have found that of 61 people who claimed to be MSG sensitive, over a third reacted to a double blind challenge of 2.5g of MSG, with symptoms of headache, nunbness, tingling and flushing. With the average person consuming between 1.5 and 2.5g of free glutamate a day (according to the industry's own figures), everyone's intake is close to sensitivity levels (J Allergy Clin Immun, 1997; 99: 757-62).
But petitions to compel manufacturers to label and quantify free glutamate contents have been ignored by the US Food and Drug Administration.
All those who figure they can avoid MSG by giving up processed food and buying fresh instead are in for a rude shock. Last year, a US company called Auxein was given leave by the Environmental Protection Agency to market a new agricultural product containing 30 per cent MSG for spraying on food crops before harvest.