Monosodium glutamate (MSG) is a food ingredient used as a flavour enhancer. Its popularity within the food industry stems from the fact that it can cause an increase in the appeal of certain foods and therefore increases the profits of the food companies. Monosodium glutamate has attracted attention because a sensitivity to it is present in some individuals. The exact nature of the reaction to monosodium glutamate in sensitive individuals can vary, but is thought to result from the plasma increase in the levels of glutamate. While some studies have investigated the absorption of monosodium glutamate in isolation, a number of studies have used modifiers in the diet to investigate the effects of other food components on its absorption. It is thought that carbohydrate may alter the absorption kinetics of the glutamate moiety from monosodium glutamate and in in this way lower plasma levels of glutamate. This modification effect is interesting because it gives clues as to the absorption routes of monosodium glutamate.
For example, in one study1, researchers fed a large dose of monosodium glutamate (150 mg of monosodium L-glutamate) to eight healthy subjects with and without carbohydrates. The carbohydrate was given as partially hydrolysed starch at a dose of 1.1 grams per kilogram of body weight. The addition of the carbohydrate to the monosodium glutamate decreased the mean peak plasma glutamate concentration and also the area under the glutamate time curve (a measure of the total increase in glutamate). The authors postulated that the reductions in the glutamate levels may be due to a delay in the gastric emptying of the monosodium glutamate. However, when the gastric emptying rate was analysed, it was found not to be a factor. The authors therefore suggested that the carbohydrate may have served as a pyruvate source in enterocytes, and that this may have allowed the transamination of some of the glutamate in enterocytes, thus lowering its subsequent appearance in plasma. Carbohydrate food may therefore decrease the appearance of glutamate in the peripheral circulation.
Another finding from this study was that the erythrocyte glutamate concentrations were not affected greatly by the administration of the monosodium glutamate. This is in contrast to rodents who appear to be sensitive to monosodium glutamate in terms of their ability to load red blood cells. This glutamate may then pass to the brain and act a neurotoxin. Whether this effects is present in primates is not clear but the inability to load erythrocytes with glutamate may explain the lack of neurotoxicity in most individuals who are not sensitive to the food additive. However, the poison is often in the dose and so it cannot be discounted that monosodium glutamate does indeed have detrimental neurological health effects in humans. The effects of the rise in plasma levels of glutamate following ingestion of monosodium glutamate are reminiscent of the rise in plasma levels of phenylalanine following ingestion of aspartame. Rises in both phenylalanine and glutamate may cause changes in brain chemistry which explains the idiopathic characteristics of the neurological changes they cause.
Dr Robert Barrington’s Comments: Those who are sensitive to monosodium glutamate should avoid any foods containing the food additive. Certain individuals are sensitive to monosodium glutamate because as plasma levels of glutamate rise, brain glutamate concentrations can increase and this causes changes to neurotransmitter production. There is no firm evidence that small amounts of monosodium glutamate are problematic in healthy non sensitive humans. However, monosodium glutamate is added to foods that are generally classified as of low quality, and as such there is no real reason for its presence in the diet, as these type of foods should not be present in a high quality diet.
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