Glutamic Acid

 

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Glutamic [glew-tam-ick] acid is a flavor-enhancing compound which provides an umami (savory) taste to food. Glutamic acid is a natural constituent of many fermented or aged foods, including soy sauce, fermented bean paste (a component of miso), and cheese, and is also a component of hydrolyzed protein such as yeast extract (e.g. Vegemite).

The sodium salt of glutamic acid, monosodium glutamate (MSG), is a widely used additive in the food industry. Although they occur naturally in many foods, the flavor contributions made by glutamic acid and other amino acids were only scientifically identified early in the twentieth century.

The substance was discovered and identified in the year 1866, by the German chemist Karl Heinrich Leopold Ritthausen. In 1907 Japanese researcher Kikunae Ikeda of the Tokyo Imperial University identified brown crystals left behind after the evaporation of a large amount of kombu (kelp) broth as glutamic acid. These crystals, when tasted, reproduced the ineffable but undeniable flavor he detected in many foods, most especially in seaweed. Professor Ikeda termed this flavor ‘umami.’ He then patented a method of mass-producing a crystalline salt of glutamic acid, monosodium glutamate.

Glutamic acid stimulates specific receptors located in taste buds such as amino acid receptors, which induce the taste known as umami, one of the five basic tastes. Glutamate itself is a widespread amino acid. It is found naturally in all living cells, primarily in the bound form as part of proteins. Part of the flavor-enhancing effect of tomatoes, fermented soy products, yeast extracts, certain sharp cheeses, and fermented or hydrolyzed protein products (such as soy sauce and fermented bean paste) is due to the presence of free glutamate ions. In the Roman Empire glutamic acid was found in a sauce called ‘Garum,’ made from fermenting fish in saltwater. It was used so widely that it has been called ‘the ketchup of the ancient Romans.’ The flavor enhancing properties of glutamic acid allowed Romans to reduce the use of expensive salt.

In 1968, Robert Ho Man Kwok wrote a letter to the ‘New England Journal of Medicine,’ coining the term ‘Chinese restaurant syndrome’ (CRS). In this letter he claimed: ‘I have experienced a strange syndrome whenever I have eaten out in a Chinese restaurant, especially one that served northern Chinese food. The syndrome, which usually begins 15 to 20 minutes after I have eaten the first dish, lasts for about two hours, without hangover effect. The most prominent symptoms are numbness at the back of the neck, gradually radiating to both arms and the back, general weakness and palpitations…’ While many people believe that MSG is the cause of these symptoms, an association has never been demonstrated under rigorously controlled conditions, even in studies with people who were convinced that they were sensitive to the compound.

Because glutamate is absorbed very quickly in the gastrointestinal tract (unlike glutamic acid-containing proteins in foods), it could spike in blood plasma. Glutamic acid is in a class of chemicals known as excitotoxins, high levels of which have been shown in animal studies to cause damage to areas of the brain unprotected by the blood–brain barrier and that a variety of chronic diseases can arise out of this neurotoxicity. There has been debate among scientists on the significance of these findings since the early 1970s, when John Olney found that high levels of glutamic acid caused damage to the brains of infant mice. The debate is complex and has focused mainly on whether the increase in plasma glutamate levels from typical ingestion levels of glutamate is enough to cause neurotoxicity and on whether humans are susceptible to the neurotoxicity from glutamic acid seen in some animal experiments.

Some scientists believe that humans and other primates are not as susceptible to excitotoxins as rodents and therefore there is little concern with glutamic acid as a food additive. While they agree that the combined effects of all food-based excitotoxins should be considered, their measurements of the blood plasma levels of glutamic acid after ingestion of monosodium glutamate and aspartame demonstrate that there is not a cause for concern. Other scientists around John Olney felt that primates are susceptible to excitotoxic damage and that humans concentrate excitotoxins in the blood more than other animals. While they agree that typical use of monosodium glutamate does not spike glutamic acid to extremely high levels in adults, they are particularly concerned with potential effects in infants and young children and the potential long-term neurodegenerative effects of small-to-moderate spikes on plasma excitotoxin levels.

MSG has been shown to indirectly cause obesity in lab rats by downregulating hypothalamic appetite suppression and, thus, increasing the amount of food the lab rats consumed. However, one study (1978) found that this obesity effect (widely used in obesity research using rats and mice, and also observed in Chinese hamsters) was not dependent on additional food intake. Animal research demonstrating an inverse relationship between increased glutamate intake via maternal feeding and serum levels of growth hormone, combined with an epidemiological survey of German adults demonstrating an inverse relationship between height and morbid obesity, compels some researchers to theorize (2006) that monosodium glutamate has a role in the occurrence of obesity in humans. In contrast, a 2008 Japanese study sponsored by a MSG manufacturer found that rats lost body fat when allowed to freely drink a solution of MSG and water.

Under 2003 U.S. Food and Drug Administration regulations, when monosodium glutamate is added to a food, it must be identified as ‘monosodium glutamate’ in the label’s ingredient list. Because glutamate is commonly found in food, primarily from protein sources, the FDA does not require foods and ingredients that contain glutamate as an inherent component to list it on the label. Examples include tomatoes, cheeses, meats, hydrolyzed protein products such as soy sauce, and autolyzed yeast extracts. These ingredients are to be declared on the label by their common or usual names. The term ‘natural flavor’ is now used by the food industry when using glutamic acid. Because of lack of regulation, it is impossible to determine what percentage of ‘natural flavor’ is actually glutamic acid.

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