MSG (Glutamic acid flavor) Health effects

Glutamic acid and its ions and salts, called glutamates, are flavor-enhancing compounds which provide an umami (savory) taste to food. Glutamic acid is a natural constituent of many fermented or aged foods, including soy sauce, fermented bean paste, and cheese, and is also a component of hydrolyzed protein such as yeast extract. The sodium salt of glutamic acid, monosodium glutamate (MSG), is a widely used additive in the food industry.

Glutamic acid versus glutamate

When glutamic acid or one of its salts is dissolved in aqueous solutions, a pH-dependent instantaneous chemical equilibrium of the amino acid's ionized forms, including zwitterionic forms, will result. These ions are called glutamates. salts exist only in a dry and crystallized form. The form ultimately responsible for the taste is the glutamate ion, and the form of glutamic acid at the time of the addition is not important. However, crystalline glutamic acid salts such as monosodium glutamate dissolve much better and faster than crystalline glutamic acid, a property important for use as a flavor enhancer.

Concentration in foods

The following list illustrates the glutamate content of some selected common foods. Free glutamate is metabolized differently from glutamate bound in protein, so they are listed separately.

Food   Free glutamate (mg/100 g)   Protein glutamate (mg/100 g)  

• Marmite 1960 
• Vegemite 1431 
• Roquefort cheese 1280 
• Parmesan cheese 1200, 9847
• Chinese soy sauce 1090 
• Japanese soy sauce 782 
• Grape juice 258 
• Peas 200, 5583
• Tomatoes 140, 238
• Corn 130, 1765
• Cow milk 2, 819
• Human milk 22, 229
• Eggs 23, 1583
• Chicken 44, 3309
• Duck 69, 3636
• Beef 33, 2846
• Pork 23, 2325
• Salmon 20, 2216

Research into health effects

In April 1968, Robert Ho Man Kwok wrote a letter to the New England Journal of Medicine, coining the term "Chinese restaurant syndrome", 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…

In 1969 the "Chinese restaurant syndrome" was attributed to the flavor enhancer glutamate largely due to the widely-cited article "Monosodium L-glutamate: its pharmacology and role in the Chinese Restaurant Syndrome" published in the journal Science. The syndrome is often abbreviated as CRS and also became known under the names "Chinese food syndrome" and "monosodium glutamate symptom complex."

Symptoms attributed to the Chinese restaurant syndrome are rather common and unspecific. They have included burning sensations, numbness, tingling, feelings of warmth, facial pressure or tightness, chest pain, headache, nausea, rapid heartbeat, bronchospasm in people with asthma, drowsiness, and weakness.

While many people believe that monosodium glutamate (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. Adequately controlling for experimental bias includes a placebo-controlled double-blinded experimental design and the application in capsules because of the strong and unique after-taste of glutamates.

In the 2004 version of his book, On Food and Cooking, noted food scientist Harold McGee states, "[after many studies], toxicologists have concluded that MSG is a harmless ingredient for most people, even in large amounts." (p. 342)

Timeline

In 1959, the U.S. Food and Drug Administration (FDA) classified monosodium glutamate as generally recognized as safe (GRAS). This action stemmed from the 1958 Food Additives Amendment to the Federal Food, Drug, and Cosmetic Act that required premarket approval for new food additives and led the FDA to promulgate regulations listing substances, such as monosodium glutamate, which have a history of safe use or are otherwise GRAS.

Since 1970, FDA has sponsored extensive reviews on the safety of monosodium glutamate, other glutamates, and hydrolyzed proteins, as part of an ongoing review of safety data on GRAS substances used in processed foods. One such review was by the Federation of American Societies for Experimental Biology (FASEB) Select Committee on GRAS Substances. In 1980, the committee concluded that monosodium glutamate was safe at current levels of use but recommended additional evaluation to determine monosodium glutamate's safety at significantly higher levels of consumption. Additional reports attempted to look at this.

In 1986, FDA's Advisory Committee on Hypersensitivity to Food Constituents concluded that monosodium glutamate poses no threat to the general public but that reactions of brief duration might occur in some people. Other reports have given the following findings:

The 1987 Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization and the World Health Organization placed monosodium glutamate in the safest category of food ingredients.

• A 1991 report by the European Community's (EC) Scientific Committee for Foods reaffirmed monosodium glutamate's safety and classified its "acceptable daily intake" as "not specified", the most favorable designation for a food ingredient. In addition, the EC Committee said, "Infants, including prematures, have been shown to metabolize glutamate as efficiently as adults and therefore do not display any special susceptibility to elevated oral intakes of glutamate."
• A 1992 report from the Council on Scientific Affairs of the American Medical Association stated that glutamate in any form has not been shown to be a "significant health hazard".
• A 1995 FDA-commissioned report by the FASEB confirmed the safety assessments of the aforementioned Committees, but stated that an unknown percentage of the population may react to monosodium glutamate and develop a monosodium glutamate symptom complex when consuming more than 3 grams of monosodium glutamate alone. The report compiled several, mostly very non-specific and common, symptoms from anecdotal reports, including burning sensation in the back of the neck, forearms and chest, numbness in the back of the neck, radiating to the arms and back, tingling, warmth and weakness in the face, temples, upper back, neck and arms, facial pressure or tightness, chest pain, headache, nausea, rapid heartbeat, bronchospasm, drowsiness, weakness.
• A 2000 review found that large doses of MSG given without food may elicit more symptoms than a placebo in individuals who believe that they react adversely to MSG. However, the frequency of the responses was low and the responses reported were inconsistent and were not reproducible. The responses were not observed when MSG was given with food.
• A 2002 report found that rats fed on diets supplemented with 10% and 20% pure monosodium glutamate suffered retina degeneration, possibly through glutamate accumulation in the vitreous humor. However, such extreme amounts are more than one order of magnitude higher than those used for flavoring or found in foods.

Excitotoxicity

Because glutamate is absorbed very quickly in the gastrointestinal tract (unlike glutamic acid-containing proteins in foods), glutamate could spike blood plasma levels of glutamate. 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 ("Olney's lesions"). 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.

At a meeting of the Society for Neuroscience in 1990, the delegates had a split opinion on the issues related to neurotoxic effects from excitotoxic amino acids found in some additives such as monosodium glutamate.

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. Based on these findings, they feel that humans are approximately 5-6 times more susceptible to the effects of excitotoxins than rodents are. 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.

Obesity

In 2008, a collaboration between American and Chinese researchers found a positive statistical association between MSG intake and obesity in humans: Prevalence of overweight was significantly higher in MSG users than in non-users. The population in this study was 752 healthy rural Chinese villagers between the ages of 40 and 59, of whom 48.7% were women.

Previously, monosodium glutamate 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, at least 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 2,239,960 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 fed MSG ad lib.

However, an earlier study (1973) did not find a similar effect in humans. The epidemiological survey of 4,938 ethnically Japanese men drawn from the Honolulu heart program in Hawaii found that self-reported dietary monosodium glutamate consumption was not statistically linked with obesity. Researchers furthermore found that frequent monosodium glutamate consumption did not significantly affect blood sugar or serum cholesterol levels among the participants.

Monosodium glutamate may worsen non-alcoholic fatty liver disease caused by trans fats.

A rat study conducted by the Institute of Life Sciences in Kawasaki, Japan during which the experimental rats were allowed to freely drink a solution of MSG and water showed a reduction in abdominal fat and weight gain compared to control rats.