Soy sauce naturally contains amounts of MSG and other glutamates. Some brands of soy sauce use MSG as an additive, though the FDA stipulates that the ingredients label must list their presence in this scenario.
Because MSG occurs naturally in soy sauce, food companies are not required to label products with a warning about MSG unless they add it in later. However, the companies also cannot label the products as MSG-free.
MSG is thought to be the cause of Chinese Restaurant Syndrome, or CRS, which began in April 1968 after an article listed various side effects caused by eating at a Chinese restaurant. The author, Dr. Ho Man Kwok, did not specify MSG as the culprit, but a later study by Dr. John Olney announced a link between MSG and brain lesions in newborn mice. These results were not duplicated in other studies, especially in human studies. CRS remains unknown, but the symptoms supposedly caused by MSG have been attributed to mass psychosis or other allergens in Chinese food such as peanuts and shellfish.
MSG occurs naturally in Parmesan cheese, walnuts, tomatoes, mushrooms, chicken and other foods. On ingredients labels, it can appear as monopotassium glutamate, autolyzed yeast extract, Ajinomoto, sodium caseinate or under other names. The FDA considers MSG to be safe.