MSG is predominately used as a flavor enhancer. It was originally extracted from kombu seaweed although free glutamic acid is present in brewer’s yeast, milk, tomatoes, cheese and other common foods. Glutamic acid is a neurotoxic neurotransmitter that has many receptors in the body and not just on the tongue. The brain has many receptor sites and so does the hypothalamus, which has no impermeable blood barrier. Some 25% of the US population exhibit reactions to free glutamate acids in foods. Glutamate is an amino acid and a common component of many edibles, especially proteins. Monosodium Glutamate is the sodium salt of glutamic acid, created by mixing sodium, water and glutamate.
As a taste element, MSG contributes a sensation recently identified as a fifth flavor element (the others being salt, sour, bitter and sweet). Called unami, it is a silky perception normally associated with oleaginous or buttery foods. Mushrooms are a rich source of glutamates. It is for this reason that they frequent accompany steaks and other meats. They enhance the flavor and textural perception (i.e., mouth feel) of the meat, making it seem and taste like a more expensive cut. If you wish to perform a small experiment, prepare a common cut of beef, like sirloin. Taste an untreated piece of the cooked meat. Then dip another similar piece in some melted butter and consume it. The butter enhances mouth feel in ways similar to MSG.
MSG will not make bad food taste good. It is able to further enhance good tastes and also mask off flavors which is why it is quite popular with the food processing industry. MSG also allows restaurants to create sauces that have a rich texture without the time or expense of using reduced meat stocks. Although a Japanese discovery, Chinese cuisine is one of the more well known food types that uses MSG. Widely distributed in proteins, it is in a bound form vital to metabolism and brain function. Metabolic functions require about 50 grams per day. Free glutamic acid is another matter entirely.
Although not an allergen, many people do show reactions to large doses of MSG, especially in the absence of food. An excerpt from this article mentions some of the toxic side effects MSG can have:
“According to Dr. Samuels, the evidence of toxicity is overwhelming. Exposed laboratory animals suffer brain lesions and neuroendocrine disorders. Scientists studying retinal degeneration in mice treated with free glutamic acid have noted that these mice also became grotesquely obese following administration of free glutamic acid. The vulnerable hypothalamus in our brains regulates weight control, as well as other endocrine functions. When the brain is deluged with more free glutamic acid than it can handle, scientists know that problems and diseases can develop. For example, they know that a diverse number of disease conditions such as ALS (amyotrophic lateral sclerosis, a progressive degeneration of neurons and motor cells of the brain), Alzheimer’s disease, seizures, and stroke are associated with the glutamate cascade.”
Personally, I do not use MSG at all and avoid it whenever possible. I believe that any competent chef can create wholesome and appealing foods without using such flavor enhancers. MSG currently shows up in some 40 different “natural flavorings” used in the processed food industry. It is one more reason to avoid fast food and preprocessed or snack food items.