Monosodium glutamate, or MSG, has been in use for 100 years. However it has faced increased controversy as it has seen increased use, especially in processed foods including many frozen entrees and snack foods. While it does much to add fullness and the distinctive flavor to these foods, it is also neurologically active as a result of its chemical composition. Understanding this chemistry is crucial to understanding the source of the controversy.

Monosodium glutamate is officially a salt, meaning it is composed of atoms and molecules that have been ionized, or become electrically charged. One unit of monosodium glutamate contains five carbon atoms, four oxygen atoms, eight hydrogen atoms and one sodium atom. Everything aside from the sodium are combined into one molecule, known as glutamic acid, bonded together with covalent bonds, or electrically neutral bonds. The ionic bond in MSG is between this molecule and sodium; sodium "donates" one electron to this molecule.

The main molecule in monosodium glutamate is glutamic acid, one of the nonessential amino acids. The full chemical formula for it in isolation is HOOC-(CH2)2-CH(NH2)-COOH, however one of the end hydrogens is lost when the molecule combines with sodium to form the salt. In this formula, O designates oxygen, H designates hydrogen, C carbon and N nitrogen. While it is one of the 20 amino acids critical for proper human cell function, it can be generated by the body, and thus is not an "essential" part of the diet.

Glutamic acid, or glutamate, is found naturally in a variety of proteins, in all animal products and a number of plant proteins, such as those found in tomatoes, soybeans and corn. It is necessary for a variety of bodily processes; for example, it has a key role in detoxifying ammonia in the brain. It can be combined with other amino acids to form polypeptides and eventually proteins, and on its own performs a variety of functions, especially in its role as an excitatory neurotransmitter.

Because monosodium glutamate separates into its component ions -- sodium and glutamic acid -- almost instantly when it comes into contact with saliva, it is the actions of these two separate compounds that affect the body. Both compounds are naturally present in the body and indeed necessary for function. However, due to glutamic acid's role as an excitatory neurotransmitter, high doses could "overexcite" the brain in people more sensitive to the compound. According to Katherine Zeratsky, a licensed and registered dietician with the Mayo Clinic, short-term reactions to MSG reported to the FDA have included headaches, nausea, heart palpitations and sweating. The FDA thus requires MSG to be labeled if it is present in a food. There is little evidence for longer-term neurotoxicity or other dangers, however, and the FDA has declared MSG to be "generally recognized as safe."