Monosodium glutamate (MSG) is the sodium salt of glutamic acid, a nonessential amino acid. High levels of MSG are naturally found in a range of food sources, including seaweed, soy sauce, parmesan cheese, tomatoes, and breast milk.
The uniquely savory flavor associated with these foods is called "umami," which is now widely accepted as the fifth taste.
Interestingly, glutamic acid itself does not have umami flavoring, but MSG in food activates glutamate receptors in the taste buds. These transmit signals to distinct regions of the brain, causing the characteristic taste.
But does MSG have a role beyond creating taste sensations? And why is there ongoing controversy over using MSG as a food additive?
Glutamate in the body
Your stomach and gut lining are rich in glutamate receptors. MSG and other forms of glutamate are absorbed through interaction with these receptors. Once in the gut, glutamate is either broken down to act as fuel, or incorporated into other molecules.
Glutamate is also an essential neurotransmitter in the brain. However, dietary glutamate is believed to be unable to cross the blood-brain barrier, suggesting that all brain glutamate is created there.
But there is evidence from studies in mice that the blood-brain barrier in newborns is immature, and that some glutamate can pass into the brain. High levels of glutamate injected into newborn mice caused significant brain damage.
A recent study showed that high levels of MSG also caused severe effects in fruit flies, leading to premature death in a significant number of them.
While the levels used in these studies far exceed normal daily consumption reported among humans, it is important to point out that restaurants and food manufacturers are not required to declare the levels of MSG added to food.
So is it safe for us to consume MSG?
'Generally recognized as safe'
The United States Food and Drug Administration (FDA) have classified MSG as "generally recognized as safe."
In 1907, scientist Kikunae Ikeda, a professor at the University of Tokyo in Japan, was the first to extract MSG from seaweed. Nowadays, MSG is produced by fermentation of carbohydrates, in a process likened to making yogurt and wine by the FDA.
The FDA require food manufacturers to list MSG as a component. But ingredients such as hydrolyzed vegetable protein, autolyzed yeast, soy extract, and protein isolate also contain naturally occurring MSG.
The MSG controversy
But what about "Chinese Restaurant Syndrome?" The controversy surrounding the use of MSG in food - mostly Chinese food - is ongoing.
Consumption of MSG has been linked to itching, hyperactivity, headache, and swelling of the tongue and throat, in what has been dubbed Chinese Restaurant Syndrome.
Therefore, the question of whether MSG is at the root of adverse food reactions or whether there is another culprit, especially in today's highly processed foods, remains to be answered.