What Is MSG?Glutamate, or glutamic acid, is an amino acid. Amino acids are the basic building block of proteins, which are in turn the building blocks of living things in the form of muscle and tissue. Monosodium glutamate is a salt of that acid -- basically, sodium combines at an atomic level with glutamate to form the salt. This is the same as how table salt is a combination, at the atomic level, of sodium and chlorine. So far so good?
Glutamate occurs naturally in your body, where it helps keep your neurotransmitters firing efficiently. If you end up with extra glutamate in your system, though -- or any other amino acid -- it becomes processed by your liver so that it can be stored for other things. If your liver and immune system work properly, this process shouldn't cause any problems. There can be some complications, though, if everything doesn't work the way it should. One of the most common is an allergic reaction.
Although glutamate occurs naturally in the body, monosodium glutamate has to be manufactured. There's two basic ways to accomplish this. The first is hydrolizing vegetable protein. The second is fermenting certain starches. Most MSG you find in food is manufactured, but it does occur as a natural by-product in some fermented foods like soy sauce.
What Does MSG Do In Food?MSG was most likely identified in the foods where it's naturally produced -- ie, fermented foods like soy sauce or worcestershire. There it creates that hard-to-pin-down flavor profile, umami. You're familiar with salty, sour, sweet and bitter, right? Umami is that delicious "savory" taste that helps complement other flavors and make food taste better. You know, the flavor you get when you eat something protein-filled like a well-cooked piece of meat? Glutamate is pretty much solely responsible for creating that flavor.
If you know anything about processed food, it really shouldn't shock you that "flavor that makes food taste better" is something that food manufacturers are eager to pump into their products.
This is why you find MSG in so many processed foods. It's way cheaper to put a little MSG in something than it is to put actual protein in there. So, for example, canned soup or Ramen Noodle packets can taste meaty and delicious even if there isn't actually any meat in them. Cheetos can taste cheesy despite there not actually being any cheese.
Is It Harmful?Your taste buds work pretty hard to make sure that the things you put in your mouth are nutritious. Different foods have different tastes, and giving you a preference for certain flavors helps make sure you eat what you're supposed to. In the case of glutamate, your body recognizes the umami flavor and says, "hey, this is full of delicious protein! eat up!"
Except, of course, if the glutamate is added to the food and there's not actually any protein in it.
So you're happily shoveling away food that your body thinks is nutritious and delicious when, in fact, you could be eating the nutritional equivalent of cardboard sprinkled with MSG. This is no different than the way processed foods trick you with added salt, sugar, and fat. The added flavors make fake food taste like yummy real food instead of tasteless fake food infused with artificial ingredients.
MSG has a few other issues. A small portion of the population is allergic to it, and you can develop a sensitivity to it if you eat a lot of it. There's also a bunch of genetic problems that can increase your glutamate sensitivity. For people with a genuine MSG sensitivity, excess glutamate can cause some serious medical problems, including neurological damage -- but this is a tiny portion of people. To so-called "Chinese Restaurant Syndrome" isn't nearly as widespread as people think it is.
Why Should You Avoid MSG?It's true: A small percentage of people in the world have an MSG intolerance that leads to headaches, nausea, fatigue and other nasty problems. That doesn't mean that MSG itself is necessarily bad any more than peanut allergies mean that peanuts are unhealthy. If you have a diagnosed MSG intolerance, then you absolutely should avoid glutamate. If you don't, though, glutamate isn't going to hurt you any more than, say, wheat gluten hurts people who don't have a gluten allergy. So all of the fear-mongering about MSG is, to put it lightly, inflated. It's not going to cause cancer or give you birth defects or make your eyeballs explode or anything. Relax, eat some soy sauce.
The real reason to avoid MSG is because its presence in your food is probably a really good sign that the food isn't something you should be eating. Remember, glutamate occurs naturally in foods. But it doesn't show up in the ingredients label that way. It only shows up in the ingredients if someone actually put the MSG in there. Which means that the food is obviously highly processed, and thus probably not something you really want to put in your mouth.