Monosodium glutamate is a food additive that enhances flavor. Although it's frowned upon today, the ancient Romans loved it and ate it with almost every meal.
There's been some debate over what exactly monosodium glutamate does to people. Some people say it gives them severe headaches, numbness, weakness, and even heart palpitations. Scientists could confirm that it does give some people short-lived reactions, but no tests showed that it had long-term health effects. Some say that it's an addictive substance which causes people to crave it repeatedly if they try it too often. Others say it's just a flavor enhancer, and people crave it because it makes things taste better.
The chemical is listed as 'safe' by the FDA, although they do require it to be clearly listed as an additive in any food product that uses it. It's most often found in canned soups and vegetables. Modern MSG is manufactured using genetically engineered bacteria. They take in nutrients and excrete glutamic acid. The acid is concentrated, and sodium is added to make the final product.
The process sounds very modern, but MSG has been around for a very long time. It was a common food additive during the time of ancient Rome, added to almost all Roman dishes. The Romans had a lot of technology for their time, but they couldn't genetically engineer bacteria. So how did they come up with MSG? Believe it or not, they used an even more disgusting process than bacteria excretions. The Romans had a fish paste called garum that they exported everywhere. They made it by filling pots with alternating layers of fish - or just fish guts - and salt and letting those pots lie out in the sun for a while.
As the mixture lay out in the sun, the stomach acids for the fish ate through their bodies. They eventually broke down the entire fish, turning the whole thing into a dark brown oily goo. When protein is broken down, the amino acid chains in the protein are freed up. One of these acid chains contains glutamic acid, which meets up with sodium from the salt and forms MSG. The Romans were such fans of the flavor enhancer that they even put it in sweets like custard. They also died off in droves, so anyone who wishes to recreate garum Roman-style — don't do it. Try organizing gladiator-style games in the back yard as a safer alternative.