The actual troublemaker is free glutamic acid, a molecule with flavor-enhancing properties. It's one of the basic amino acids that make up proteins. No one reac

The actual troublemaker is free glutamic acid, a molecule with flavor-enhancing properties. It's one of the basic amino acids that make up proteins. No one reacts badly when they eat it if it's linked up to other amino acids, but when it's running around free it causes reactions in some people. A largish dose of free glutamic acid will trigger a migraine for me (and my dad) consistently. It's anecdotal, but I'm confident that this particular anecdote would hold up in a double-blind test. Monosodium glutamate (MSG) disassociates into sodium and free glutamic acid in food; that's why it's often added. But any ingredient that contains broken up protein can be an issue; e.g. hydrolyzed soy protein or autolyzed yeast extract. MSG has a bad rep, so often foodmakers will try to slip free glutamic acid in some other way (often while shouting on the package, "NO MSG"). Food that benefits a lot from the flavor enhancer, such as canned soups, will almost always have a lot of it. Lots of stuff has _some_ amount of free glutamic acid (such as broccoli, like the article mentions) but (speaking personally), it's not enough to trigger a reaction. There aren't many foods our ancestors would've eaten that contained lots of free glutamic acid. We're talking about the same molecule that serves as an excitatory neurotransmitter in your brain. It turns out that if you eat a lot of it, it's possible to get a headache (other people claim other ailments). Not much is known about the specifics of the mechanism, but it's not that implausible.