Monosodium glutamate gets a bad rap. Commonly known as MSG, the chemical compound is a synthetic production of glutamates that are naturally occurring in foods like tomatoes, Parmesan, and mushrooms. And it's the unjustly stereotyped whipping boy for every problem associated with cheap Chinese food: headaches, bubble guts, rapidly reoccurring hunger, impotence (probably).
But the hate against MSG is as unfair as one fortune cookie with two orders of General Tso's. MSG is almost definitely not bad for you -- double-blind studies show little correlation between it and adverse symptoms, meaning the so-called "Chinese Restaurant Syndrome" is in your head. If you're not feeling so hot after that cheap takeout, it's probably the same reason that you don't feel so great after housing a meat lover's pizza solo: a ton of salt, saturated fats, and simple starches.