It's a naturally occurring 'excitotoxin,' which occurs in many various foods like tomatoes, kelp, and yeast. The problem is when it's isolated and added to low quality, non-nutritious food, tricking the brain into believing it's eating something nutrient rich and and a consequence, delicious. In this unnatural configuration, the dosage easily approaches levels causing significant brain cell death, hence the term excitotoxin, and the widely reported headaches from over-consumption of crap like "chinese food."
