Dont worry just wiki-ed it. Monosodium glutamate. Non-essential fatty acid. Flavour enhancer.
Occurs naturally in food, but has been isolated and added to many different types of food, especially chinese. Different glutamates don't have the same negative effects as MSG.
Negative effects = higher BMI and asthmatic symptoms, and if there is a sensitivity it can cause one or more of the following for a couple of hours: burning sensations, numbness, tingling, feelings of warmth, facial pressure or tightness, chest pain, headache, nausea, rapid heartbeat, bronchospasm in people with asthma, drowsiness, and weakness.
BUT it only tends to cause these in a small percentage of the population, those that are sensitive to it.