There are quite a few people that are sensitive to the effects of monosodium glutamate (MSG). It is a big enough health concern that many food manufacturers stopped putting MSG in their foods. However, because of the flavor enhancing characteristics of MSG, food processors found a way to still use the components. By separating out the components of MSG, they are able to still gain the same effects, but not have to include MSG in the label.
The component of MSG that causes the adverse reactions is called free glutamic acid. There are currently over 40 different ingredients that are commonly used in food processing that contain free glutamic acid. Because most people are looking specifically for MSG, they are eating foods that are causing them harm without even realizing it.
Free glutamic acid is refereed to as an excitotoxin, which means it over-excites the cells, causing damage to them. It mostly effects brain cells, and it's been linked to many neural disorders, including Lou Gehrig's disease, Alzhiemer's, and Parkinson's disease. It has also causes many other problems including obesity, eye damage, headaches, and depression.
The best way to avoid this in your diet is to learn where it is hidden in your foods. Some of the more common ingredients that contain free glutamic acid include: yeast extract, hydrolyzed protein, yeast food, glutamic acid, and textured protein. The Truth in Labeling website has a much more exhaustive list.
This shows once again how important it is for you to learn as much as you can about your food, and what is being added without your knowledge. The more you can learn, the healthier you can be.