The salt of an amino acid which is used by the body as a neurotransmitter. There are receptors for glutamate in the hypothalamus, the eyes, the ears, the pancreas. Thus, the food additive MSG, which when ingested raises blood levels of this amino acid in the blood, affects a myriad of body processes. Because free glutamate is the currency of nerve transmission in the body, MSG ingestion can result in ringing in the ears, blurred vision, dizziness, and an insulin increase. Seemingly disparate reactions which all have a neurotransmitter link to glutamate. In addition, glutamate is a calcium channel OPENER, which affects the cardiovascular system. (Most of the public is familiar with calcium channel BLOCKERS - which lower blood pressure.) MSG may cause headaches by affecting the constriction of blood vessels. By also increasing histamine response, MSG ingestion has also been associated with diseases of inflammation such as rosacea. By affecting the pancreas and hypothalamus - the seat of hunger in the brain, MSG injected in mice has been proven to cause obesity. MSG causes leptin resistance in mice. (Leptin is the substance released by fat cells and active in the hypothalamus which controls appetite by suppressing AMPK. Mice with activated AMPK eat more, and reduce activity by 30% in effect becoming "lazy".) Thus MSG indirectly activates AMPK, the substance that increases weight due to the body trying to conserve enrgy any way it can.
MSG is a harmful, and unecessary, food additive.