Glutamic acid is critical for proper cell function, but it is not considered an essential nutrient in humans because the body can manufacture it from simpler compounds.
In addition to being one of the building blocks in protein synthesis, it is also important in brain function, as an excitatory neurotransmitter. Free glutamic acid cannot cross the blood-brain barrier in appreciable quantities; instead it is converted into L-glutamine, which the brain uses for fuel and protein synthesis.
The sodium salt of glutamic acid, monosodium glutamate (MSG) is responsible for one of the five basic tastes of the human sense of taste, and MSG is extensively used as a food additive.