Glutamate (neurotransmitter)
Glutamate is a major constituent of a wide variety of proteins; consequently it is one of the most abundant amino acids in the human body. Under ordinary conditions enough is obtained from the diet that there is no need for any to be synthesized. Nevertheless, glutamate is formally classified as a non-essential amino acid, because it can be synthesized from alpha-Ketoglutaric acid, which is produced as part of the citric acid cycle by a series of reactions whose starting point is citrate. Glutamate cannot cross the blood-brain barrier unassisted, but it is actively transported into the nervous system by a high affinity transport system, which maintains its concentration in brain fluids at a fairly constant level.
Protein (nutrient)
The table below presents the most important food groups as protein sources, from a worldwide perspective. It also lists their respective performance as source of the commonly limiting amino acids, in milligrams of limiting amino acid per gram of total protein in the food source. The table reiterates the need for a balanced mix of foods to ensure adequate amino acid source.