GABA is a naturally occurring amino acid found in the brain, where it functions as a neurotransmitter, inhibiting neural excitation. While GABA is present in some green plants, yeasts, and bacteria, and is also available as a dietary supplement, dietary sources are not thought to significantly affect brain GABA levels, as the nutrient is not transported efficiently from the bloodstream into the brain. Most GABA found in the brain is manufactured there from the simple sugar glucose and the amino acid glutamine.
Although GABA supplements have been promoted for a wide variety of benefits such as increasing lean muscle mass, reducing body fat, reducing stress, and promoting sleep, there is little evidence to support such claims. As dietary GABA does not readily cross the blood/brain barrier, it is unlikely to influence mood or sleep patterns. While some studies indicate that GABA supplements can stimulate growth hormone and prolactin secretion, effects on muscle mass and body fat have not been demonstrated.
Although a therapeutic dosage level for GABA has not been demonstrated, a common recommendation is 200 mg, four times per day. Human studies on GABA have used single doses of 5 grams with no overt signs of toxicity. The long-term safety of GABA supplementation is unknown.