Gamma aminobutyric acid (GABA) is a neurotransmitter in the central nervous system of mammals and is directly responsible for the regulation of muscle tone. GABA is synthesized primarily from glutamate by glutamic acid decarboxylase (enzyme that catalyzes the decarboxylation of glutamate to GABA).
Cavagnini, F. and co-workers  examined the effect of 5 g gamma aminobutyric acid on growth hormone and prolactin secretion in 19 healthy men. Blood samples that were taken before and 3 hours after administration showed a significant elevation of plasma growth hormone levels (P < 0.001). Prolactin levels were not consistently altered since only 5 out of 15 subjects showed an increase.
Powers, Michael E. and assistants  evaluated the effect of gamma aminobutyric acid ingestion at rest and in combination with exercise. Eleven bodybuilders (18-30 yr) ingested either 3 g of GABA or sucrose placebo. Fasting blood samples showed that at rest GABA ingestion elevated growth hormone levels by about 400%. Exercise and GABA had up to 200% greater growth hormone levels than placebo and exercise.
As a dietary supplement GABA is also sold for its relaxing effect and sleep support. However, these claims are not scientifically proven.
More evidence is needed to prove the effectiveness of GABA as ergogenic aid. There is also not enough scientific information to determine the right dose. However, 3-5 grams show an increase in GH levels at rest and in combination with exercise.[1,2]
Watanabe, Masahito, et al. “GABA and GABA receptors in the central nervous system and other organs.” International review of cytology 213 (2002): 1-47.
- Cavagnini, F., et al. “Effect of acute and repeated administration of gamma aminobutyric acid (GABA) on growth hormone and prolactin secretion in man.” Acta Endocrinologica 93.2 (1980): 149-154.
Powers, Michael E., et al. “Growth hormone isoform responses to GABA ingestion at rest and after exercise.” Medicine and science in sports and exercise 40.1 (2008): 104.