Powers, M. E., Yarrow, J. F., McCoy, S. C., & Borst, S. E., (2008). Growth hormone isoform responses to GABA ingestion at rest and after exercise. Medicine and Science in Sports and Exercise, 40, 104-110.

"Oral administration of the amino acid/inhibitory neurotransmitter gamma aminobutyric acid (GABA) reportedly elevates resting serum growth hormone (GH) concentrations". This study assessed if GABA ingestion stimulates immunoreactive GH and immunofunctional GH release at rest and that GABA augments the resistance exercise-induced immunoreactive GH/immunofunctional GH responses. Resistance-trained men (N = 11) ingested either 3 g of GABA or a sucrose-placebo, followed either by resting or resistance exercise sessions. Fasting venous blood samples were acquired immediately before and at 15, 30, 45, 60, 75, and 90 minutes after GABA or P ingestion and were assayed for immunoreactive GH and immunofunctional GH.

At rest, GABA ingestion elevated both immunoreactive GH and immunofunctional GH compared with placebo. Specifically, peak concentrations of both hormones were elevated by about 400%. Resistance exercise elevated time-point (15-60 minutes) immunoreactive GH and immunofunctional GH concentrations compared with rest. The combination of GABA and resistance exercise also elevated the peak concentrations and the 15- to 60-minute time-point immunoreactive GH and immunofunctional GH responses compared with resting conditions. Additionally, 200% greater immunoreactive GH and 175% greater immunofunctional GH concentrations were observed in the exercise-GABA condition than in the exercise-placebo condition, 30 minutes after ingestion. GABA ingestion did not alter the immunoreactive GH to immunofunctional GH ratio, and, under all conditions, immunofunctional GH represented approximately 50% of immunoreactive GH.

Implication. Ingested GABA elevates resting and post-exercise immunoreactive GH and immunofunctional GH concentrations. "The extent to which immunoreactive GH/ifGH secretion contributes to skeletal muscle hypertrophy is unknown, although augmenting the postexercise immunoreactive GH/immunofunctional GH response may improve resistance training-induced muscular adaptations".

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