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Gamma aminobutyric acid (GABA) is one of the amino acids and is an inhibitory neurotransmitter. Oral administration GABA reportedly elevates resting serum growth hormone (GH) concentrations. GH is an important regulator of body composition including muscle protein synthesis, although the effect of GABA on muscle anabolism is unclear. On the other hand, it is well known that ingestion of protein after exercise stimulates protein synthesis in skeletal muscles. In addition to post-exercise protein supplementation, the ingestion of GABA may have additive effect in training-induced muscle hypertrophy.PURPOSE: To examine the effect of oral administration of GABA combined with whey protein on muscular hypertrophy during progressive resistance training in men.METHODS: Twenty-six healthy male volunteers (26-48 yrs) were divided into one of two groups; whey protein (WP) group ingesting 10g of whey protein, or whey protein + GABA (WP+G) group ingesting 10g of whey protein and 100 mg of GABA every day for 12 weeks. Both groups were subjected to a resistance training twice a week in which they performed three sets of 12 repetitions at 60% of one-repetition maximum on the following exercises: leg press, leg extension, leg curl, chest press and pull down. Body composition was assessed by dual-energy X-ray absorptiometry at baseline and 12wk after the training period. Resting plasma GH concentration was assessed at baseline, 4, 8 and 12wk.RESULTS: In WP+G group, plasma GH level in resting state was elevated significantly at 4 and 8wk compared with week 0 (689 ± 203, 661 ± 199 vs. 264 ± 93 pg/mL, p<0.05 respectively). On the other hand, plasma GH level in WP group was elevated significantly only at 8wk as compared with baseline (589 ± 179 vs. 237 ± 86 pg/mL, p<0.05). After 12 wk, change in whole body lean mass was significantly higher in WP+G group compared to that in WP group (1340 ± 465 vs. 146 ± 218 g, p<0.05).CONCLUSIONS: Combined ingestion of GABA with whey protein increased lean mass more effectively than ingestion of whey protein alone in resistance-trained men. Therefore, dairy supplementation with GABA may be a useful addition to whey protein for augmenting exercise-induced muscle hypertrophy.