Only recently Wang et al. were able to show that dietary glycine supplementation enhances skeletal muscle growth in young pigs (Wang. 2014a,b) - an excellent model of human metabolism. In view of the fact that the mechanism has hitherto not been fully understood, it was difficult to judge whether the smallest possible amino, i.e. glycine, could / would be useful for trainees as well.
With their latest study, the same team of scientists, this time with KaiJi Sun as their lead author (Sun. 2016), provide an experimentally supported explanation of the anabolic effects of glycine - an explanation that is not just convincing but would also suggest that glycine could be an interesting supplement for bodybuilders and fitness freaks, as well as their grandparents.
|Table 1: Physiological functions of glycine in animals and humans (Wang. 2013).|
Where's the human study in trained athletes? Early evidence for its ergogenic effects comes from Buchman, et al. (1999) who actually wanted to use glycine as a control for the beneficial effects of arginine on marathon runners, and then realized that it was glycine that enhanced the subjects' performance. Similar, albeit inconsistent effects have been reported for GPLC, i.e. Glycine Propionyl-L-Carnitine which improved resistance training performance in Jacobs, et al. 2009, but failed to yield ergogenic effects in a long-term follow-up with negative effects on aerobic acid with high dose (4.5g/day) supplementation of GPLC. Another glycine-compound with mixed evidence of its ergogenic prowess is GAKIC, i.e. glycine-arginine--ketoisocaproate, which has been shown to improve the performance of repeated cycling sprints in Buford, et al. (2004) - a result that could not be confirmed in a follow-up by Beis, et al. (2011), though.It is also an amino acid of which the previously cited studies showed that its use as a dietary supplement will (Wang. 2014b)...
At this point, the existing evidence is thus clearly insufficient to recommend glycine itself or any of the supplement industry's favorite glycine compounds as go-to supplements for athletes. The only thing that comes close is a recent study showing a decrease in nitrogen excretion (a marker of, you guessed it, protein breakdown) and thus reversal of the negative nitrogen balance with high glycine collagen protein - and by the way, the negative nitrogen balance persisted when the older women who participated in this 2-week study consumed a whey supplement (see figure on the left | Hays. 2009).
|Collagen hydrolysate, a high glycine protein (right bars), turned cata- into anabolism in a recent study in older women on low protein diets whey (left bars) did not (Hays. 2009).|
- increased small-intestinal villus height (could reverse leaky gut and will improve nutrient transport), intestinal transport of glycine, plasma concentrations of glycine and GSH, as well as whole-body growth and protein accretion, while
- reducing plasma concentrations of ammonia, urea, and glutamine,
|Figure 1: Effects of different doses of glycine on muscle anabolism (blue, left axes) and catabolism (gray, right axes) in C2C12 myotubes - all differences were highly significant (p = 0.001 | Sun. 2016).|
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- Buchman, A. L., et al. "The effect of arginine or glycine supplementation on gastrointestinal function, muscle injury, serum amino acid concentrations and performance during a marathon run." International journal of sports medicine 20.05 (1999): 315-321.
- Buford, BRITNI N., and Alexander J. Koch. "Glycine-arginine--ketoisocaproic acid improves performance of repeated cycling sprints." Med Sci Sports Exerc 36 (2004): 583-587.
- Hays, Nicholas P., et al. "Effects of whey and fortified collagen hydrolysate protein supplements on nitrogen balance and body composition in older women." Journal of the American dietetic association 109.6 (2009): 1082-1087.
- Jacobs, Patrick L., et al. "Glycine propionyl-L-carnitine produces enhanced anaerobic work capacity with reduced lactate accumulation in resistance trained males." Journal of the International Society of Sports Nutrition 6.1 (2009): 1.
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- Sun, KaiJi, et al. "Glycine Regulates Protein Turnover by Activating Akt/mTOR and by Inhibiting MuRF1 and Atrogin-1 Gene Expression in C2C12 Myoblasts." The Journal of Nutrition (2016): jn231266.
- Wang, Weiwei, et al. "Glycine metabolism in animals and humans: implications for nutrition and health." Amino acids 45.3 (2013): 463-477.
- Wang, Weiwei, et al. "Glycine stimulates protein synthesis and inhibits oxidative stress in pig small intestinal epithelial cells." The Journal of nutrition 144.10 (2014a): 1540-1548.
- Wang, Weiwei, et al. "Glycine is a nutritionally essential amino acid for maximal growth of milk-fed young pigs." Amino Acids 46.8 (2014b): 2037-2045.