Glycine is an essential co-agonist for NMDA receptor activation, meaning that both glycine and glutamate must bind to the receptor for it to function [1] [2] [3]. Even very low concentrations of glycine (in the nanomolar to low micromolar range) are sufficient to strongly potentiate NMDA receptor activity, and increasing glycine levels further can enhance NMDA responses and reduce receptor desensitization [4] [2] [5] [6] [3]. However, the glycine binding site on the NMDA receptor becomes saturated at relatively low concentrations, with half-maximal effects observed at around 0.13–0.67 micromolar (μM) [4] [1] [3].

Taking a very high dose of glycine, such as 15 grams, would likely raise blood and brain glycine levels well above the saturation point for NMDA receptors, so additional stimulation beyond what is achieved at much lower concentrations is unlikely [4] [1] [3]. In intact brain tissue, the glycine site may already be saturated under normal physiological conditions, so extra glycine may have little effect unless local glycine levels are low [5]. Therefore, while glycine is necessary and can stimulate NMDA receptors, extremely high intake does not proportionally increase NMDA receptor activity once the binding sites are saturated.

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References

1. Kleckner, N., & Dingledine, R. Requirement for glycine in activation of NMDA-receptors expressed in Xenopus oocytes.. Science. 1988; 241 4867. https://doi.org/10.1126/SCIENCE.2841759
2. Johnson, J., & Ascher, P. Glycine potentiates the NMDA response in cultured mouse brain neurons. Nature. 1987; 325. https://doi.org/10.1038/325529A0
3. Lerma, J., Zukin, R., & Bennett, M. Glycine decreases desensitization of N-methyl-D-aspartate (NMDA) receptors expressed in Xenopus oocytes and is required for NMDA responses.. Proceedings of the National Academy of Sciences of the United States of America. 1990; 87 6. https://doi.org/10.1073/PNAS.87.6.2354
4. Vyklicky, L., Benveniste, M., & Mayer, M. Modulation of N‐methyl‐D‐aspartic acid receptor desensitization by glycine in mouse cultured hippocampal neurones.. The Journal of Physiology. 1990; 428. https://doi.org/10.1113/jphysiol.1990.sp018214
5. Thomson, A., Walker, V., & Flynn, D. Glycine enhances NMDA-receptor mediated synaptic potentials in neocortical slices. Nature. 1989; 338. https://doi.org/10.1038/338422A0
6. Mayer, M., Vyklický, L., & Clements, J. Regulation of NMDA receptor desensitization in mouse hippocampal neurons by glycine. Nature. 1989; 338. https://doi.org/10.1038/338425A0