The mean intake of glycine among Americans is 3.2 g/d, but young men (presumably consuming more connective tissu) intake up to 7.8 g/d without evident harm.
Glycine potentiates NMDA-receptor mediated neurotransmission, and I looked for evidence of adverse effects in the psychiatric trials with glycine at 30-60 g/d. Aside from some who didn't care for the sweetness, and some gastrointestinal distress, I couldn't find anything serious, and some patients continued past the trials for a year. The last case-study administered ~40 g/d glycine to a patient for 5 years.
Heresco-Levy U. & Lindenmayer JP. 1994. Amelioration of negative symptoms in schizophrenia by glycine. The American journal of psychiatry,151(8), 1234-1236.
Leiderman E et al. 1996. Preliminary investigation of high-dose oral glycine on serum levels and negative symptoms in schizophrenia: an open-label trial. Biological psychiatry,39(3), 213-215.
Heresco-Levy U et al. 1999. Efficacy of high-dose glycine in the treatment of enduring negative symptoms of schizophrenia. Archives of general psychiatry,56(1), 29-36.
Greenberg WM et al. 2009. Adjunctive glycine in the treatment of obsessive-compulsive disorder in adults. Journal of psychiatric research, 43(6), 664-670.
Cleveland WL et al. 2010. High-dose glycine treatment of refractory obsessive-compulsive disorder and body dysmorphic disorder in a 5-year period. Neural plasticity, 2009.
In rats, chronic high-dose (5 g/kg) glycine does cause the adaptation of a reduction of N-type Ca2+ channels in parietal cortex and hippocampus, but no evidence of neurodegeneration. 5 g/d is would represent most caloric intake in humans, with the standard 1/6 conversion its 50 g/d.
Shoham S et al. 2001. Chronic high-dose glycine nutrition: effects on rat brain cell morphology. Biological psychiatry,49(10), 876-885.
