Glycine
DEGlycin
Glycine is the smallest, simplest amino acid. It is 'non-essential' under normal conditions. But it is 'conditionally essential' in aging, pregnancy, and disease. In those states, demand can outrun what your body makes (from serine and threonine). Glycine is the most abundant amino acid in collagen. It is also one of the three building blocks of glutathione (the 'Gly' in the γ-Glu-Cys-Gly tripeptide). That is why it can be a rate-limiting ingredient for glutathione in older adults, who typically run low on glycine. Glycine has other jobs too. It is an inhibitory neurotransmitter in the spinal cord and brainstem. It tunes NMDA receptor activity. And it takes part in one-carbon metabolism, bile-acid conjugation, and creatine synthesis. Dietary sources are gelatin, skin, bones, and connective tissue. Modern lean-meat-heavy diets provide relatively little. In mice, glycine supplements extended lifespan in the rigorous ITP (Miller 2019). The worm (C. elegans) evidence is more indirect. It comes mostly from methionine-restriction and one-carbon studies, not direct glycine trials. In humans, glycine deficiency in older adults is increasingly recognized. And small pilot trials (especially as part of GlyNAC, about 8 people each) suggest it restores glutathione and improves multiple aging-related markers.
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