Glycine

Glycine is the simplest amino acid and serves multiple roles: as an inhibitory neurotransmitter in the spinal cord and brainstem, as a co-agonist at NMDA glutamate receptors in the forebrain, as a precursor for glutathione (a major antioxidant), and as a structural component of collagen and bile acids. It is classified as conditionally essential because endogenous synthesis is typically insufficient to meet metabolic demand, particularly in individuals with increased collagen turnover or oxidative stress.

Clinical evidence for supplemental glycine is strongest for sleep quality. Randomized trials (Yamadera et al., 2007; Bannai et al., 2012) demonstrate that 3g glycine taken 30-60 minutes before bedtime improves subjective sleep quality, reduces sleep onset latency, and increases slow-wave sleep. Proposed mechanisms include peripheral vasodilation that reduces core body temperature (which facilitates sleep onset) and central effects via NMDA receptor co-agonism. Effect sizes are modest but meaningful and safety is excellent.

Additional indications with emerging evidence include glutathione repletion in aging adults (combined with N-acetylcysteine as GlyNAC, showing cellular aging marker improvements), metabolic support in type 2 diabetes, and collagen synthesis support. Glycine is classified as Generally Recognized As Safe by the FDA, with no known clinically significant drug interactions at standard doses. It is well-tolerated up to 30g daily in clinical studies. Slight sweetness makes it easy to consume in powder form mixed with water, and it can be combined with magnesium glycinate formulations where glycine contributes both the delivery mechanism and the calming effect.