Supplement analysis backed by scientific evidence · PT

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Limited evidenceSleep & relaxation

Glycine

Glycine's sleep reputation comes from a small cluster of Japanese crossover studies with roughly a dozen participants each that have never been independently replicated at scale; the metabolic and glutathione research is early but genuinely interesting.

What the science says

  • Subjective sleep quality improves in small crossover trials Limited evidence

    Trials from a single Japanese research group gave 3 g of glycine before bed to volunteers with poor sleep and reported better subjective sleep quality, reduced next-day fatigue and shortened polysomnographic sleep latency. Sample sizes were around 10-15 people per study, and no large independent replication has been published in the years since.

  • GlyNAC (glycine plus NAC) improved oxidative stress markers and physical function in older adults Limited evidence

    A randomized clinical trial in older adults supplementing glycine plus N-acetylcysteine for 16 weeks reported corrected glutathione deficiency, lower oxidative stress and inflammation, and improvements in gait speed, muscle strength and cognition. The trial was small (around two dozen participants) and comes from one research group, so it needs replication before the anti-aging framing is credible.

  • Conditionally essential rather than truly optional Moderate evidence

    Human glycine synthesis appears insufficient to cover the demands of collagen and glutathione turnover on typical diets, with an estimated shortfall of several grams per day. This makes a physiological case for supplementation, but a plausible mechanism is not the same as demonstrated clinical benefit.

Dosage & safety

Studied doseSleep studies used 3 g taken 30-60 minutes before bed. GlyNAC protocols used approximately 100 mg/kg/day each of glycine and N-acetylcysteine (roughly 7-8 g glycine daily) for 12-24 weeks.
SafetyVery well tolerated; glycine is a normal dietary amino acid and the main complaints at 3-10 g/day are mild nausea, soft stools and a sweet taste. It has been given at much higher doses (up to 60 g/day) in schizophrenia trials without serious toxicity, though GI upset rises steeply. Caution in people taking clozapine, where glycine has been reported to reduce clozapine's efficacy. People with significant kidney or liver impairment should discuss gram-level amino acid loads with a clinician.

Scientific references

Where to buy

NOW Foods, Glycine, Pure Powder, 1 lb (454 g)

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