Glycine

Glycine is the simplest amino acid in the body and the most underrated. Most people take 1-3g before bed, where it gently improves sleep quality without sedation. The other reason to consider it is glutathione, your body's main antioxidant, which is rate-limited by glycine availability and tends to drop with age, chronic stress, and metabolic dysfunction.

The practical use case is two-fold. As a sleep aid before bed helps you fall asleep faster, sleep more deeply, and wake up less foggy, useful especially when stress, travel, or short sleep windows are the issue. As a foundational amino acid for older adults or anyone with metabolic issues, 5-7g per day (often paired with N-acetylcysteine) supports glutathione synthesis, mitochondrial function, and insulin sensitivity. It's one of the cheapest, safest supplements you can take.

Glycine is also the most abundant amino acid in collagen, so it has a theoretical role in supporting skin and connective tissue. In practice, if connective tissue is the goal, collagen peptides plus vitamin C is the better-evidenced protocol because it delivers proline and hydroxyproline alongside glycine. More on that in the deep-dive.

Deep-dive

Glycine has a split personality in the central nervous system that explains everything about how it works. In the spinal cord and brainstem, it's the main inhibitory neurotransmitter alongside GABA, binding to strychnine-sensitive glycine receptors and opening chloride channels that quiet motor and sensory firing. In the higher brain (hippocampus, cortex), it's a required co-agonist at the NMDA receptor, where it has to be present for glutamate to actually open the channel. So glycine is simultaneously inhibitory in some regions and a permissive amplifier of excitatory transmission in others. This dual role is why the effects can look paradoxical depending on what's being measured.

Why it helps sleep without sedating. The original 2007 polysomnography trial showed 3g of glycine before bed reduced sleep onset latency, increased slow-wave sleep, and improved subjective sleep quality in people with sleep complaints. The proposed mechanism: glycine acts on NMDA receptors in the suprachiasmatic nucleus to lower core body temperature (a precondition for sleep onset) and to support the natural NREM transition. It doesn't act on GABA-A receptors the way benzos or zolpidem do, which is why there's no sedation, no morning hangover, and no dependence. A follow-up sleep-restriction study found 3g before a short night reduced next-day fatigue and improved performance, suggesting it improves the restorative quality of sleep rather than just sleep duration. Almost every meaningful trial uses 3g. You'll see lower doses (500-1500mg) suggested in some popular content, but there's no published RCT supporting sleep benefits at that range, and individual response below 3g is mostly anecdotal.

Glutathione and antioxidant capacity, with caveats. Glycine is one of three amino acids (alongside cysteine and glutamate) that form glutathione (GSH), the body's main intracellular antioxidant. There's a biochemical case that glycine availability can be rate-limiting in older adults, vegetarians, and people on low-protein diets. The human evidence is mixed and worth being honest about.

The positive data: the Baylor GlyNAC trial (glycine + N-acetylcysteine, ~100mg/kg of each, ~7g glycine for a 70kg adult) in 24 older adults over 16 weeks showed improvements in red blood cell glutathione, oxidative stress markers, mitochondrial function, insulin sensitivity, walking speed, and grip strength. Earlier pilot work from the same group showed similar effects.

The counterweight: a larger 114-person Nestlé-led RCT in 2022 tested GlyNAC at 2.4g, 4.8g, and 7.2g/day for 2 weeks and missed its primary endpoint, GlyNAC did not raise circulating glutathione vs. placebo. The benefit only appeared in post-hoc analysis of people with both high baseline oxidative stress and low baseline glutathione. So the effect is real in deficient populations but not a guaranteed glutathione lift in healthy older adults.

The other thing worth being clear about: in the GlyNAC protocol, NAC is doing most of the heavy lifting. NAC alone has the strongest standalone evidence for raising glutathione in humans. Adding glycine on top is biochemically reasonable, especially if dietary glycine is low, but human trials of isolated glycine on glutathione outcomes are sparse. For a healthy younger adult eating adequate protein, taking glycine specifically to raise glutathione is largely theoretical.

Metabolic effects. People with metabolic syndrome, type 2 diabetes, obesity, and HIV all show lower plasma glycine levels than healthy controls. Recent supplementation trials in severe obesity found that 2 weeks of dietary glycine raised plasma glycine, reduced triglycerides and aminotransferases, and improved markers of metabolic dysfunction-associated liver disease (MASLD). It didn't move fasting glucose or insulin. The metabolic story is real but modest, and probably more relevant to people who are already metabolically unhealthy than to lean trained people.

Collagen and connective tissue, and why this isn't the main use case. Glycine makes up roughly one-third of the amino acid residues in collagen, the most abundant protein in your body. The triple helix structure of collagen requires glycine at every third position, and endogenous synthesis falls roughly 10g/day short of what's needed for collagen production in a 70kg adult, based on biochemical calculations. So at the biochemistry level, more glycine should mean more collagen building blocks. The problem is that collagen synthesis also needs proline, hydroxyproline, and lysine, and the human evidence for tendon, ligament, and skin outcomes lives in the collagen peptide literature, not the isolated glycine literature. There's one rat Achilles tendon study showing 5% dietary glycine improved tendon healing, but no human RCT of isolated glycine on tendon, ligament, or skin outcomes. If connective tissue is the goal, the better-evidenced protocol is 10-15g of collagen peptides with 50-100mg vitamin C, taken 30-60 min before training. Adding extra free glycine on top is theoretical and probably harmless but doesn't have its own evidence base.

Schizophrenia, NMDA, and a useful caveat. Because glycine is the obligatory co-agonist at the NMDA receptor, it's been studied at very high doses (60g/day, ~0.8g/kg) as an adjunctive treatment for the negative symptoms of schizophrenia. Early trials at 60g/day showed roughly 30% reductions in negative symptoms when added to standard antipsychotics. Trials of glycine added to clozapine showed no benefit and possibly interference with the antipsychotic effect, suggesting the NMDA modulation is real and dose-dependent enough to actually push receptor function around. This is far above any practical supplement dose, but it's evidence that glycine crosses the blood-brain barrier in meaningful amounts at high oral doses and can shift CNS receptor activity. At 3-10g/day this is irrelevant to anyone without an NMDA-related condition.

Women. Glycine demand rises significantly in late pregnancy, where endogenous synthesis can't keep up with fetal demand and glycine becomes "conditionally indispensable." Outside of pregnancy, the sleep and glutathione effects look comparable between sexes. The original sleep trials included a female-majority sample and found the same subjective improvements. Women in perimenopause and menopause where stress-driven sleep disruption is common often pair glycine with magnesium and L-theanine. No female-specific dose adjustment is needed.

Limitations. Most of the strongest evidence is in older adults, sleep-disturbed populations, or people with metabolic dysfunction. In healthy young adults with good sleep and adequate protein, the noticeable benefit may be small beyond the sleep effect. The schizophrenia data is at doses 20-30x above what anyone supplements normally. Glycine is sweet-tasting and bulky as a powder, which most people don't realize until they try to take 10g of it daily.

Dosage:

For sleep: 3g is the dose that's been studied. Some people report effects at 500mg-1,500mg. 30-60 min before bed.

For glutathione/antioxidant support (older adults, chronic stress, metabolic dysfunction): 50-100 mg/kg/day (3.5-7g for a 70kg adult), often combined with NAC (the "GlyNAC" protocol). This is the chronic, daily, foundational dose used in the Baylor aging trials.

Upper end: Doses up to 60-90g/day have been used in clinical trials (schizophrenia) without serious adverse events, but you don't need anywhere near this. Most practical use sits between 3g (sleep only) and 7g (sleep + glutathione).

Timing: Glycine is rapidly absorbed (plasma rises within 10 min, peaks around 30-60 min). Sleep dose: take 30-60 min before bed. Glutathione protocol: split AM and PM, doesn't matter much when. No need to cycle.

Form: Plain glycine powder is by far the cheapest. Sweet-tasting (the name comes from the Greek for "sweet"), dissolves easily in water or tea. Capsules are convenient but expensive at chronic doses. Magnesium glycinate provides ~14% glycine by weight.

Stacks: Pairs well with magnesium glycinate or magnesium threonate plus L-theanine for sleep. Pairs with NAC for the glutathione protocol. If connective tissue support is the goal, collagen peptides + vitamin C is the better-evidenced choice, with or without extra free glycine on top.

Here's what you can expect:

For sleep, you should notice falling asleep slightly faster (5-15 min reduction in sleep onset is typical), sleeping more deeply, and waking up feeling more rested rather than groggy within the first few nights. The effect is subtle, not knockout-style. If you're expecting heavy sedation you'll be disappointed. If you're expecting to feel more recovered the next morning, that's the realistic target.

For glutathione, metabolic health, and antioxidant capacity, there's no subjective signal at all. You take it (with NAC) because the mechanistic case is reasonable in older or metabolically struggling populations, not because you'll feel different. If you're under 40, lean, eating adequate protein, and metabolically healthy, this use case is largely theoretical for you, and NAC alone may give you most of the benefit anyway.

No tolerance, no withdrawal. You can stop and restart at any time. You can also forget a dose with no real consequence.

Side effects & risks:

GI upset is the main side effect, usually only above 5-10g taken at once. Mild nausea, loose stools, or stomach discomfort. Splitting the dose across the day fixes it.

Drowsiness at the sleep dose (3g) is mild and only occurs around bedtime. Don't take a sleep dose mid-day if you need to drive or focus.

Clozapine interaction is real and not theoretical. The Evins 2000 trial showed glycine actively interfered with clozapine's antipsychotic effect at 30-60g/day. Anyone on clozapine should not supplement glycine. General caution with other antipsychotics at high doses.

Non-ketotic hyperglycinemia (rare genetic disorder) is a hard contraindication.

Kidney and liver: No reported toxicity at any practical dose. In rats, the no-observed-adverse-effect level is 5g/kg/day; humans have tolerated 60-90g/day in clinical trials for weeks without major adverse events. People with significant kidney or liver dysfunction should still discuss high doses with their team.

Pregnancy: Glycine demand rises in late pregnancy, and dietary intake from food (including collagen) is generally fine. High-dose isolated supplementation (10g+/day) hasn't been formally studied in pregnancy. Default to dietary sources.

Excellent overall safety profile in healthy adults at typical doses (3-10g/day). The exceptions above are narrow.

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Blood markers

For typical use (3-10g/day for sleep or general support), no specific bloodwork is needed.

For older adults running the GlyNAC protocol, a GGT (gamma-glutamyl transferase) test is a reasonable proxy for systemic glutathione status and oxidative load at baseline and 3 months. Hs-CRP for inflammation is also reasonable to track.

Plasma glycine and amino acid panels exist but are rarely useful in healthy people, the dose-response is well-established and you don't need to confirm absorption.

Sold widely as a dietary supplement and food ingredient.