Glycine

Glycine is a simple amino acid found mostly in connective tissue: collagen, gelatin, skin, bone and bone broth. It helps the liver clear excess methionine, an amino acid concentrated in meat, and in a rigorous mouse programme it modestly extended lifespan. No human trial has tested it as a methionine buffer against any clinical outcome, so for people it remains a cheap, low-risk bet rather than a finding.

If your protein comes mostly from muscle meat, eat the connective tissue too: bone broth, slow-cooked stews with cartilaginous cuts, gelatin, or 5–10 g of plain glycine a day. Collagen powder works, but it takes roughly 25–50 g to supply the same glycine.

What it is

Glycine is abundant in connective tissue and largely missing from modern Western diets, which rely on muscle meat rather than skin, cartilage and bone. Collagen and gelatin are rich in it; muscle meat is not.

Why it might matter: the methionine connection

Methionine is a sulfur-containing amino acid concentrated in meat, poultry, eggs and dairy, and scarcer in legumes. In yeast, worms, flies and rodents, restricting it, without cutting calories, extends lifespan and improves metabolic health; in rats it added about 30% to lifespan. No primate has been tested, so applying this to people is an inference, not a finding.^[1]

Methionine is the raw material for S-adenosylmethionine (SAM), the molecule cells use to hand out methyl groups, and high SAM switches on the growth sensor mTOR (mechanistic target of rapamycin). Glycine gives the liver a way to dump the excess: an enzyme called glycine N-methyltransferase moves a methyl group from SAM onto glycine, making sarcosine. That is why dietary glycine is described as a partial mimic of methionine restriction.^[2]

The mouse evidence: replicated, but small

The US National Institutes of Health runs an Interventions Testing Program that tries candidate anti-ageing compounds in genetically mixed mice at three separate sites at once, a high bar for replication. A diet of 8% glycine extended lifespan in both males and females at all three sites. The effect was small: median lifespan rose about 6% in males and 4% in females, and one of the three sites only just reached significance.^[3]

The 8% was a share of the whole diet by weight, far more than the few grams a day suggested here.

What hasn't been tested

No human trial has tested glycine as a methionine buffer against any clinical outcome: not lifespan, not heart disease, not cancer. The case rests on liver biochemistry, the animal methionine-restriction work and the mouse lifespan result. It is a mechanistic bet, and a cheap and safe one.

Collagen is not the same dose

Collagen is only about a fifth glycine by weight. A 5–10 g scoop of collagen therefore supplies only about 1–2 g of glycine; matching 5–10 g of glycine takes roughly 25–50 g of collagen. Collagen is also poor for building muscle: it lacks tryptophan and carries very little leucine, so it is a weak stand-in for whey, dairy, fish or legumes. The point of collagen here is the glycine, not your tendons.

As a sleep aid: small trials from one company

Glycine is also sold as a sleep aid, at 3 g before bed. In small trials it improved how people rated their sleep, shortened the time to fall asleep and improved next-day alertness.^[4] The proposed mechanism, worked out in rats, is cooling rather than sedation: glycine acts on the brain's master clock to raise blood flow to the skin, so the body sheds heat and core temperature falls, which is what triggers sleep.^[5]

The caveat is the source. The human trials are small, and essentially all of this work, mechanism and clinical, comes from one research group at a single amino-acid manufacturer, with no independent replication and no meta-analysis. It is very well tolerated and not addictive.

If you also take magnesium bisglycinate, count the glycine: the flagship magnesium sleep trial delivered about 1.5 g of glycine a day alongside the magnesium, so the two effects can't be separated, and someone also taking 3 g of glycine gets roughly 4.5 g. Other sleep aids are compared in Sleep supplements.

What to do

If your protein is meat-heavy, eat the connective tissue as well: bone broth, stews made with cartilaginous cuts, gelatin.
Or take 5–10 g of plain glycine a day. It is the cheapest way to get the dose.
Don't rely on a scoop of collagen to supply it; you would need roughly 25–50 g.
Treat it as an add-on to the bigger choice of which protein you eat; see Protein.
For sleep, 3 g before bed is a low-risk thing to try, but the evidence is small and comes from one company's research group.