Full mechanism, in text

Five jobs for one amino acid

1 · The hub and five spokes

The glycine pool is filled by diet and synthesis and drained by five uses; by one estimate, use runs 10-15 g a day against 4-6 g of supply.Cells hold a glycine pool, and its size depends on both sides of the ledger:

· Inflow: diet (about 1.5-3 g a day in a typical Western diet) + the body's own synthesis (the SHMT pathway from serine, about 2.5-3 g a day)
· Outflow: five main uses drawing on the same pool

The ledger: collagen, glutathione, creatine, heme and neurotransmitters together use about 10-15 g a day, while supply is only about 4-6 g. On that basis Meléndez-Hevia 2009 (J Biosci) estimated that a 70 kg adult may fall about 10 g a day short.

This is an estimate, not a measured shortfall: it adds up published production and use figures item by item, the result depends on how each use is estimated, and other researchers put the body's own synthesis differently. The argument itself is simple — glycine has no special new use; add up the known uses and the supply side does not match, which is why some researchers call it conditionally essential.

The next three steps light up the five uses one by one and show where the glycine actually goes.

2 · Collagen: every third residue

The collagen triple helix needs glycine at every third position, because only its tiny side chain fits the tight centre, which makes collagen synthesis the biggest glycine use.By that estimate, the largest use is collagen synthesis

Collagen is the most abundant protein in the body, and its triple helix has a strict chemical requirement: every third amino acid must be glycine — only glycine's tiny side chain (a single H atom) fits into the tightest centre of the triple helix.

Chemical consequence:

· About 33% of collagen's residues are glycine, with a lot of proline and hydroxyproline besides
· Collagen is constantly renewed, so this production line draws on the glycine pool every day

What it means:

· Training, injury and ageing speed up collagen turnover and raise glycine demand
· Joint pain, thinning skin and slow healing may be partly limited by too little glycine — an inference from Meléndez-Hevia 2009 that has not been tested directly in people
· In the small Shaw 2017 trial (8 men, crossover), 15 g of vitamin C-enriched gelatin taken 1 h before rope-skipping doubled a blood marker of type I collagen synthesis; it measured a synthesis marker, not recovery or injury outcomes

Reframe: collagen is rich in glycine not because glycine is a cofactor but because collagen's structure forces one glycine into every third position, which makes collagen synthesis the biggest glycine use.

3 · GSH tripeptide

Glutathione is a tripeptide of glutamate, cysteine and glycine, and in the second step of its synthesis GSS attaches the glycine.Another large use: glutathione (GSH) synthesis

GSH = γ-Glu-Cys-Gly — a tripeptide of three amino acids, the third of which is glycine.

Synthesis pathway (the story covers the first step):

1. GCL is rate-limiting: glutamate + cysteine + → γ-glutamylcysteine
2. GSS adds glycine: γ-glutamylcysteine + glycine + ATP → GSH

· Cells hold GSH at millimolar levels (about 1-10 mmol/L); after neutralizing oxidants it becomes GSSG, part is repaired and part is lost, so it has to be made continuously

Clinical relevance:

· NAC raises cysteine, the first step's bottleneck; with chronically low glycine the second step can stall too
· In ageing, diabetes and chronic inflammation, GSH synthesis capacity falls, and a low glycine pool is thought to be one reason (Pérez-Torres 2017 review)
· GlyNAC (glycine + NAC together): Kumar 2023 (*The Journals of Gerontology: Series A*) — 24 older adults + 12 younger adults; the older adults were randomized to GlyNAC (n=12) or an isonitrogenous control. This was not a glycine on top of NAC add-on trial, so it cannot separate the two contributions

The chemical difference: NAC alone feeds only the first step (GCL); NAC plus glycine also supplies the substrate for the second step (GSS).

4 · Three more fates light up

The remaining three uses — each an independent metabolic or signalling pathway, all drawing on the same glycine pool.Creatine synthesis:

· AGAT (kidney): glycine + arginine → guanidinoacetate (GAA) + ornithine
· GAMT (liver): GAA + → creatine + SAH
· Vegetarians and vegans get no creatine from food and rely entirely on making it
· This step also spends SAM methyl groups, a sizeable share of the body's methyl budget

Heme synthesis:

· ALAS (mitochondria): glycine + succinyl-CoA → δ-aminolevulinic acid (δ-) + CO₂ (the first step of heme synthesis)
· 8 ALA → porphyrin ring + Fe²⁺ → heme → packed into red cells
· Bone marrow makes about 2 million new red cells every second, so this line never stops
· Demand rises with menstruation, pregnancy and blood loss

Neurotransmitter (tiny amounts, large effects):

· Inhibitory (spinal cord + brainstem): glycine receptors are chloride channels that quiet motor neurons; tetanus toxin blocks glycine release → muscle rigidity
· Excitatory co-agonist (brain): NMDA receptors open only when glycine and glutamate are both bound → learning, memory
· Sleep: Bannai 2012 found in rats that a large oral dose of 2 g per kg raised plasma glycine to 13 times control and cerebrospinal fluid to 6 times within 30 minutes; it acts on NMDA receptors in the suprachiasmatic nucleus, widening skin blood vessels and lowering core temperature. How far blood glycine rises after 3 g in people, and whether the same route works in humans, has not been measured directly

Overall picture: 1.5-3 g from the diet plus 2.5-3 g made by SHMT feed one cellular glycine pool, which splits five ways: collagen (largest), GSH, creatine, heme and neurotransmitters (tiny). Total use is about 10-15 g/day, an estimated gap of about 10 g/day (70 kg adult).

This is the core of Meléndez-Hevia's argument that glycine is conditionally essential — demand > supply, five uses compete for one pool, and heavier use on any one squeezes the others.

In practice: glycine is cheap and well tolerated. The evidence for 3 g before bed as a sleep aid comes from a few small crossover trials by the same research group (Yamadera 2007 is one) and still needs larger trials; GlyNAC is an emerging direction with only small trials so far. It is not a muscle-building supplement — people take it for the collagen, glutathione and sleep routes.

Full mechanism, in text

1 · The hub and five spokes

2 · Collagen: every third residue

3 · GSH tripeptide

4 · Three more fates light up

Glycine hub — five fates from one pool

Full mechanism, in text

1 · The hub and five spokes

2 · Collagen: every third residue

3 · GSH tripeptide

4 · Three more fates light up

Glycine hub — five fates from one pool