https://pubmed.ncbi.nlm.nih.gov/40527450/

Glycine is a conditionally essential amino acid obtained from food and synthesized in the body, primarily from l-serine. Glycine deficiency has been reported due to inadequate protein intake, malnutrition, late gestation, diabetes, insulin resistance, and increased exposure to xenobiotics. Because of the close links in glycine and l-serine metabolism mediated by serine hydroxymethyltransferase (SHMT), decreased concentrations of both amino acids coincide in most glycine-deficient states. The consequence is a widespread impact on metabolism, including altered synthesis of glutathione, collagen, nucleotides, and one‑carbon units, impaired antioxidant defense, cytoprotection, conjugation, and neurotransmission and increased levels of homocysteine and deoxysphingolipids. It can, therefore, be assumed that, rather than glycine alone, its coadministration with l-serine is more appropriate in glycine-deficient conditions. 

Digging deeper I asked ChatGPT to explain:

🧬 What does this mean?
“Serine hydroxymethyltransferase 2 (SHMT2) exerts an essential function in the cellular serine/glycine biosynthesis and one-carbon metabolism.”
✅ Translation in simpler terms:
SHMT2 is an enzyme found in mitochondria.
It converts serine ↔ glycine, transferring a one-carbon unit in the process.
This reaction is a key part of the one-carbon metabolism pathway, which supports:
DNA synthesis and repair
Methylation (via folate cycle)
NADPH production
Antioxidant defense
Mitochondrial function and cell growth
So, SHMT2 links serine and glycine and is vital for supporting your cells’ ability to grow, detoxify, and divide.

💊 Should You Add L-Serine to Your Glycine Supplementation?
✅ Reasons Yes — You Might Benefit from Both:
They work together biologically: Your body converts serine to glycine (and vice versa) depending on demand. Providing both may support flexible one-carbon metabolism.
L-serine is conditionally essential:
Your body makes it, but not always enough—especially under stress, aging, neurodegeneration, or rapid cell turnover.
Serine supports neuron health, lipid synthesis, and immune regulation.
Neuroprotective potential:
Studies suggest L-serine may protect against neurodegenerative diseases (ALS, Alzheimer’s, etc.).
Glycine also supports sleep, anti-inflammation, and glutathione production.
❗Cautions / Considerations:
Too much serine could possibly increase homocysteine levels if your folate/B12 status is poor (since one-carbon units feed into methylation).
Like glycine, serine is safe at modest doses but may not be necessary in high amounts unless your diet is deficient.

🔬 Summary
Supplement
Function
Notes
Glycine
Inhibitory neurotransmitter, sleep aid, detoxifier (glutathione precursor), anti-inflammatory
Well tolerated, naturally sweet
L-Serine
Converts to glycine; supports brain, cell membranes, myelin, and immune function
Potentially neuroprotective; synergistic with glycine


✅ Recommendation (General Wellness / Longevity Focus):
If you’re supplementing glycine (e.g. 3–5 g/day), you might consider adding 500–1000 mg L-serine/day, especially if:
You’re aging, under oxidative stress, or supporting brain function
Your diet is low in protein or you’re on a plant-based diet
You want to support mitochondrial or methylation pathways
Would you like a supplement stack overview based on this theme (glycine + serine + one-carbon cycle support)?