Glycine

Urinary glycine rises when there’s more glycine in circulation, reduced renal reabsorption in the kidney’s proximal tubule, or simply higher dietary/supplement intake with renal “spill.” Results are easiest to compare when normalized to urine creatinine (e.g., µmol/mmol creatinine); very dilute urine can exaggerate patterns.

Common, non-worrisome reasons

• Diet & supplements: Collagen/gelatin, bone broth, glycine powder, or high-protein meals in the prior 24–72 hours.

• Specimen factors: Very dilute urine (high fluid intake) or non–first-morning samples.

• Convalescence/illness or stress: Transient amino-aciduria can occur during recovery.

Medical causes to consider

• Renal proximal tubule dysfunction (global or partial amino-aciduria)

  • Fanconi-type patterns: glycosuria with normal blood glucose, phosphaturia, uricosuria, proteinuria/low-molecular-weight proteins.

• Reduced kidney function or tubular injury (medications, toxins, ischemia).

• Inborn errors of metabolism (rare):

  • Nonketotic hyperglycinemia (NKH): classically very high glycine in plasma and CSF; urine can be elevated but diagnosis hinges on plasma/CSF and clinical features (infancy onset—lethargy, seizures, apnea).

  • Organic acidemias (e.g., propionic or methylmalonic): secondary elevations of glycine may appear with other metabolic markers.

• One-carbon metabolism shifts (serine↔glycine): usually subtle and accompanied by other lab changes (folate/B12).

How to sanity-check the result

1. Was it creatinine-normalized? Prefer values reported as a ratio (e.g., µmol glycine per mmol creatinine). If it’s a raw concentration and the urine was very dilute (low creatinine/specific gravity), repeat with a first-morning sample.

2. Look for pattern: Are other amino acids also elevated (generalized amino-aciduria) or is glycine isolated?

3. Cross-check kidney indices: Urinalysis, urine specific gravity, urine creatinine, and serum creatinine/eGFR.

Practical next steps

• If you used glycine/collagen/gelatin recently: Stop for 3–5 days, hydrate normally, and repeat a first-morning urine amino acids test (creatinine-normalized).

• If still elevated:

  • Order/confirm serum creatinine, eGFR, urinalysis; consider urine protein and β2-microglobulin or other low-molecular-weight proteins if Fanconi-type loss is suspected.

  • Consider a plasma amino acid panel (and serine) to see systemic balance.

  • If there are red flags (infancy/childhood onset, neurologic symptoms, failure to thrive, metabolic acidosis): add urine organic acids, acylcarnitine profile, and discuss metabolic genetics.

• Medication review: Look for agents associated with tubular injury or amino-aciduria; adjust only with clinician guidance.

When to escalate

• Infants/children with neurologic signs or failure to thrive.

• Adults with persistent elevation plus kidney abnormalities (proteinuria, rising creatinine), systemic symptoms, or multi-analyte amino-aciduria.

Bottom line

Most adult elevations in urinary glycine are due to recent intake/supplements or dilution. Persistent, creatinine-normalized elevations—especially with abnormalities in other amino acids or kidney markers—warrant a renal/tubular evaluation and, when appropriate, metabolic workup.

Glycine is the simplest amino acid and is classified as non-essential because the body can synthesize it from other compounds. Beyond its structural roles (e.g., collagen and glutathione synthesis), glycine functions as an inhibitory neurotransmitter in the spinal cord, brainstem, and retina. Loss of glycinergic inhibition—such as with tetanus toxin, which blocks the release of inhibitory transmitters—can cause spastic paralysis due to unopposed muscle contraction. Glycine also serves as a co-agonist at NMDA receptors, where it is required alongside glutamate for receptor activation.