What homocysteine is
Homocysteine is a sulfur-containing amino acid produced as an intermediate in methionine metabolism. It's normally cleared rapidly through one of two pathways:
- Re-methylation to methionine (uses methylated folate and B12)
- Trans-sulfuration to cysteine (uses B6)
When these pathways are impaired, homocysteine accumulates.
The methylation cycle
Methylation is essential for many biological processes, DNA repair, neurotransmitter synthesis, detoxification, gene expression. The methyl donor cycle requires:
- Folate (specifically 5-MTHF, methylated form)
- B12 (specifically methylcobalamin)
- B6 (for trans-sulfuration backup)
- Betaine (alternative methyl donor)
Inadequate cofactors or genetic variants impair the cycle.
What raises homocysteine
- B12, folate, or B6 deficiency
- MTHFR genetic variants (slow methylation)
- Kidney dysfunction
- Hypothyroidism
- Certain medications (methotrexate, metformin long-term, some seizure meds)
- Alcohol
- Smoking
- Aging
- Chronic inflammation
Reference ranges
- Optimal: <7 µmol/L
- Normal: 7-10
- Borderline: 10-13
- Elevated: >13
- Severe: >20
Treatment
- Methylfolate 1-5 mg daily
- Methylcobalamin 1,000-5,000 µg daily
- B6 (P5P form) 25-50 mg daily
- Betaine (TMG) 500-3,000 mg daily for poor responders
- Address underlying drivers (alcohol, kidney, thyroid)
- Re-test at 3 months
MTHFR genetic variants
The MTHFR gene encodes an enzyme that produces methylfolate. Common variants (C677T, A1298C) reduce enzyme activity:
- Heterozygous C677T: ~30% reduced activity
- Homozygous C677T: ~70% reduced activity
- About 30-40% of population has at least one variant
Variants raise homocysteine somewhat but the practical implication is mainly to use methylated B-vitamins (which bypass the slow enzyme).
The clinical pearl: Homocysteine is one of the easiest cardiovascular risk markers to address. Most cases respond to methylated B-vitamins within 3 months. Test routinely; treat if elevated.
Bottom line
Homocysteine reflects methylation status and is an independent cardiovascular risk marker. Elevation is common and treatable with B-vitamin supplementation. Standard panels often miss it; specific request needed.