# Homocysteine

> Homocysteine is an amino acid in the blood that's a byproduct of protein metabolism. Elevated levels of homocysteine, known as hyperhomocysteinemia, have b

*Source: [https://www.health3.app/biomarkers/homocystei](https://www.health3.app/biomarkers/homocystei)*

### On This Page

- What it measures
- Measurement units
- Reference ranges
- Health impact
- Related biomarkers
- Scientific references

## What is Homocysteine?

Homocysteine is an amino acid in the blood that's a byproduct of protein metabolism. Elevated levels of homocysteine, known as hyperhomocysteinemia, have been associated with an increased risk of cardiovascular diseases, such as heart attacks and strokes. High homocysteine levels can damage the lining of arteries and promote blood clot formation, leading to atherosclerosis, a condition characterized by narrowed and hardened arteries. Additionally, elevated homocysteine has been linked to an increased risk of venous thrombosis, where clots form in the veins.

Apart from cardiovascular risks, **high levels of homocysteine** have been implicated in other health issues. These include osteoporosis, where high homocysteine levels may interfere with collagen cross-linking, weakening bones and increasing the risk of fractures. There's also evidence suggesting that hyperhomocysteinemia may be associated with Alzheimer's disease, dementia, and decreased cognitive function, although the exact mechanisms are not fully understood. Elevated homocysteine levels can result from a **deficiency of vitamins B6, B9 (folate), and B12**, kidney disease, genetic factors, certain medications, and lifestyle factors such as smoking and excessive alcohol consumption.

On the other hand, **very low homocysteine levels** are rare and generally not considered a health risk. The focus in clinical practice is typically on managing high homocysteine levels, often through dietary changes and supplementation with B vitamins, particularly B6, B9, and B12, which help break down homocysteine in the body. Maintaining balanced homocysteine levels is important for cardiovascular and bone health, as well as for cognitive function.

**Factors that support healthy homocysteine levels**:

- An **adequate intake of B vitamins**, particularly B6, B9 (folate), and B12, can come through a balanced diet or supplementation if necessary. Good sources include leafy greens, fortified grains, legumes, and lean meats.
- A **heart-healthy lifestyle** is supportive, including regular exercise, maintaining a healthy weight, and avoiding smoking and excessive alcohol consumption.
- For people with a family history of hyperhomocysteinemia or at increased risk for cardiovascular disease, a healthcare provider can help monitor homocysteine levels and develop a personalized management plan.
- For people with a chronic condition such as kidney disease that may affect homocysteine levels, working closely with a healthcare team can help manage the condition and monitor homocysteine levels.

## Measurement Units

Homocysteine can be measured in: mg/100mL, mg/dL, mg/L, mg%, µg/mL, µmol/L

## Reference Ranges by Age and Gender

Reference ranges represent typical values for healthy individuals. Your healthcare provider must interpret your specific results.

| Age Range | Gender | Unit | Optimal | Normal | Source |
| --- | --- | --- | --- | --- | --- |
| 15 - 65 | All genders | µmol/L | - | 0 - 15 | Burnham, 2023 |
| 65 - ∞ | All genders | µmol/L | - | 0 - 20 | Burnham, 2023 |

## Related Biomarkers

- **Total Cholesterol** (Coming Soon)

 Elevated homocysteine levels can negatively affect lipid metabolism, leading to unfavorable changes in lipid profiles. These changes can exacerbate the risk of cardiovascular disease[Daly, 2009][Momin, 2017].
- [**Vitamin B6 (Pyridoxine)**](https://www.health3.app/biomarkers/vitaminb6p)

 Vitamin B6 is involved in the conversion of homocysteine to cysteine. Adequate levels of vitamin B6 are necessary to maintain normal homocysteine levels. A deficiency in vitamin B6 can result in elevated homocysteine, which is a risk factor for cardiovascular disease.[Selhub, 1999]
- **LDL Cholesterol** (Coming Soon)

 Elevated homocysteine levels can negatively affect lipid metabolism, leading to unfavorable changes in lipid profiles. These changes can exacerbate the risk of cardiovascular disease[Daly, 2009][Momin, 2017].

## Related tools

- [**Blood Test Unit Converter**](https://www.health3.app/tools/blood-test-unit-converter)

## Academic References

1. Selhub J. Homocysteine metabolism (1999). *Annu Rev Nutr.*. [DOI: 10.1146/annurev.nutr.19.1.217](https://doi.org/10.1146/annurev.nutr.19.1.217)
2. Kasturiratne A, Simmonds M, and Wald DS. Serum homocysteine and dementia: meta-analysis of eight cohort studies including 8669 participants (2011). *Alzheimers Dement*. [DOI: 10.1016/j.jalz.2010.08.234](https://doi.org/10.1016/j.jalz.2010.08.234)
3. Collins P, Cooney MT, Daly C, Fitzgerald AP, Graham IM, and O'Callaghan P. Homocysteine increases the risk associated with hyperlipidaemia (2009). *Eur J Cardiovasc Prev Rehabil*. [DOI: 10.1097/HJR.0b013e32831e1185](https://doi.org/10.1097/HJR.0b013e32831e1185)
4. Burnham C.-A. D., Chiu R. W. K., Rifai N., Wittwer C., and Young I.. Tietz Textbook of Laboratory Medicine (2023). *Tietz Textbook of Laboratory Medicine*.
5. Breteler MM, de Groot LC, de Jonge R, Dhonukshe-Rutten RA, Hofman A, Lindemans J, Lips P, Pluijm SM, Pols HA, Uitterlinden AG, van der Klift M, van Leeuwen JP, van Meurs JB, and Witteman JC. Homocysteine levels and the risk of osteoporotic fracture (2004). *N Engl J Med*. [DOI: 10.1056/NEJMoa032546](https://doi.org/10.1056/NEJMoa032546)
6. Refsum H. and Smith A. D.. Homocysteine, B Vitamins, and Cognitive Impairment (2016). *Annual Review of Nutrition*. [DOI: 10.1146/annurev-nutr-071715-050947](https://doi.org/10.1146/annurev-nutr-071715-050947)
7. Nygård O., Refsum H., Ueland P. M., and Vollset S. E.. Homocysteine and cardiovascular disease (1998). *Annual Review of Medicine*. [DOI: 10.1146/annurev.med.49.1.31](https://doi.org/10.1146/annurev.med.49.1.31)
8. Clarke R., McPartlin J., Nexø E., Scott J.M., Smith A. D., and Ueland P. M.. Facts and recommendations about total homocysteine determinations: an expert opinion (2006). *Clinical Chemistry*. [DOI: 10.1373/clinchem.2003.021634](https://doi.org/10.1373/clinchem.2003.021634)
9. Broe KE, Cupples LA, Hannan MT, Jacques PF, Kiel DP, McLean RR, Samelson EJ, Selhub J, and Tucker KL. Homocysteine as a predictive factor for hip fracture in older persons (2004). *N Engl J Med*. [DOI: 10.1056/NEJMoa032739](https://doi.org/10.1056/NEJMoa032739)
10. Fan F., Jia J., and Momin M.. Relationship between plasma homocysteine level and lipid profiles in a community-based Chinese population (2017). *Lipids Health Dis*. [DOI: 10.1186/s12944-017-0441-6](https://doi.org/10.1186/s12944-017-0441-6)
11. Arnold MJ, Fodor G, Genest J Jr., Held C, Lonn E, McQueen MJ, Micks M, Pogue J, Probstfield J, Sheridan P, and Yusuf S. Homocysteine lowering with folic acid and B vitamins in vascular disease (2006). *N Engl J Med*. [DOI: 10.1056/NEJMoa060900](https://doi.org/10.1056/NEJMoa060900)
12. Cupples LA, Fredman L, Hannan MT, Jacques PF, Kiel DP, McLean RR, Samelson EJ, Selhub J, and Tucker KL. Plasma B vitamins, homocysteine, and their relation with bone loss and hip fracture in elderly men and women (2008). *J Clin Endocrinol Metab*. [DOI: 10.1210/jc.2007-2710](https://doi.org/10.1210/jc.2007-2710)
13. Kim DJ, Kim GS, Kim HH, Kim YS, Koh JM, Lee KU, Lee YS, and Park JY. Homocysteine enhances bone resorption by stimulation of osteoclast formation and activity through increased intracellular ROS generation (2006). *J Bone Miner Res*. [DOI: 10.1359/jbmr.060406](https://doi.org/10.1359/jbmr.060406)
14. Cornett EM, Fuller MC, Jeha GM, Kaye AD, Kevil CG, Lerner ZI, Pham AD, Sibley GT, Urits I, and Viswanath O. Folic Acid Supplementation in Patients with Elevated Homocysteine Levels (2020). *Adv Ther*. [DOI: 10.1007/s12325-020-01474-z](https://doi.org/10.1007/s12325-020-01474-z)

## What Homocysteine Measures

Homocysteine is a sulphur-containing amino acid produced during the metabolism of methionine (an essential amino acid found in protein-rich foods). Unlike dietary amino acids, homocysteine is not obtained from food — it is an intermediate metabolite. In a healthy body, homocysteine is rapidly converted back to methionine (requiring folate and vitamin B12) or converted to cystathionine (requiring vitamin B6). When these B-vitamin cofactors are insufficient — or certain enzymes are defective — homocysteine accumulates in the blood.

Elevated homocysteine (hyperhomocysteinaemia) is a well-established risk marker for cardiovascular disease, stroke, and venous thromboembolism. It has also been linked to cognitive decline and bone fragility. Because homocysteine is directly modifiable through B-vitamin supplementation, it is both an informative marker and a potential intervention target. See the [Cardiovascular Health](https://www.health3.app/topics/cardiovascular-health) topic page and the [B vitamins blood test guide](https://www.health3.app/blog/b-vitamins-blood-test-guide).

## Homocysteine Reference Ranges

| Category | Plasma Level (µmol/L) | Clinical Note |
| --- | --- | --- |
| Normal / Desirable | <10 µmol/L | Most guidelines; some labs extend to <15 µmol/L |
| Mild elevation | 10 – 30 µmol/L | Associated with increased cardiovascular risk |
| Moderate elevation | 30 – 100 µmol/L | Warrants investigation; possible renal disease, enzyme defect |
| Severe elevation | >100 µmol/L | Rare; suggests homocystinuria (genetic enzyme defect) |

Reference ranges vary between laboratories. Some experts use an optimal target of <7 µmol/L for cardiovascular prevention, particularly in people with other risk factors. Homocysteine increases with age, renal impairment, and is typically higher in men than women. A [blood test unit converter](https://www.health3.app/tools/blood-test-unit-converter) can help when a lab reports in mg/L (multiply µmol/L by 0.135).

## What High Homocysteine Means: Causes and Risks

The most common cause of elevated homocysteine is **nutritional deficiency** — particularly low folate (B9), vitamin B12, and vitamin B6, all of which are required for homocysteine metabolism. Other causes include:

- **Chronic kidney disease (CKD)** — the kidneys clear homocysteine; reduced kidney function leads to accumulation. Homocysteine often rises in parallel with declining GFR
- **MTHFR gene variants** — particularly the C677T polymorphism, which reduces the activity of the enzyme that converts folate to its active form (5-methyltetrahydrofolate); very common (30–40% of the general population carry one or two copies)
- **Hypothyroidism** — thyroid hormones regulate homocysteine metabolism; untreated hypothyroidism elevates homocysteine. See [TSH](https://www.health3.app/biomarkers/tsh) and the [thyroid blood tests guide](https://www.health3.app/blog/thyroid-blood-tests-explained)
- **Medications** — methotrexate (interferes with folate metabolism), metformin (reduces B12 absorption), proton pump inhibitors (reduce B12 absorption), and anticonvulsants
- **Lifestyle factors** — smoking, excessive alcohol consumption, high-protein/low-vegetable diets
- **Homocystinuria** — a rare inherited disorder (CBS enzyme defect) causing very high homocysteine levels (>100 µmol/L); associated with lens dislocation, intellectual disability, and severe vascular disease

**What elevated homocysteine does to the body:** High homocysteine damages vascular endothelium, promotes oxidative stress, activates platelet aggregation, and impairs colagen cross-linking. These mechanisms contribute to atherosclerosis, arterial stiffness, and thrombosis risk.

## Conditions Associated with Elevated Homocysteine

The following associations are well-established in observational literature; note that reducing homocysteine via B-vitamin supplementation has shown mixed results in RCTs for some endpoints:

- **Cardiovascular disease:** Each 5 µmol/L increase in homocysteine is associated with approximately 20–25% increased risk of coronary heart disease and stroke in meta-analyses
- **Cognitive decline and dementia:** Elevated homocysteine is one of the most consistent risk markers for Alzheimer's disease and vascular dementia in observational studies. B-vitamin supplementation slows brain atrophy in people with mild cognitive impairment and high homocysteine (VITACOG trial)
- **Osteoporosis and fracture:** Homocysteine interferes with collagen cross-linking in bone matrix; elevated levels are independently associated with hip fracture risk (McLean et al. and van Meurs et al., NEJM, 2004). See [Bone Health](https://www.health3.app/topics/bone-health)
- **Venous thromboembolism:** Associated with deep vein thrombosis and pulmonary embolism
- **Renal disease:** Homocysteine is often the first marker to suggest developing CKD; correlates with GFR decline

## Lowering Homocysteine: Evidence and Approach

Elevated homocysteine driven by B-vitamin deficiency is highly responsive to supplementation. The standard approach:

- **Folate (B9):** 400–1,000 µg/day; methylfolate (5-MTHF) may be preferable for people with MTHFR variants
- **Vitamin B12:** 500–1,000 µg/day; methylcobalamin form often used; important in people on metformin or with atrophic gastritis
- **Vitamin B6:** 25–100 mg/day (also needed for the transsulphuration pathway)

Dietary changes — increasing folate-rich foods (leafy greens, legumes, fortified cereals) and B12 sources (meat, fish, dairy, or fortified plant milks for vegans) — are the first-line approach. Monitoring homocysteine after 8–12 weeks of intervention confirms response.

## Tracking Homocysteine Over Time

Since homocysteine is a risk marker that responds to intervention, tracking it over time is particularly informative. A single elevated reading is commonly followed up clinically by checking folate, B12, and B6 status, renal function (creatinine, eGFR), and thyroid function (TSH). Retesting after 2–3 months of supplementation confirms whether levels have normalised.

People with the MTHFR C677T variant may find their homocysteine remains borderline elevated despite standard supplementation, in which case a methylated B-vitamin formula and attention to dietary folate may be more effective.

## Related Markers

- [**Vitamin B12**](https://www.health3.app/biomarkers/vitaminb12) — deficiency is the most common cause of elevated homocysteine; always check together
- [**Folate (plasma)**](https://www.health3.app/biomarkers/vitaminb9_plasma) or [**red cell folate**](https://www.health3.app/biomarkers/vitaminb9_rbc) — the other primary cofactor for homocysteine remethylation
- [**Vitamin B6 (P5P)**](https://www.health3.app/biomarkers/vitaminb6p) — cofactor for the transsulphuration pathway
- [**TSH**](https://www.health3.app/biomarkers/tsh) — hypothyroidism raises homocysteine; treat the thyroid and levels may normalise
- **eGFR / creatinine** — kidney function; renal impairment elevates homocysteine. See the [eGFR calculator](https://www.health3.app/tools/egfr-calculator)

## Frequently Asked Questions About Homocysteine

### What is a normal homocysteine level?

Most laboratories define normal as below 10–15 µmol/L. Many cardiovascular prevention specialists prefer a target below 10 µmol/L, and some use an optimal target of <7 µmol/L. Levels typically increase with age and are higher in men than women by about 2–3 µmol/L on average.

### What does high homocysteine mean?

High homocysteine most commonly indicates deficiency in folate, B12, or B6. It can also reflect kidney disease, hypothyroidism, MTHFR gene variants, or medication effects. It is an independent cardiovascular and neurocognitive risk marker. Importantly, it is one of the few cardiovascular risk factors that is both measurable with a simple blood test and easily modifiable through dietary changes and supplementation.

### What foods lower homocysteine?

Folate-rich foods (leafy green vegetables, lentils, beans, asparagus, fortified cereals), B12-rich foods (meat, fish, dairy, eggs, or fortified plant milks), and B6-rich foods (poultry, fish, potatoes, bananas). A Mediterranean-style diet is associated with lower homocysteine levels.

### Does the MTHFR gene mutation cause high homocysteine?

The MTHFR C677T variant reduces enzyme activity by 30–60%, which can impair folate conversion to its active form (5-MTHF) and lead to mildly elevated homocysteine. It is common (30–40% of the general population carry at least one copy). Most people with this variant can normalise homocysteine with adequate folate and B12 intake; methylated forms of these vitamins may be particularly beneficial.

### Does lowering homocysteine reduce heart disease risk?

B-vitamin supplementation reliably lowers homocysteine (by 20–30%), but large RCTs have not consistently shown reduced rates of heart attacks or strokes in people who were not severely deficient. One important exception: studies suggest that B-vitamin supplementation reducing homocysteine in people with cognitive impairment does slow brain atrophy — suggesting the cognitive pathway may be more responsive. The field is still evolving.

### How quickly does homocysteine respond to supplementation?

Plasma homocysteine typically drops within 2–4 weeks of starting folate and B12 supplementation, with maximum response (20–30% reduction) usually achieved by 8–12 weeks. Retesting at 2–3 months after starting supplements is recommended to confirm response.

#### Medical Disclaimer

This page is for **educational purposes only** and does not constitute medical advice. Reference ranges vary between laboratories. Elevated homocysteine requires clinical evaluation to identify the cause before starting treatment. Always discuss results with a healthcare provider. Health3 is an educational tracking tool, not a diagnostic service.

### ⚠️ Important Medical Information

This reference page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Reference ranges vary between laboratories. Always review your lab results with a qualified healthcare provider.