Plasma total homocysteine (Homocysteine)

Updated October 31, 2026 · 59 sources · Written by the AumniVIDA team

Units

What is Homocysteine?

Homocysteine is an amino acid your body makes when it uses methionine from protein. Folate, vitamin B12 and vitamin B6 clear it, so a blood level above about 15 µmol/L often signals a vitamin shortfall, kidney decline or a genetic variant. High levels track with heart disease, stroke and dementia, but B-vitamin trials did not prevent heart attacks.

Abstract illustration of a circular metabolic cycle drawn as interlocking gears, with one small molecule at a branch point leading off to a second pathway. — Homocysteine sits at a fork: folate and B12 recycle it into methionine, and B6 routes it toward cysteine. When either path slows, it builds up in blood.

At a glance

What it measures: Total homocysteine in plasma or serum: the free amino acid plus the larger share bound to proteins and other thiols.
Why it is ordered: To check functional folate and B12 status, investigate unexplained blood clots or neuropathy, screen for homocystinuria, and, less often now, assess vascular risk.
Units: µmol/L, the same in US and SI reporting. 1 mg/L is about 7.4 µmol/L.
Sample handling: Separate plasma promptly. Cells keep releasing homocysteine at room temperature, so labs centrifuge at once or keep the tube on ice and spin within an hour.
Fasting: Preferred. A protein-rich meal can raise the result by about 13% for several hours.
Typical reference interval: About 5 to 15 µmol/L in adults; one large US lab uses up to 14.5 at 18 to 60 years and up to 21.3 after 80.
Hyperhomocysteinemia: Moderate 15 to 30, intermediate 30 to 100, severe over 100 µmol/L. Some sources start the bands at 16 and 31.
Main causes of high levels: Low B12 or folate, reduced kidney function, hypothyroidism, metformin, the MTHFR 677TT genotype with low folate, smoking, heavy coffee and older age.

Questions people ask about Homocysteine

Key takeaways

1
Homocysteine is a by-product of methionine metabolism, cleared by folate and B12 (back to methionine) and by B6 (on to cysteine), so it works as a functional readout of B-vitamin status [1][2].
2
Adult values usually fall between 5 and 15 µmol/L; experts behind the dementia consensus treat 10 µmol/L or below as probably safe and 11 or above as worth acting on [1][3].
3
Each 25% lower homocysteine was associated with 11% less heart disease and 19% less stroke in cohorts [4], yet trials that cut it by about 25% did not reduce heart attacks [5][6].
4
The trial evidence is more favourable for stroke, especially in low-folate China (21% fewer first strokes in CSPPT) [7], and for brain atrophy in older adults with high homocysteine [8].
5
Folic acid at 0.4 to 0.8 mg a day captures most of the possible reduction, B12 adds about 7%, and B6 adds little to fasting levels [9].

What is homocysteine?

Homocysteine is an amino acid made when the body uses methionine from protein. It is recycled by folate and B12 or converted to cysteine with B6, so its blood level reflects how well those vitamins work.

Homocysteine is a sulfur-containing amino acid that the body makes from methionine, an essential amino acid in dietary protein. It is not built into proteins and you do not eat it in meaningful amounts. It exists only briefly, as a crossroads between two pathways, and the cells either recycle it back into methionine or convert it to cysteine [1].

A blood test reports total homocysteine (often written tHcy). In plasma, most homocysteine is bound to albumin or paired with other thiols such as cysteine, and only a small share circulates free. Total homocysteine counts all of these forms together, and that is what reference intervals and studies use [10][11].

Two features make the marker useful:

It reflects vitamin function, not just intake. Because folate, vitamin B12 and vitamin B6 all act on homocysteine, a high level shows that one of these pathways is not keeping up, even when blood vitamin levels look borderline [12][2].
It tracks many risks at once. Raised homocysteine is associated with more than 100 conditions, from heart disease and stroke to cognitive decline and fractures [3]. That breadth is also the reason to be cautious: a marker linked to everything is often a marker of general health, not a single cause.

Homocysteine is spelled differently in older papers. Homocystine is two homocysteine molecules joined, which is what spills into urine in the inherited disease homocystinuria, first reported in 1962 [13]. "Homocyst(e)ine" was used for total plasma measures before the term total homocysteine settled [14].

Minimal illustration of a dashboard warning light glowing beside a set of tangled threads that lead away to several separate objects. — Homocysteine rises with many things that harm arteries and brains, which makes it a good warning light even where lowering it alone changes little.

How is homocysteine made and cleared?

Methionine donates a methyl group and becomes homocysteine. Folate and B12 recycle it back to methionine, betaine offers a liver shortcut, and B6 turns it into cysteine.

Homocysteine is the turning point of the methionine cycle. Methionine is first activated to S-adenosylmethionine (SAM), the body's main methyl donor for DNA, neurotransmitters and many other molecules. Once SAM gives up its methyl group it becomes S-adenosylhomocysteine, which is split to release homocysteine [1].

From there, homocysteine takes one of two routes.

Remethylation: back to methionine

Methionine synthase adds a methyl group taken from 5-methyltetrahydrofolate, the main circulating form of folate. The enzyme needs vitamin B12 as its cofactor [2].
MTHFR (methylenetetrahydrofolate reductase) makes that 5-methyltetrahydrofolate. A common variant, C677T, makes the enzyme less stable, and people with two copies (TT) have higher homocysteine [15]. The enzyme uses a riboflavin-derived cofactor, which is why riboflavin lowers homocysteine in TT carriers [16].
In the liver and kidney, betaine offers a second remethylation route that does not depend on folate or B12 [17].

Transsulfuration: on to cysteine

Cystathionine beta-synthase joins homocysteine to serine to form cystathionine, which becomes cysteine and then glutathione and taurine. This step needs vitamin B6. Inherited loss of this enzyme causes classical homocystinuria, with values often far above 100 µmol/L [1].

Why B12 and folate matter most for the blood test

The remethylation loop handles most of the daily load, so shortfalls of folate or B12 raise fasting homocysteine far more than a shortfall of B6. Trials agree: adding B6 to folic acid did not lower fasting homocysteine further [9]. B12 is also the cofactor for methylmalonyl-CoA mutase, so low B12 raises methylmalonic acid (MMA) as well, while low folate does not [2]. That difference is the basis of the B12 and MTHFR page.

Where each vitamin acts

Methionine cycle

Methionine

Supplied by protein in food.

SAM

S-adenosylmethionine, the body's main methyl donor.

SAH

S-adenosylhomocysteine, left after SAM gives up its methyl group.

Homocysteine

Exit to cystathionine and cysteine via cystathionine beta-synthase, which needs B6.

Remethylation

Methionine synthase needs B12 and 5-methyl-THF, made by MTHFR with riboflavin. Betaine offers a liver shortcut.

Methionine cycle
1MethionineSupplied by protein in food.
2SAMS-adenosylmethionine, the body's main methyl donor.
3SAHS-adenosylhomocysteine, left after SAM gives up its methyl group.
4HomocysteineExit to cystathionine and cysteine via cystathionine beta-synthase, which needs B6.
5RemethylationMethionine synthase needs B12 and 5-methyl-THF, made by MTHFR with riboflavin. Betaine offers a liver shortcut.
Back to step 1

B12 is also the cofactor for methylmalonyl-CoA mutase, so low B12 raises both homocysteine and MMA, while low folate raises homocysteine only. The MTHFR C677T variant makes that enzyme less stable.

One diagram shows why B12, folate, riboflavin, B6 and betaine all move homocysteine, and why MMA points to B12 specifically.

Is high homocysteine bad for your heart and brain?

High homocysteine predicts heart disease, stroke, dementia and fractures. B-vitamin trials lowered it by about a quarter without preventing heart attacks, but showed some benefit for stroke and brain atrophy.

High homocysteine predicts heart disease, stroke, dementia and fractures in cohort studies. The harder question is whether homocysteine causes these outcomes or travels with things that do. Three kinds of evidence answer that question differently.

Cohort studies: consistent associations

Heart disease and stroke. Pooling 30 studies, the Homocysteine Studies Collaboration found that a 25% lower homocysteine (about 3 µmol/L) was associated with 11% lower ischaemic heart disease risk and 19% lower stroke risk, after adjustment for other risk factors [4]. Associations were stronger in studies that measured homocysteine after disease began, which hints at reverse causation.
Dementia. In the Framingham Study, each standard deviation higher homocysteine carried 1.4 times the risk of dementia, and levels above 14 µmol/L nearly doubled Alzheimer risk [18]. An international consensus later put the relative risk at 1.15 to 2.5 [12].
Bone. Older adults in the top homocysteine quartile had about 1.9 times the fracture risk in Dutch cohorts [19], and men had almost four times the hip fracture risk in Framingham [20].

Genetic studies: weaker than expected

The MTHFR TT genotype raises homocysteine for life, independent of lifestyle, so it works as a natural experiment (Mendelian randomization). An early analysis read the genetic and cohort data as evidence of causation [21]. Larger, less biased datasets changed the picture. In 48,175 coronary cases from unpublished datasets, TT carried no extra heart disease risk (OR 1.02), and the authors attributed earlier positive findings to publication bias [22]. For stroke, the TT genotype mattered in low-folate Asia (OR 1.68) but not where food is fortified (OR 1.03) [23], and in 156,000 Chinese adults TT carried 13% higher stroke risk with no excess heart disease [24]. Genetic analyses point at small vessel stroke in particular [25].

Randomised trials: no fewer heart attacks

Large trials lowered homocysteine by roughly a quarter and followed people for years. Pooled across 8 trials and 37,485 people, folic acid had no effect on major vascular events (RR 1.01), coronary events (1.03) or stroke (0.96) [5]. The 2017 Cochrane review of 15 trials and 71,422 people found no effect on heart attack (RR 1.02) or death (RR 1.01) and a small reduction in stroke (RR 0.90) [6]. Individual trials tell the same story. VISP found no fewer recurrent strokes after a 2 µmol/L greater reduction [26]. HOPE-2 found no fewer major events but 25% fewer strokes [27]. NORVIT found no benefit after a heart attack [28], SEARCH none after 6.7 years [29], and VITATOPS a borderline 9% reduction in its combined outcome [30]. The table below brings the main trials together.

The two clearest positive signals came from settings the earlier trials did not test: people with low folate intake and high blood pressure in China, where folic acid cut first strokes by 21% [7], and older adults with mild cognitive impairment, where B vitamins slowed brain shrinkage, mainly when homocysteine was above 11 to 13 µmol/L [8][31].

Reviewers also point out that most trials enrolled people with established disease and only mildly raised homocysteine, so primary prevention and severe hyperhomocysteinemia remain largely untested [32].

B-vitamin trials and pooled analyses: how far homocysteine fell and what happened to outcomes. Abstract figures; RR is relative risk, HR hazard ratio.
Trial (source)	Who	Daily regimen	Homocysteine change	Main result
VISP (Toole 2004, JAMA)	3,680 adults after ischaemic stroke	Folic acid 2.5 mg, B6 25 mg, B12 0.4 mg vs low dose	2 µmol/L more than low dose	Recurrent stroke 9.2% vs 8.8%; no effect on stroke, heart events or death
HOPE-2 (Lonn 2006, NEJM)	5,522 aged 55+ with vascular disease or diabetes	Folic acid 2.5 mg, B6 50 mg, B12 1 mg	Down 2.4 vs up 0.8 µmol/L on placebo	Major events RR 0.95 (0.84 to 1.07); stroke RR 0.75 (0.59 to 0.97); more unstable angina admissions
NORVIT (Bønaa 2006, NEJM)	3,749 after heart attack	Folic acid 0.8 mg + B12 0.4 mg, with or without B6 40 mg	27% lower	RR 1.08 (0.93 to 1.25); all three vitamins RR 1.22 (1.00 to 1.50)
SEARCH (2010, JAMA)	12,064 heart attack survivors, UK	Folic acid 2 mg + B12 1 mg	3.8 µmol/L (28%) lower	Major vascular events RR 1.04 (0.97 to 1.12); stroke RR 1.02
VITATOPS (2010, Lancet Neurol)	8,164 after stroke or TIA, 20 countries	Folic acid 2 mg, B6 25 mg, B12 0.5 mg	Lowered (not given in abstract)	Stroke, heart attack or vascular death RR 0.91 (0.82 to 1.00), borderline
CSPPT (Huo 2015, JAMA)	20,702 adults with hypertension, China, no prior stroke	Enalapril 10 mg + folic acid 0.8 mg vs enalapril	Lowered (not given in abstract)	First stroke 2.7% vs 3.4%, HR 0.79 (0.68 to 0.93); heart attack HR 1.04
VITACOG (Smith 2010, PLoS One)	Adults over 70 with mild cognitive impairment, 168 scanned	Folic acid 0.8 mg, B12 0.5 mg, B6 20 mg	Lowered (not given in abstract)	Brain atrophy 0.76% vs 1.08% a year; 53% slower when homocysteine was above 13 µmol/L
Pooled 8 trials (Clarke 2010)	37,485 at raised vascular risk	Folic acid based	25% lower on average	Major vascular events RR 1.01; coronary 1.03; stroke 0.96; cancer and death unchanged
Cochrane (Martí-Carvajal 2017)	15 trials, 71,422 people	B6, B9 and B12 alone or combined	Lowered	Heart attack RR 1.02; death RR 1.01; stroke RR 0.90 (0.82 to 0.99)

Why lowering homocysteine did not cut heart attacks

Outcome down, Type of evidence across

Heart attack

Cohort studiesAbout 11% lower risk per 25% lower homocysteine (HSC 2002)
Genetic (MTHFR) studiesNo extra risk: OR 1.02 in unpublished datasets (Clarke 2012)
Randomised B-vitamin trialsNo fewer heart attacks: RR 1.02 (Cochrane 2017)

Stroke

Cohort studiesAbout 19% lower risk (HSC 2002)
Genetic (MTHFR) studiesOR 1.68 in low-folate Asia, 1.03 where food is fortified (Holmes 2011)
Randomised B-vitamin trialsRR 0.90 overall (Cochrane 2017); HR 0.79 in CSPPT, China (Huo 2015)

	Cohort studies	Genetic (MTHFR) studies	Randomised B-vitamin trials
Heart attack	About 11% lower risk per 25% lower homocysteine (HSC 2002)	No extra risk: OR 1.02 in unpublished datasets (Clarke 2012)	No fewer heart attacks: RR 1.02 (Cochrane 2017)
Stroke	About 19% lower risk (HSC 2002)	OR 1.68 in low-folate Asia, 1.03 where food is fortified (Holmes 2011)	RR 0.90 overall (Cochrane 2017); HR 0.79 in CSPPT, China (Huo 2015)

Shading marks the strength of the link. Possible reasons for the gap, all hypotheses: homocysteine partly marks kidney function, smoking and diet; most trials ran in fortified populations with established disease; NORVIT hinted at small harm from high doses.

Cohorts, genes and trials point in different directions for heart disease, but line up better for stroke in low-folate populations.

What do homocysteine levels mean?

Most adults have 5 to 15 µmol/L. Above 15 is hyperhomocysteinemia: moderate to 30, intermediate to 100, severe beyond. Prevention researchers aim lower, at 10 or below.

Labs report total homocysteine in µmol/L; 1 mg/L is about 7.4 µmol/L, so 3 µmol/L equals 0.41 mg/L [4]. Most adults fall between 5 and 15 µmol/L, and a value above 15 µmol/L is called hyperhomocysteinemia [1]. Raised values are then graded as moderate (15 to 30), intermediate (30 to 100) and severe (over 100 µmol/L) [33]; some references draw the lines at 16 and 31 [1].

The table compiles the cutoffs you will meet in lab reports, guidelines and trials, side by side.

The upper limit of "normal" depends on who wrote it. Lab ranges describe where most tested people fall, and they rise with age: Labcorp's limit is 14.5 µmol/L at 18 to 60 years and 21.3 µmol/L after 80 [34], while ARUP uses 0 to 15 µmol/L for adults [35]. Researchers who study prevention set lower lines. Smith and Refsum judge 10 µmol/L or below probably safe and 11 or above worth acting on [3], and the VITACOG brain benefit appeared above 11 to 13 µmol/L [8][31].

Three practical points:

One result is a snapshot. Homocysteine varies by about 8% within the same person, and lying down for 30 minutes before the draw lowers it by about 6% [36]. Season shifts it by about 0.3 µmol/L [37].
Handling matters. Left at room temperature in whole blood, homocysteine rises about 0.7 µmol/L per hour [38]. A sample that sat on a counter can read falsely high.
Very low values are uncommon and poorly studied. One Korean cohort found more dementia at both ends (8.9 or below and 10.6 or above) than in the middle [39], but this is a single observational study.

For ranges by age and sex and what counts as low, see what is a normal homocysteine level.

Homocysteine thresholds by source, in µmol/L.
Source	Type	Threshold	What it means
Smith and Refsum 2021	Expert review	10 or below; 11 or above	Probably safe; may justify intervention
Homocysteine and dementia consensus 2018	Consensus statement	Above 11	Moderately raised; linked to cognitive decline in older people
VITACOG trial 2010	Randomised trial	Above 13	B vitamins slowed brain atrophy by 53% in this group
Framingham 2002	Cohort	Above 14	Alzheimer risk nearly doubled
Fortification study 1999	Cohort	Above 13	Defined as high; share fell from 18.7% to 9.8%
Labcorp	Lab reference interval	Up to 14.5 (18 to 60 y); 17.2 (61 to 70); 19.2 (71 to 80); 21.3 (over 80)	Upper limit of normal by age
ARUP Laboratories	Lab reference interval	0 to 15	Adult reference interval
StatPearls	Clinical reference	5 to 15 typical; 16 to 30; 31 to 100; over 100	Normal; moderate; intermediate; severe
Levy 2021	Clinical study	15 to 30; 30 to 100; over 100	Moderate; intermediate; severe

Homocysteine categories in µmol/L

10 or below: probably safe (Smith and Refsum): 0 to 10
10 to 15: within lab range, above expert target: 10 to 15
15 to 30: moderate: 15 to 30
30 to 100: intermediate (over 100 severe): 30 to 40

10 µmol/L: Probably safe at or below
11 µmol/L: Dementia risk threshold in consensus
13 µmol/L: VITACOG benefit above
14.5 µmol/L: Labcorp upper limit, age 18 to 60
15 µmol/L: ARUP upper limit
30 µmol/L: Intermediate starts

Most adults sit between 5 and 15 µmol/L. Moderate, intermediate and severe hyperhomocysteinemia start at about 15, 30 and 100 µmol/L, and the cutoffs that experts and labs use for action differ by several units.

What causes high homocysteine?

Low B12 or folate, reduced kidney function, hypothyroidism, metformin, the MTHFR TT genotype with low folate, smoking, heavy coffee and age. Very high values point to inherited disorders or severe deficiency.

Most high results have an ordinary explanation, and the cause matters more than the number.

Low vitamin B12. B12 is the cofactor for remethylation, so deficiency raises homocysteine and methylmalonic acid together [2]. Vegetarians and especially vegans who do not supplement have higher homocysteine and lower B12 than omnivores [40].
Low folate. Before US grain fortification, 18.7% of Framingham adults had homocysteine above 13 µmol/L; after it, 9.8% did [41].
Reduced kidney function. The kidney clears homocysteine, and higher creatinine tracks higher levels [42]. Severe kidney failure explained 15% of cases in a hospital series of mostly intermediate and severe hyperhomocysteinemia [33].
Hypothyroidism. Raised levels are well documented in overt hypothyroidism, and even subclinical hypothyroidism adds about 1.16 µmol/L [43]. See the TSH hub.
Medications. Metformin lowered B12 by 19% over 4.3 years, and patients who became B12 deficient averaged 23.7 µmol/L [44]. Proton pump inhibitors are often blamed, but in 25,953 adults the difference was 12.0 versus 11.6 µmol/L, too small to matter [45].
MTHFR C677T TT genotype, mainly with low folate. TT raised homocysteine by about 3.1 µmol/L in low-folate Asia but barely at all in fortified countries [23].
Smoking, coffee and age. Homocysteine rises with age, is higher in men and climbs with each cigarette per day [46]. A litre a day of filtered coffee raised it by 1.5 µmol/L (18%) [47].
Inherited disorders. Rare enzyme defects such as cystathionine beta-synthase deficiency cause severe hyperhomocysteinemia, often far above 100 µmol/L [1][33].

The high homocysteine page walks through which causes fit which range.

What lowers homocysteine?

Folic acid lowers homocysteine by up to about 25%, B12 adds about 7%, and riboflavin helps MTHFR TT carriers. Diet changes move it by under 1 µmol/L.

Homocysteine responds quickly and predictably to B vitamins. Folic acid alone lowered it by 13% at 0.2 mg a day, 20% at 0.4 mg and about 23 to 25% at 0.8 mg or more; B12 added about 7% and B6 added nothing significant to fasting levels [9]. Higher starting levels and lower folate give bigger drops [48].

Other levers are smaller but real:

Riboflavin lowered homocysteine by 22% in people with the MTHFR TT genotype, and not at all in CC or CT [16].
Betaine at 4 g a day or more lowered it by about 1.2 µmol/L [49].
Diet pattern. A DASH-style diet rich in fruit, vegetables and low-fat dairy lowered it by about 0.8 µmol/L relative to a typical American diet [50]. Plant-forward patterns such as the Mediterranean diet and MIND diet supply folate, and a whole-food plant-based diet needs a reliable B12 source.
Coffee and smoking. Heavy coffee raises homocysteine [47], and smoking tracks with higher levels [46].

What lowering achieves is the separate question. It has not prevented heart attacks [6]. The full ranking of levers, doses and timelines is on how to lower homocysteine. Decisions about supplements, especially for people with kidney disease, heart disease or on medication, belong with a clinician.

How much each B vitamin lowers homocysteine

Folic acid 5 mg/day25 % lower homocysteine
Folic acid 2 mg/day23 % lower homocysteine
Folic acid 0.8 mg/day23 % lower homocysteine
Smallest dose with close to the full effect
Folic acid 0.4 mg/day20 % lower homocysteine
About 90% of the maximum effect
Folic acid 0.2 mg/day13 % lower homocysteine
Vitamin B12 0.4 mg/day, added to folic acid7 % lower homocysteine
Vitamin B6, added to folic acid0 % lower homocysteine
No significant effect on fasting homocysteine

Show the data

How much each B vitamin lowers homocysteine
Item	Value (% lower homocysteine)	Low	High	Note
Folic acid 5 mg/day	25	22	28
Folic acid 2 mg/day	23	20	26
Folic acid 0.8 mg/day	23	21	26	Smallest dose with close to the full effect
Folic acid 0.4 mg/day	20	17	22	About 90% of the maximum effect
Folic acid 0.2 mg/day	13	10	16
Vitamin B12 0.4 mg/day, added to folic acid	7	4	9
Vitamin B6, added to folic acid	0			No significant effect on fasting homocysteine

Folic acid does most of the work and plateaus near 0.8 mg a day; B12 adds about 7 more points, and B6 adds nothing measurable to fasting levels. Values are standardised to a starting homocysteine of 12 µmol/L.

What does the evidence not show?

It does not show that lowering homocysteine prevents heart attacks or dementia, that it causes heart disease, or that MTHFR testing helps most people.

Some claims about homocysteine go further than the evidence.

It does not show that B vitamins prevent heart attacks. Across 15 trials, heart attack risk was unchanged (RR 1.02) [6], and in NORVIT the three-vitamin arm trended toward more events (RR 1.22) [28].
It does not prove homocysteine causes heart disease. Unbiased genetic data show little or no coronary effect of lifelong higher homocysteine [22], and a Korean Mendelian randomization study found no causal effect on death [51].
It does not show B vitamins prevent dementia. Trials show slower brain atrophy in people with high homocysteine [8][31], but conversion to dementia has not been tested at scale, and the pooled effect on global cognition is very small (Hedges g 0.11) [52].
It does not mean MTHFR testing is useful. The American College of Medical Genetics advises against routine MTHFR testing [53].
It does not settle stroke for everyone. The CSPPT benefit came from a low-folate population taking an antihypertensive [7]; in fortified countries genetic and trial data agree on little benefit [23].
It does not mean lower is always better. One cohort linked low values to more dementia [39], and a folate umbrella review rated no cardiometabolic link as convincing [54].

What the evidence does support: homocysteine is a sensitive check on B12 and folate function, a sign worth explaining when high, and possibly a target in older adults with cognitive decline and in low-folate settings.

From a rare disease to the vitamin trials

1962
Homocystinuria first reported in Northern Ireland
1969
McCully links homocysteine to arteriosclerosis
1995
Common MTHFR C677T variant identified
1998
Folic acid shown to lower homocysteine about 25%
1999
US folic acid fortification halves high homocysteine
2002
Cohorts tie homocysteine to heart disease, stroke and dementia
2004
VISP: no fewer recurrent strokes
2006
HOPE-2 and NORVIT: no fewer heart attacks
2010
VITACOG slows brain atrophy; SEARCH null
2013
ACMG advises against MTHFR testing
2015
CSPPT: folic acid cuts first stroke 21% in China
2017
Cochrane: no effect on heart attack, small stroke benefit
2018
Consensus calls homocysteine a modifiable dementia risk
2025
Meta-analysis finds very small cognitive benefit

The homocysteine hypothesis grew for four decades from rare inherited disease and cohort studies, then met large trials that lowered the marker without cutting heart attacks.

The research

Foundational studies

The work that established what this marker means.

1962Metabolic abnormalities detected in a survey of mentally backward individuals in Northern Ireland.Arch Dis ChildA metabolic survey in Northern Ireland reported the first patients with the inherited disorder that became known as homocystinuria.
1969Vascular pathology of homocysteinemia: implications for the pathogenesis of arteriosclerosis.Am J PatholArterial damage in children with inherited disorders of homocysteine metabolism led to the hypothesis that homocysteine promotes arteriosclerosis.
1992Hyperhomocyst(e)inemia as a risk factor for occlusive vascular disease.Annu Rev NutrAn early review of the evidence that raised blood homocysteine is a risk factor for occlusive vascular disease.
1995A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase.Nat GenetIdentified the common MTHFR C677T variant, carried on about 38% of chromosomes, which makes a heat-sensitive enzyme and raises homocysteine in people with two copies.
1995Total plasma homocysteine and cardiovascular risk profile. The Hordaland Homocysteine Study.JAMAIn 16,176 Norwegian adults, homocysteine was higher in men, rose with age and rose steeply with cigarettes smoked per day.
1998Lowering blood homocysteine with folic acid based supplements: meta-analysis of randomised trials. Homocysteine Lowering Trialists' Collaboration.BMJFolic acid at 0.5 to 5 mg a day lowered homocysteine by about 25%, vitamin B12 added about 7% more, and vitamin B6 added no significant effect.
1999The effect of folic acid fortification on plasma folate and total homocysteine concentrations.N Engl J MedAfter US grain fortification, mean homocysteine fell from 10.1 to 9.4 µmol/L and the share above 13 µmol/L fell from 18.7% to 9.8%.
2002Homocysteine and risk of ischemic heart disease and stroke: a meta-analysis.JAMAAcross 30 studies, a 25% lower homocysteine (about 3 µmol/L) was associated with 11% lower heart disease risk and 19% lower stroke risk in prospective data.
2002Homocysteine and cardiovascular disease: evidence on causality from a meta-analysis.BMJGenetic and prospective studies gave similar odds ratios per 5 µmol/L of homocysteine, which the authors read as evidence of causation.
2002Plasma homocysteine as a risk factor for dementia and Alzheimer's disease.N Engl J MedIn the Framingham Study, each standard deviation higher homocysteine carried 1.4 times the risk of dementia, and levels above 14 µmol/L nearly doubled Alzheimer risk.
2004Homocysteine levels and the risk of osteoporotic fracture.N Engl J MedIn 2,406 older Dutch adults, homocysteine in the top age-specific quartile was linked to 1.9 times the fracture risk.
2004Homocysteine as a predictive factor for hip fracture in older persons.N Engl J MedIn Framingham, the highest homocysteine quartile had almost four times the hip fracture risk in men and 1.9 times in women.

Guidelines and reviews

Where expert bodies and systematic reviews have landed.

2004Facts and recommendations about total homocysteine determinations: an expert opinion.Clin ChemAn international expert group set out how to collect, measure and interpret total homocysteine, including sample handling, reference intervals and target groups for testing.
2005Dose-dependent effects of folic acid on blood concentrations of homocysteine: a meta-analysis of the randomized trials.Am J Clin NutrIn 25 trials, folic acid 0.2, 0.4, 0.8, 2 and 5 mg a day lowered homocysteine by 13%, 20%, 23%, 23% and 25%; B12 added 7% and B6 added nothing significant.
2009The treatment of hyperhomocysteinemia.Annu Rev MedB vitamins lower homocysteine well, but trials in people with mildly raised levels fell short, and severe hyperhomocysteinemia remained untested.
2010Effects of lowering homocysteine levels with B vitamins on cardiovascular disease, cancer, and cause-specific mortality: Meta-analysis of 8 randomized trials involving 37 485 individuals.Arch Intern MedFolic acid lowered homocysteine by 25% on average but did not change major vascular events (RR 1.01), coronary events (1.03), stroke (0.96), cancer or death over 5 years.
2011Effect modification by population dietary folate on the association between MTHFR genotype, homocysteine, and stroke risk: a meta-analysis of genetic studies and randomised trials.LancetThe MTHFR TT genotype raised homocysteine by 3.12 µmol/L and stroke odds by 1.68 times in low-folate Asia but had almost no effect where food is fortified with folic acid.
2012Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias.PLoS MedIn 48,175 coronary cases from unpublished datasets, the MTHFR TT genotype carried no extra heart disease risk (OR 1.02), suggesting earlier positive studies reflected publication bias.
2013ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing.Genet MedThe American College of Medical Genetics and Genomics advised against MTHFR variant testing in routine thrombophilia workups because it has minimal clinical utility.
2014Guidelines for the diagnosis and treatment of cobalamin and folate disorders.Br J HaematolThe British Society for Haematology lists methylmalonic acid as a second-line B12 test and homocysteine as a less specific second-line test.
2016Biomarkers and Algorithms for the Diagnosis of Vitamin B12 Deficiency.Front Mol BiosciB12 deficiency inactivates methionine synthase and methylmalonyl-CoA mutase, so homocysteine and methylmalonic acid rise, and serum B12 alone has limited diagnostic value.
2017Homocysteine-lowering interventions for preventing cardiovascular events.Cochrane Database Syst RevIn 15 trials with 71,422 people, B-vitamin treatment did not change heart attack (RR 1.02) or death (RR 1.01) and slightly lowered stroke (RR 0.90).
2018Homocysteine and Dementia: An International Consensus Statement.J Alzheimers DisAn expert panel concluded that raised homocysteine is a modifiable risk factor for cognitive decline and dementia in older people, with relative risks of 1.15 to 2.5.
2021Homocysteine - from disease biomarker to disease prevention.J Intern MedThe authors list more than 100 conditions linked to raised homocysteine and judge adult values of 10 µmol/L or below probably safe and 11 µmol/L or above worth acting on.
2022Hyperhomocysteinemia.StatPearlsA clinical reference giving a usual range of 5 to 15 µmol/L and grouping raised values as moderate (16 to 30), intermediate (31 to 100) and severe (over 100 µmol/L).

Recent research

Notable work from the last three years.

2023MTHFR and risk of stroke and heart disease in a low-folate population: a prospective study of 156 000 Chinese adults.Int J EpidemiolIn the China Kadoorie Biobank, 21% carried MTHFR TT, which carried 13% higher stroke risk but no higher risk of ischaemic heart disease.
2023Association Between Plasma Homocysteine Level and Mortality: A Mendelian Randomization Study.Korean Circ JMeasured homocysteine predicted death in 10,005 Koreans, but genetically predicted homocysteine did not, which argues against a causal effect on mortality.
2025Efficacy of B Vitamin Supplementation on Global Cognitive Function in Older Adults: A Systematic Review and Meta-analysis.Nutr RevAcross 17 trials in older adults, B vitamins had a very small benefit on global cognition (Hedges g 0.11) once outliers and single-blind studies were removed.
2026Folate and global health review series, part 3: syntheses on cardiovascular, cerebrovascular, and metabolic diseases.J Glob HealthAn umbrella review of 49 reviews found no convincing folate and cardiometabolic association, with suggestive evidence for lower heart disease and stroke at higher folate intake.
2026Efficacy and safety of folic acid on homocysteine and cardiovascular surrogate biomarkers in hyperhomocysteinemia: a systematic review and meta-analysis of RCTs.BMC NutrIn 36 trials of people with hyperhomocysteinemia, folic acid lowered homocysteine dose-dependently, but no trial measured hard cardiovascular outcomes.
2026Season of sampling and the interpretation of serum homocysteine results: a within-person analysis of 268,877 health-screening examinations.Clin Chem Lab MedWithin the same people, homocysteine followed a small seasonal cycle of about 0.3 µmol/L, peaking in early November, enough to shift results near 15 µmol/L.

What well-known voices say about Homocysteine

Their positions, summarized in our words. Follow each link for the full context.

A. David SmithPharmacologist at the University of Oxford and lead author of the VITACOG trial
Smith argues that homocysteine is more than a bystander marker. With Helga Refsum he concludes that adult values of 10 µmol/L or below are probably safe, that 11 or above may justify B-vitamin treatment, and that lowering it can help prevent primary stroke and cognitive impairment in older people.
Smith and Refsum, Homocysteine: from disease biomarker to disease prevention, J Intern Med 2021
Robert ClarkeEpidemiologist at the Clinical Trial Service Unit, University of Oxford
Clarke led the pooled analyses of the B-vitamin trials and of MTHFR genetics. His reading is that lifelong moderate homocysteine elevation has little or no effect on coronary heart disease, and that earlier positive genetic studies reflected publication bias.
Clarke et al., Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, PLoS Med 2012
Michael GregerPhysician and founder of NutritionFacts.org
Greger treats homocysteine as an important stroke risk factor and argues that the higher stroke signal seen in some vegetarians reflects low B12. He recommends that people on plant-based diets take a regular B12 supplement or eat B12-fortified foods.
NutritionFacts.org, What About Homocysteine, Vitamin B12, and Vegetarians' Stroke Risk?

Watch

Talks and explainers worth your time. Nothing loads from YouTube until you press play.

Methionine Metabolism and Activated Methyl Cycle | Pathway and Purpose
JJ MedicineOpen on YouTube
A clear whiteboard walk through methionine, SAM and homocysteine that matches the biochemistry section on this page.
One Carbon Metabolism | Tetrahydrofolate and the Folate Cycle
JJ MedicineOpen on YouTube
Explains the folate cycle that supplies the methyl group for recycling homocysteine, including where MTHFR fits.
Understanding Vitamin B12 metabolism
Armando HasudunganOpen on YouTube
Shows why low B12 raises both homocysteine and methylmalonic acid, the logic behind second-line B12 testing.
Classical Homocystinuria (NORD)
Osmosis from ElsevierOpen on YouTube
Covers the rare inherited disease where homocysteine runs far above 100 µmol/L, the origin of the whole homocysteine story.
Friday Favorites: How to Test for Functional Vitamin B12 Deficiency
NutritionFacts.orgOpen on YouTube
A plain-language look at using MMA and homocysteine to judge B12 status beyond a serum B12 number.

Protocols that act on Homocysteine

Common questions

What is homocysteine in simple terms?

It is a short-lived amino acid made when your body uses methionine from protein. Folate, B12 and B6 clear it, so a high level usually means one of those pathways is slow [1].

What is the meaning of homocysteine levels on a blood test?

The number shows how much total homocysteine is in your plasma. Most adults have 5 to 15 µmol/L; above 15 is called hyperhomocysteinemia [1].

Are homocysteine levels important?

They are useful for checking B12 and folate function and for finding rare inherited disorders. As a heart risk target they matter less, because lowering them did not reduce heart attacks in trials [6].

What are the homocysteine guidelines?

No major cardiology guideline recommends routine homocysteine screening. Expert groups describe how to measure it [10], haematology guidance uses it as a second-line B12 test [55], and a dementia consensus calls raised levels a modifiable risk in older people [12].

Is homocysteine linked to blood pressure?

Yes, in cohorts homocysteine is higher in people with higher blood pressure [46]. In CSPPT, adding folic acid to blood pressure treatment lowered first strokes by 21% [7].

What is the difference between homocysteine and cysteine?

Homocysteine has one more carbon in its side chain than cysteine. With vitamin B6, the body converts homocysteine into cysteine, which it uses to build proteins and glutathione [1].

How does homocysteine turn back into methionine?

Methionine synthase adds a methyl group from 5-methyltetrahydrofolate, using B12 as a cofactor; betaine offers a second route in the liver [2][17].

Is homocysteine related to autism?

Studies have looked at homocysteine and methylation in autism, but this page does not review that evidence, and a single homocysteine test is not a diagnostic tool for autism.

Is there an at-home homocysteine test?

Some services offer finger-prick or mail-in kits, but homocysteine rises if blood sits unseparated at room temperature [38], so ask how the kit handles the sample.

Sources

1.Son P, Lewis L. Hyperhomocysteinemia. StatPearls, 2022. PubMed 32119295
2.Hannibal L, Lysne V, Bjørke-Monsen AL, et al. Biomarkers and Algorithms for the Diagnosis of Vitamin B12 Deficiency. Front Mol Biosci, 2016. doi:10.3389/fmolb.2016.00027
3.Smith AD, Refsum H. Homocysteine - from disease biomarker to disease prevention. J Intern Med, 2021. doi:10.1111/joim.13279
4.Homocysteine Studies Collaboration. Homocysteine and risk of ischemic heart disease and stroke: a meta-analysis. JAMA, 2002. doi:10.1001/jama.288.16.2015
5.Clarke R, Halsey J, Lewington S, et al. Effects of lowering homocysteine levels with B vitamins on cardiovascular disease, cancer, and cause-specific mortality: Meta-analysis of 8 randomized trials involving 37 485 individuals. Arch Intern Med, 2010. doi:10.1001/archinternmed.2010.348
6.Martí-Carvajal AJ, Solà I, Lathyris D, et al. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane Database Syst Rev, 2017. doi:10.1002/14651858.CD006612.pub5
7.Huo Y, Li J, Qin X, et al. Efficacy of folic acid therapy in primary prevention of stroke among adults with hypertension in China: the CSPPT randomized clinical trial. JAMA, 2015. doi:10.1001/jama.2015.2274
8.Smith AD, Smith SM, de Jager CA, et al. Homocysteine-lowering by B vitamins slows the rate of accelerated brain atrophy in mild cognitive impairment: a randomized controlled trial. PLoS One, 2010. doi:10.1371/journal.pone.0012244
9.Homocysteine Lowering Trialists' Collaboration. Dose-dependent effects of folic acid on blood concentrations of homocysteine: a meta-analysis of the randomized trials. Am J Clin Nutr, 2005. doi:10.1093/ajcn/82.4.806
10.Refsum H, Smith AD, Ueland PM, et al. Facts and recommendations about total homocysteine determinations: an expert opinion. Clin Chem, 2004. doi:10.1373/clinchem.2003.021634
11.Guttormsen AB, Schneede J, Fiskerstrand T, et al. Plasma concentrations of homocysteine and other aminothiol compounds are related to food intake in healthy human subjects. J Nutr, 1994. doi:10.1093/jn/124.10.1934
12.Smith AD, Refsum H, Bottiglieri T, et al. Homocysteine and Dementia: An International Consensus Statement. J Alzheimers Dis, 2018. doi:10.3233/JAD-171042
13.Carson NA, Neill DW. Metabolic abnormalities detected in a survey of mentally backward individuals in Northern Ireland. Arch Dis Child, 1962. doi:10.1136/adc.37.195.505
14.Kang SS, Wong PW, Malinow MR. Hyperhomocyst(e)inemia as a risk factor for occlusive vascular disease. Annu Rev Nutr, 1992. doi:10.1146/annurev.nu.12.070192.001431
15.Frosst P, Blom HJ, Milos R, et al. A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase. Nat Genet, 1995. doi:10.1038/ng0595-111
16.McNulty H, Dowey le RC, Strain JJ, et al. Riboflavin lowers homocysteine in individuals homozygous for the MTHFR 677C->T polymorphism. Circulation, 2006. doi:10.1161/CIRCULATIONAHA.105.580332
17.Steenge GR, Verhoef P, Katan MB. Betaine supplementation lowers plasma homocysteine in healthy men and women. J Nutr, 2003. doi:10.1093/jn/133.5.1291
18.Seshadri S, Beiser A, Selhub J, et al. Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med, 2002. doi:10.1056/NEJMoa011613
19.van Meurs JB, Dhonukshe-Rutten RA, Pluijm SM, et al. Homocysteine levels and the risk of osteoporotic fracture. N Engl J Med, 2004. doi:10.1056/NEJMoa032546
20.McLean RR, Jacques PF, Selhub J, et al. Homocysteine as a predictive factor for hip fracture in older persons. N Engl J Med, 2004. doi:10.1056/NEJMoa032739
21.Wald DS, Law M, Morris JK. Homocysteine and cardiovascular disease: evidence on causality from a meta-analysis. BMJ, 2002. doi:10.1136/bmj.325.7374.1202
22.Clarke R, Bennett DA, Parish S, et al. Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias. PLoS Med, 2012. doi:10.1371/journal.pmed.1001177
23.Holmes MV, Newcombe P, Hubacek JA, et al. Effect modification by population dietary folate on the association between MTHFR genotype, homocysteine, and stroke risk: a meta-analysis of genetic studies and randomised trials. Lancet, 2011. doi:10.1016/S0140-6736(11)60872-6
24.Bennett DA, Parish S, Millwood IY, et al. MTHFR and risk of stroke and heart disease in a low-folate population: a prospective study of 156 000 Chinese adults. Int J Epidemiol, 2023. doi:10.1093/ije/dyad147
25.Larsson SC, Traylor M, Markus HS. Homocysteine and small vessel stroke: A mendelian randomization analysis. Ann Neurol, 2019. doi:10.1002/ana.25440
26.Toole JF, Malinow MR, Chambless LE, et al. Lowering homocysteine in patients with ischemic stroke to prevent recurrent stroke, myocardial infarction, and death: the Vitamin Intervention for Stroke Prevention (VISP) randomized controlled trial. JAMA, 2004. doi:10.1001/jama.291.5.565
27.Lonn E, Yusuf S, Arnold MJ, et al. Homocysteine lowering with folic acid and B vitamins in vascular disease. N Engl J Med, 2006. doi:10.1056/NEJMoa060900
28.Bønaa KH, Njølstad I, Ueland PM, et al. Homocysteine lowering and cardiovascular events after acute myocardial infarction. N Engl J Med, 2006. doi:10.1056/NEJMoa055227
29.Armitage JM, Bowman L, Clarke RJ, et al. Effects of homocysteine-lowering with folic acid plus vitamin B12 vs placebo on mortality and major morbidity in myocardial infarction survivors: a randomized trial. JAMA, 2010. doi:10.1001/jama.2010.840
30.VITATOPS Trial Study Group. B vitamins in patients with recent transient ischaemic attack or stroke in the VITAmins TO Prevent Stroke (VITATOPS) trial: a randomised, double-blind, parallel, placebo-controlled trial. Lancet Neurol, 2010. doi:10.1016/S1474-4422(10)70187-3
31.Douaud G, Refsum H, de Jager CA, et al. Preventing Alzheimer's disease-related gray matter atrophy by B-vitamin treatment. Proc Natl Acad Sci U S A, 2013. doi:10.1073/pnas.1301816110
32.Maron BA, Loscalzo J. The treatment of hyperhomocysteinemia. Annu Rev Med, 2009. doi:10.1146/annurev.med.60.041807.123308
33.Levy J, Rodriguez-Guéant RM, Oussalah A, et al. Cardiovascular manifestations of intermediate and major hyperhomocysteinemia due to vitamin B12 and folate deficiency and/or inherited disorders of one-carbon metabolism: a 3.5-year retrospective cross-sectional study of consecutive patients. Am J Clin Nutr, 2021. doi:10.1093/ajcn/nqaa432
34.Labcorp. Homocyst(e)ine, Plasma (test 706994). Labcorp test menu, 2026. Link
35.ARUP Laboratories. Homocysteine, Total (test 0099869). ARUP Laboratory Test Directory, 2026. Link
36.Rasmussen K, Moller J, Lyngbak M. Within-person variation of plasma homocysteine and effects of posture and tourniquet application. Clin Chem, 1999. doi:10.1093/clinchem/45.10.1850
37.Kim SH, Chae JH, Chae CH. Season of sampling and the interpretation of serum homocysteine results: a within-person analysis of 268,877 health-screening examinations. Clin Chem Lab Med, 2026. doi:10.1515/cclm-2026-1304
38.Korzun WJ, Ho QV. Stability of plasma total homocysteine concentrations in EDTA-whole blood kept on ice. Clin Lab Sci, 2000. PubMed 11586504
39.Bae JB, Han JW, Song J, et al. Hypohomocysteinemia may increases the risk of dementia and Alzheimer's disease: A nationwide population-based prospective cohort study. Clin Nutr, 2021. doi:10.1016/j.clnu.2021.05.034
40.Obersby D, Chappell DC, Dunnett A, et al. Plasma total homocysteine status of vegetarians compared with omnivores: a systematic review and meta-analysis. Br J Nutr, 2013. doi:10.1017/S000711451200520X
41.Jacques PF, Selhub J, Bostom AG, et al. The effect of folic acid fortification on plasma folate and total homocysteine concentrations. N Engl J Med, 1999. doi:10.1056/NEJM199905133401901
42.Refsum H, Nurk E, Smith AD, et al. The Hordaland Homocysteine Study: a community-based study of homocysteine, its determinants, and associations with disease. J Nutr, 2006. doi:10.1093/jn/136.6.1731S
43.Zhang SF, Li LZ, Zhang W, et al. Association Between Plasma Homocysteine Levels and Subclinical Hypothyroidism in Adult Subjects: A Meta-Analysis. Horm Metab Res, 2020. doi:10.1055/a-1199-2633
44.de Jager J, Kooy A, Lehert P, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ, 2010. doi:10.1136/bmj.c2181
45.Lerman TT, Cohen E, Sochat T, et al. Proton pump inhibitor use and its effect on vitamin B12 and homocysteine levels among men and women: A large cross-sectional study. Am J Med Sci, 2022. doi:10.1016/j.amjms.2022.07.006
46.Nygård O, Vollset SE, Refsum H, et al. Total plasma homocysteine and cardiovascular risk profile. The Hordaland Homocysteine Study. JAMA, 1995. doi:10.1001/jama.1995.03530190040032
47.Urgert R, van Vliet T, Zock PL, et al. Heavy coffee consumption and plasma homocysteine: a randomized controlled trial in healthy volunteers. Am J Clin Nutr, 2000. doi:10.1093/ajcn/72.5.1107
48.Homocysteine Lowering Trialists' Collaboration. Lowering blood homocysteine with folic acid based supplements: meta-analysis of randomised trials. Homocysteine Lowering Trialists' Collaboration. BMJ, 1998. PubMed 9569395
49.McRae MP. Betaine supplementation decreases plasma homocysteine in healthy adult participants: a meta-analysis. J Chiropr Med, 2013. doi:10.1016/j.jcm.2012.11.001
50.Appel LJ, Miller ER 3rd, Jee SH, et al. Effect of dietary patterns on serum homocysteine: results of a randomized, controlled feeding study. Circulation, 2000. doi:10.1161/01.cir.102.8.852
51.Choi CK, Kweon SS, Lee YH, et al. Association Between Plasma Homocysteine Level and Mortality: A Mendelian Randomization Study. Korean Circ J, 2023. doi:10.4070/kcj.2023.0089
52.Berg J, Grant R, Siervo M, et al. Efficacy of B Vitamin Supplementation on Global Cognitive Function in Older Adults: A Systematic Review and Meta-analysis. Nutr Rev, 2025. doi:10.1093/nutrit/nuaf155
53.Hickey SE, Curry CJ, Toriello HV. ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genet Med, 2013. doi:10.1038/gim.2012.165
54.Yoo S, Montazeri A, Bennett D, et al. Folate and global health review series, part 3: syntheses on cardiovascular, cerebrovascular, and metabolic diseases. J Glob Health, 2026. doi:10.7189/jogh.16.04160
55.Devalia V, Hamilton MS, Molloy AM. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol, 2014. doi:10.1111/bjh.12959
56.McCully KS. Vascular pathology of homocysteinemia: implications for the pathogenesis of arteriosclerosis. Am J Pathol, 1969. PubMed 5792556
57.Zighetti ML, Chantarangkul V, Lombardi R, et al. Effects of some pre-analytical conditions on the measurement of homocysteine and cysteine in plasma. Clin Chem Lab Med, 2004. doi:10.1515/CCLM.2004.037
58.Long ZP, Wang CW, Sun FF, et al. Efficacy and safety of folic acid on homocysteine and cardiovascular surrogate biomarkers in hyperhomocysteinemia: a systematic review and meta-analysis of RCTs. BMC Nutr, 2026. doi:10.1186/s40795-026-01343-y
59.Cullen CE, Carter GT, Weiss MD, et al. Hypohomocysteinemia: a potentially treatable cause of peripheral neuropathology?. Phys Med Rehabil Clin N Am, 2012. doi:10.1016/j.pmr.2011.11.001