Homocysteine: Levels, Causes and What the Research Really Shows

What is homocysteine?

Where does homocysteine come from?

Homocysteine is a sulphur-containing amino acid – one of the small building blocks of proteins, here carrying a sulphur atom. The body makes it continuously from methionine, an essential amino acid supplied by protein-rich foods (meat, fish, eggs, dairy). It is therefore a normal intermediate of metabolism: not “toxic” in itself, and everyone has some in their blood, more precisely in the plasma^[1]. Laboratories measure total homocysteine (abbreviated Hcy); a high homocysteine usually causes no obvious symptoms and is only found on a blood test.

What are vitamins B9, B12 and B6 for?

They are the cogs that recycle homocysteine. Normally the body transforms it straight away in two directions: either it recycles it back into methionine – with the help of folate (vitamin B9) and vitamin B12, which transfer a small “methyl group” to it through an enzyme called methionine synthase – or it removes it by converting it into cysteine, with the help of vitamin B6^[1]. The first route feeds the methylation cycle – and thus the formation of SAM (S-adenosylmethionine), the body’s main “methyl-group donor” – a basic mechanism in every cell. When these vitamins are lacking, recycling works poorly and homocysteine builds up. This is precisely why “folate and vitamin B12 contribute to normal homocysteine metabolism” – the only wording recognised by Swiss and European regulations.

Why is there so much talk about it?

Because an elevated level was long presented as a cause of heart disease. We now know it is mainly a marker: it accompanies other situations (vitamin deficiency, ageing, kidney problems) more than it directly causes them^[1]. This is the point most articles summarise poorly, and we come back to it below.

Normal and high levels: how to read a result

What is a “normal” level?

In adults, a homocysteine level between 5 and 15 µmol/L (measured fasting) is most often considered normal. Above that, we speak of hyperhomocysteinaemia. These reference ranges vary slightly from one laboratory to another: the range printed on your report always takes precedence, and its interpretation is up to the doctor.

What do “mild”, “intermediate” or “severe” mean?

Above the normal range, hyperhomocysteinaemia is divided into three levels. The higher the value, the more likely a specific medical cause is involved.

Should you have your homocysteine measured?

Not as a routine. In practice it is a simple blood test, with a fasting sample – but it is not a systematic screening test. A doctor orders it in specific cases: to complement the search for a vitamin B12 or folate deficiency, after a stroke that occurs young or with no clear cause, or to investigate rare metabolic disorders. In a healthy person, measuring it “just to see” provides little useful information.

Why do homocysteine levels rise?

A high level almost always has an explanation – often at the crossroads of genes and environment, that is, mostly diet. The causes fall into a few families, from the most ordinary to the rarest.

B-vitamin deficiency (the most common cause)

This is by far the first lead. When the intake of folate (B9), vitamin B12 or, more rarely, B6 is insufficient, the recycling of homocysteine slows and the level rises^[1]. A marked B12 or folate deficiency can, moreover, also cause anaemia (with tiredness) and, for B12, neurological problems. The link works both ways: this is why a homocysteine test can also help the doctor detect an otherwise unnoticed vitamin B12 or folate deficiency^[11].

The MTHFR gene

MTHFR (for methylenetetrahydrofolate reductase) is a gene that makes a key enzyme in folate recycling. A very common variant – a so-called mutation, called 677TT – makes this enzyme slightly less effective: carriers tend to have higher homocysteine, especially when their folate intake is low^[3]. Good news: it is a common feature and, most of the time, without practical consequences.

Kidneys, thyroid, age and lifestyle

Other factors come into play. The kidneys clear homocysteine: kidney failure makes it rise. A sluggish thyroid (hypothyroidism) does too. Add to these age, male sex, certain medicines (methotrexate, some antiepileptics) and habits such as smoking and heavy coffee consumption^[1].

Homocysteine, heart and brain: what the research really shows

This is the heart of the matter – and the part most poorly summarised elsewhere. Two very different questions must be separated: “is a high level associated with more disease?” (yes, modestly) and “does lowering it protect?” (much less than was hoped). Homocysteine is not a disease, and no supplement treats anything.

Is a high level bad for the heart?

It is associated with a slightly higher risk. Large analyses across dozens of studies showed that lower homocysteine levels go hand in hand with better cardiovascular health (less cardiovascular disease) – but the effect is modest: about a quarter less homocysteine corresponds to roughly 11% less coronary risk (coronary heart disease) and 19% less stroke, mostly in people already at risk^[2]. And “associated” does not mean “caused”: the link may reflect other factors^[1]. Homocysteine is often called a risk factor, but the proposed mechanisms for possible damage to the blood vessels (atherosclerosis) – weakening of their wall, oxidative stress, reactive forms such as homocysteine thiolactone – remain hypothetical^[1].

Does lowering homocysteine protect the heart?

This is where the surprise comes. When the idea was actually put to the test – giving B vitamins to lower homocysteine and following participants for years – the expected benefit for the heart did not materialise. The large Cochrane review, pooling 15 trials and over 71’000 participants, finds no reduction in heart attacks (myocardial infarction) or mortality^[6]. Older analyses already pointed in the same direction^[7]. In other words: lower the number, yes; prevent heart attacks, no.

What about stroke?

This is the only area where a benefit emerges, and it remains modest. The same Cochrane review observes a small reduction in strokes^[6]. Above all, the benefit is clearer where the population’s folate intake is low: a large Chinese trial showed that adding folic acid to the treatment of adults with high blood pressure cut the risk of a first stroke by about a fifth^[8], and analyses confirm a more marked effect in countries without folic-acid-fortified flour^[9].

What about the brain and memory?

The signal is interesting, but far from proof. In older people with mild memory decline and high homocysteine, one trial showed that high doses of B vitamins slowed the natural shrinking of the brain (from 1.1% to 0.8% a year), the more so the higher the starting level^[10][11]. This does not mean B vitamins “prevent dementia” or cure Alzheimer’s disease or other neurodegenerative diseases: it is a research lead, to be confirmed, in very specific people.

What the research shows, question by question

How to lower your homocysteine level?

If your doctor has found a high level, the good news is that it almost always corrects simply and naturally – often through diet and a few lifestyle changes. The goal is not the number itself, but the possible deficiency or the cause it reveals.

B vitamins, the most effective lever

Folic acid (B9) is by far the most effective. A large analysis of clinical trials showed that a daily dose of about 0.8 mg of folic acid lowers homocysteine by about a quarter; adding vitamin B12 lowers it a little more, while vitamin B6 alone has no clear effect on the number^[5]. The effect appears within a few weeks – not a few days.

What should you eat?

Focus on folate and B12, not on a “miracle” food. Folate (B9) is abundant in green leafy vegetables (spinach, broccoli), pulses (lentils, chickpeas), wholegrains, citrus fruit and nuts. Vitamin B12 comes from animal products (meat, fish, eggs, dairy). Cutting down on smoking and excess coffee also helps, since these are factors that push the level up^[1]. No plant and no single food “melts away” homocysteine on its own.

Is a supplement useful?

Only if it fills a real deficiency. In a person who eats a balanced diet and has no deficiency, taking B vitamins to “lower” an already normal level brings no proven health benefit^[6]. If a supplement is indicated, it is the B9, B12 and B6 vitamins that are involved – and vitamin B12 in particular deserves attention in older people or those on a diet low in animal products. Ask your doctor or pharmacist for advice, especially if you are on any treatment.

When should you see a doctor?

A level slightly above normal is usually nothing to worry about and corrects with diet; often it is enough to monitor the levels over time. Some situations do call for medical advice, though:

• !An intermediate or severe level (above 30 µmol/L): a medical cause must be sought – see a doctor.
• !On treatment (methotrexate, antiepileptics, etc.): do not add any supplement without discussing it with your doctor.
• !Pregnancy or planning a pregnancy: folate needs are specific – folic acid is officially recommended before and in early pregnancy, in particular to help prevent neural tube defects (problems with the neural tube closure of the embryo). Follow medical advice.
• !A known kidney or thyroid disease: here it is the underlying cause that needs treating.

Frequently asked questions