Homocysteine

What it actually measures

Most blood markers measure a thing. Homocysteine measures a traffic jam.

Here is how it works. Your body constantly runs a process called methylation, which is the business of attaching small chemical tags to things in order to switch them on or off. It builds neurotransmitters this way. It maintains DNA this way. It processes hormones this way. It is running in you right now, billions of times a second.

In the middle of that cycle sits homocysteine. It is an intermediate, a halfway product, and the body has two routes to clear it. One recycles it back into methionine, and that route needs vitamin B12 and folate. The other converts it onward toward cysteine, and that route needs vitamin B6.

So when homocysteine builds up in your blood, it usually is not because you are making too much. It is because the exits are short of what they need to work.

That is what makes this marker unusual and genuinely useful. A serum B12 tells you how much B12 is floating in your blood. Homocysteine tells you whether the pathway that depends on B12 is actually running. Those are different questions, and the second one is closer to what you actually wanted to know.

Normal and optimal are different questions

Your lab probably draws its line somewhere between 12 and 15 micromoles per litre. Practitioners who use this marker functionally aim for under 8 or 9.

That gap is not arbitrary and it is not just wellness enthusiasm either. The reference range is built from a population, and the population in question includes a great many people whose B vitamin intake is unremarkable rather than good. So the upper end of normal is describing what is common, not what is well supplied.

The functional argument runs like this: homocysteine is a checkpoint, and a checkpoint sitting high means a cofactor is short. A cofactor being short is worth knowing about whether or not the checkpoint value itself causes any harm.

That is the reasoning I find defensible, and I want to be precise about what it does and does not claim, because the next section is where most writing on this marker goes further than it should.

What lowering it does, and what it does not

Alright. This is the part where I have to tell you something that reframes the marker, so listen close.

For years homocysteine was one of the great hopes in cardiovascular prevention. The observational evidence was consistent: people with higher homocysteine had more cardiovascular disease. The mechanism was plausible. B vitamins lower homocysteine cheaply and safely. So a whole generation of large randomised trials went out to test whether lowering it lowered events.

In 2017 Cochrane pooled that work. Fifteen randomised controlled trials, 71,422 participants, follow-up running from one year to more than seven, nine of the fifteen trials at low risk of bias.

The result: no difference against placebo for myocardial infarction, 7.1 percent versus 6.0 percent. No difference for death from any cause, 11.7 percent versus 12.3 percent. Both rated high-quality evidence.

There was one positive finding, and it deserves reporting as carefully as the negative ones. Stroke came out lower on homocysteine-lowering treatment, 4.3 percent versus 5.1 percent, a relative risk of 0.90, also high-quality evidence. Real, small, and consistent.

So what do you do with that? Two things, and they point in opposite directions from where most people land.

First, stop treating a homocysteine number as a heart attack lever. It is not one. The trials that would have shown that were run properly and at scale, and they did not show it.

Second, do not throw the marker away, because the trials answered a question about treatment and not a question about diagnosis. The trials gave B vitamins to large mixed populations, most of whom were not deficient, and asked whether events fell. That is not the same as measuring homocysteine in one person to find out whether their methylation pathway has what it needs.

The number is still telling you something true about your B vitamin status. It was just never the cardiovascular lever it was sold as, and the honest thing is to say so on the page rather than quietly leave the impression standing.

What moves it

Down: correcting a genuine shortfall in B12, folate or B6, which is the main lever. Riboflavin and betaine, which feed the same pathway. Treating hypothyroidism, since thyroid function affects it. Reducing heavy alcohol intake and stopping smoking. Adequate folate intake also blunts the effect of the common MTHFR variants.

Up: shortfalls in B12, folate or B6. Reduced kidney function, which is one of the strongest non-nutritional drivers and a reason a high value deserves a proper look. Hypothyroidism. Some medications, including metformin and methotrexate. Heavy alcohol, smoking, and increasing age. Two copies of a less efficient MTHFR variant, particularly on a low folate intake.

The honest hedge

I am not a doctor and I am not diagnosing anyone.

This is the marker where I most want you to hold two things at once. A raised homocysteine is genuinely useful information about your B vitamin status, and it is not a number to treat as a target for its own sake. Both of those are true and the second one comes from good trial evidence rather than from caution.

There is a specific practical caution here too, and it is not decorative. High dose folic acid can lower homocysteine while a B12 shortfall continues underneath, and a 2024 review in Food and Nutrition Bulletin worked through the clinical evidence that excess folic acid can exacerbate B12 deficiency and mask it. B12 deficiency left running causes neurological damage that does not always reverse. So the order matters: test B12 and folate, then correct what is actually short, rather than reaching for a high dose B complex because a number looked high.

A markedly raised homocysteine is also a reason to have kidney function checked, which is a doctor conversation rather than a supplement one.

I am not saying your methylation is broken. I am saying it is a variable, a measurable one, and one that almost nobody has ever had ordered.

So. What to actually do.

Tonight, at no cost. Look through your last blood work for the word homocysteine. It is almost certainly not there, which means the functional question has never been asked.

Test it alongside the vitamins rather than instead of them. A B12 and folate panel tells you what is in circulation. Homocysteine tells you whether the pathway depending on them is running. Together they answer a question neither can answer alone.

Draw it fasted. Homocysteine rises after a protein-containing meal, and the sample needs handling reasonably promptly, so use a lab that runs it routinely.

If it comes back high, work the list rather than the number. B12, folate, B6, thyroid, kidney function, alcohol. That is a conversation for a good functional medicine doctor, and it is a far better use of a high result than buying a methylation supplement.

Recheck in about three months. This is one of the more responsive markers, and if it has not moved on adequate B vitamins, that itself is the finding.

None of this has to happen this week. But tonight you can check whether anyone ever measured the pathway rather than just the vitamin.

You are not tired because your body forgot how to work. You are a cycle with cofactors, running billions of times a second, occasionally short of one small thing at one specific step.

Your body is not broken. It is blocked. And sometimes the block is a single missing cofactor at a junction nobody ever measured, quietly backing up a pathway that touches your mood, your DNA and your hormones all at once.

Go find out whether the pathway has what it needs.