The blood marker I never skip: homocysteine

A few years ago, my homocysteine was 4 µmol/L. I was told it was fine.

At the time, I was genuinely unwell — hives breaking out across my face, bad gut dysbiosis, reacting to everything, vomiting intermittently and sweating heavily at night. I later discovered I was living in a mould-affected environment.

Looking back, I can't say that a homocysteine of 4 told me exactly what was going on. But I can say it was a result that deserved more attention than simply being labelled "fine."

We hear a lot about high homocysteine

Most of us have heard that high homocysteine isn't ideal.

When homocysteine is running high, it can contribute to damage and inflammation inside the blood vessels. Every 5 µmol/L increase in homocysteine has been linked to a 22% higher risk of coronary heart disease [1]. It can also be influenced by diet, nutrient status, genetics and certain medications.

MTHFR variants are one of the most studied genetic contributors to elevated homocysteine. Reduced MTHFR enzyme activity can impair the conversion of folate into 5-MTHF, which can reduce the efficiency of homocysteine remethylation and contribute to higher homocysteine levels. [5]

So yes — high absolutely matters. But what about low?

This is the part I think gets overlooked

Homocysteine sits at the intersection of three systems most people never think to connect — methylation, detoxification and antioxidant defence. It's one number that gives you a window into how these pathways are working together.

Homocysteine can be recycled back towards methionine through the remethylation pathway, or directed through the transsulfuration pathway towards cysteine and ultimately glutathione. [2]

This is where the picture gets more interesting. Homocysteine isn't simply a case of "the lower, the better." There are good reasons to look at what might be driving a very low result, rather than automatically assuming it's a positive finding. [3]

So when I see low homocysteine, I'm asking why.

What can drive it lower?

• High oxidative or environmental burden — Mould exposure, environmental toxins, chronic inflammation or other stressors can increase demand for glutathione and may influence how homocysteine is being used through the transsulfuration pathway. [4]
• Methylation support — Particularly when methylfolate or other methyl donors are being used without looking at the full picture. More isn't always better, and excessive support can potentially push homocysteine lower.
• Low protein or methionine intake — Simple but important. Inadequate protein intake means less methionine entering the pathway in the first place.

This is why I'll often look at homocysteine alongside relevant methylation-related SNPs, nutritional markers and the wider metabolic picture.

Homocysteine and ageing

Homocysteine also tends to increase as we age.

That makes it a useful marker to keep an eye on as part of a longevity-focused bloodwork panel. I'm interested not just in where someone's homocysteine sits today, but how it changes over time alongside their nutritional status, cardiovascular health and overall metabolic picture.

What I actually look for

I like to see homocysteine sitting around 7–9 µmol/L.

Not pushed as low as possible. Not climbing unchecked either.

For me, that's the Goldilocks zone — a range that suggests a good balance between the pathways using homocysteine.

A number outside that range — high or low — isn't a verdict on its own. It needs to be interpreted in context: what someone is eating, what they're taking, their genetics, their symptoms and what's actually happening in their life and environment.

A low number doesn't tell me the answer on its own. But it tells me I need to look further.

The takeaway

Homocysteine isn't a number to glance at and dismiss just because it sits "within range."

High or low, it can give you valuable insight into what's happening across methylation, antioxidant defence and the wider metabolic picture — which is exactly why I never skip it, and exactly why mine being "fine" at 4 was anything but.

If it's not already part of your bloodwork, it's worth asking about. Every client in our Foundation Program has it tested — because it's the one marker I never skip.

References

1. Wang, B., Mo, X., Wu, Z., & Guan, X. (2022). Systematic review and meta-analysis of the correlation between plasma homocysteine levels and coronary heart disease. Journal of Thoracic Disease, 14(3), 646–653. https://doi.org/10.21037/jtd-22-78
2. McCaddon, A., & Miller, J. W. (2023). Homocysteine—a retrospective and prospective appraisal. Frontiers in Nutrition, 10, 1179807. https://doi.org/10.3389/fnut.2023.1179807
3. Bae, J. B., Han, J. W., Song, J., et al. (2021). Hypohomocysteinemia may increase the risk of dementia and Alzheimer's disease: A nationwide population-based prospective cohort study. Clinical Nutrition, 40(7), 4579–4584. https://doi.org/10.1016/j.clnu.2021.05.034
4. Sbodio, J. I., Snyder, S. H., & Paul, B. D. (2019). Regulators of the transsulfuration pathway. British Journal of Pharmacology, 176(4), 583–593. https://doi.org/10.1111/bph.14446
5. Lyon, P., Strippoli, V., Fang, B., & Cimmino, L. (2020). B vitamins and one-carbon metabolism: Implications in human health and disease. Nutrients, 12(9), 2867. https://doi.org/10.3390/nu12092867

Clinical disclaimer: This information is for general education and is not a substitute for individual medical advice, diagnosis or treatment. It should always be interpreted in the context of your individual symptoms, health history and other clinical findings with your qualified healthcare practitioner.