Homocysteine

Homocysteine is an amino acid produced as a normal by-product of the body's metabolism. It is cleared with the help of B vitamins — folate, B12 and B6 — so when those run low, homocysteine tends to build up in the blood.

It matters because raised homocysteine is associated with cardiovascular risk and has been studied in relation to cognitive decline. Usefully, it is often modifiable: correcting a genuine B-vitamin shortfall can bring it down.

In practice, homocysteine is read alongside folate and B12 status; whether lowering it with supplements changes hard outcomes is still debated, so it is treated as one informative marker rather than a target to chase blindly.