Vitamin B6 (Pyridoxine)
B-vitamin for amino acid metabolism, neurotransmitter production, and immune function
Also known as
Definition
Vitamin B6 (an umbrella term for pyridoxine, pyridoxal, and pyridoxamine) is a water-soluble B-vitamin whose active form pyridoxal-5-phosphate (PLP) serves as a cofactor for more than 100 enzymes in amino acid metabolism. PLP is essential for the synthesis of neurotransmitters (serotonin, dopamine, GABA) and hemoglobin, as well as for homocysteine conversion. High-dose supplementation can cause peripheral neuropathy.
| Parameter | Value |
|---|---|
| Unit | nmol/l (µg/l) |
| Reference Range | Plasma PLP: 20–150 nmol/l (~5–37 µg/l) |
| Optimal Range (lab2go) | > 40 nmol/l plasma PLP (lab2go Optimum Zone; optimal enzyme saturation and neurotransmitter synthesis) |
Plasma PLP: 1 nmol/l = 0.247 µg/l. Reference ranges usually given as plasma PLP; US: 5–50 µg/l.
↓ What a low value means
B6 deficiency occurs with inadequate intake, chronic alcoholism, autoimmune disease, B6 antagonist medications (isoniazid, penicillamine), or increased demand (pregnancy). Symptoms: seborrheic dermatitis, glossitis, microcytic anemia, peripheral neuropathy, depression, elevated homocysteine, weakened immune defense.
↑ What a high value means
Chronically high B6 doses (> 200 mg/day for months) can cause sensory peripheral neuropathy with numbness and tingling in hands and feet (paradoxical toxicity). EFSA sets the tolerable upper intake level at 25 mg/day for adults.
✓ How to optimize this marker
Dietary sources: poultry, tuna, salmon, potatoes, chickpeas, bananas, nuts. Supplementation: 1.5–2 mg/day (daily requirement); therapeutically up to 25–50 mg/day. Pyridoxal-5-phosphate (P5P) is the active form and does not require hepatic activation — advantageous with liver problems.
When to test
With neuropathy, depression, elevated homocysteine, PMS symptoms, chronic alcoholism, or certain medications (isoniazid, theophylline). Plasma PLP is the preferred marker over total B6.
Frequently asked questions
Does vitamin B6 help with PMS symptoms? +
Several studies show that B6 (50–100 mg/day) can reduce PMS symptoms such as mood swings, irritability, and anxiety, as B6 is required for serotonin and GABA synthesis. Evidence is moderate; high doses should only be used short-term due to neuropathy risk.
What is the difference between pyridoxine and pyridoxal-5-phosphate (P5P)? +
Pyridoxine (pyridoxine HCl) must be converted in the liver to P5P (the active form). P5P is immediately bioavailable and preferred with liver disease, poor conversion, or for direct effects on neurotransmitter synthesis. P5P is more expensive but better suited for sensitive users.
Can vitamin B6 cause neuropathy? +
Paradoxically yes: although B6 deficiency causes neuropathy, chronic overdosing (> 100–200 mg/day for months) can trigger sensory neuropathy. Symptoms are usually reversible after stopping. EFSA recommends a maximum tolerable intake of 25 mg/day for adults.
What is the relationship between B6 and homocysteine? +
B6 is the cofactor of cystathionine beta-synthase, which converts homocysteine via the transsulfuration pathway to cystathionine. Together with B12 and folate, B6 forms the triad of B-vitamins regulating homocysteine. Deficiency in any one of the three can cause elevated homocysteine.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.