Vitamin B6 is a cheap, essential nutrient. Correcting a true shortfall — common in smokers, older adults and women on hormonal contraceptives — reverses skin, blood and nerve problems; it also eases pregnancy nausea. Beyond that, evidence is stronger on the harms of sustained high intake than on any gain from adding more. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Plasma pyridoxal 5'-phosphate | 40–80 nmol/L | Direct measure of active vitamin B6 status |
| Plasma total homocysteine | Below 9 µmol/L | Functional indicator of one-carbon metabolism |
| 4-Pyridoxic acid to pyridoxal 5'-phosphate ratio | Below approximately 2 | Marks accelerated vitamin B6 turnover |
| High-sensitivity C-reactive protein | Below 1.0 mg/L | Determines whether a low vitamin B6 result is real |
| Complete blood count with mean corpuscular volume | Mean corpuscular volume 82–89 fL | Detects the microcytic, sideroblastic anaemia of deficiency |
| Serum alkaline phosphatase | 50–90 U/L | Determines how to read pyridoxal 5'-phosphate |
| Serum vitamin B12 and folate | Vitamin B12 above 500 pg/mL; folate above 10 ng/mL | Co-factors that must be adequate before judging vitamin B6 |
Cadence: Pyridoxal 5'-phosphate and homocysteine retested at 8–12 weeks, then every 6–12 months on maintenance intake; every 3–6 months above 50 mg/day, with the sensory examination repeated at each point.