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Vitamin B6
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In one pass B6 is the body's manager for protein and amino acids: it takes the amino acids you eat apart, remodels them, and puts them back together, and it helps make several neurotransmitters (the molecules that carry signals in the brain, tied to mood and sleep).
Educational content, not medical advice — consult a clinician.
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Chapter 1
A tool for handling amino acids
B6 is the body's manager for protein and amino acids: it takes the amino acids you eat apart, remodels them, and puts them back together, and it helps make several neurotransmitters (the molecules that carry signals in the brain, tied to mood and sleep).
B6 in food comes in several forms: pyridoxine (mostly in plants), and pyridoxal and pyridoxamine (mostly in animal foods). Inside the body, all of them are converted into the coenzyme that does the actual work: pyridoxal phosphate (PLP).
If protein is bricks, B6 (as PLP) is the toolbox that decides how those bricks are taken apart, swapped, and reassembled. Its four most-used tools are:
Moving an amino group to a new home (transamination): this is exactly what the two liver enzymes on a blood panel, and , doCutting off a carbon dioxide (decarboxylation): this is how an amino acid becomes a neurotransmitterTurning into cysteine (transsulfuration)Helping muscle break glycogen down for energy (glycogen phosphorylase)
B6 in food comes in several forms: pyridoxine (mostly in plants), and pyridoxal and pyridoxamine (mostly in animal foods). Inside the body, all of them are converted into the coenzyme that does the actual work: pyridoxal phosphate (PLP).
If protein is bricks, B6 (as PLP) is the toolbox that decides how those bricks are taken apart, swapped, and reassembled. Its four most-used tools are:
Moving an amino group to a new home (transamination): this is exactly what the two liver enzymes on a blood panel, and , doCutting off a carbon dioxide (decarboxylation): this is how an amino acid becomes a neurotransmitterTurning into cysteine (transsulfuration)Helping muscle break glycogen down for energy (glycogen phosphorylase)
Mechanism · Why liver enzyme tests involve B6
The (alanine aminotransferase) and (aspartate aminotransferase) on a blood panel are the most commonly run liver tests, and the reactions they catalyze depend entirely on PLP.ALT catalyzes alanine + α-ketoglutarate ⇌ pyruvate + glutamate. PLP is its cofactor, covalently bound at the enzyme's active site, and ALT sits mainly in the cytoplasm of liver cells.
AST catalyzes aspartate + α-ketoglutarate ⇌ oxaloacetate + glutamate. PLP is its cofactor too, and AST is widespread in the liver, heart, muscle, and red blood cells.
How they are used clinically:
When liver cells are damaged, these enzymes leak into the blood and blood ALT and AST riseALT is more specific to the liver than AST (AST also comes from heart and muscle)The AST/ALT ratio: > 2 suggests alcohol-related liver disease; < 1 is more typical of metabolic dysfunction-associated steatotic liver disease (, formerly called non-alcoholic fatty liver disease) and viral hepatitis
B6 deficiency and liver enzyme tests:
In severe B6 deficiency, the rise in ALT and AST from liver damage may be underestimated, because the enzymes lack their cofactor and are less activeThis is one reason a normal ALT does not fully rule out liver damage, and people who drink heavily over the long term and are also malnourished need particular careThe reference method recommended by the International Federation of Clinical Chemistry (IFCC) adds PLP to the reagent, which avoids the problem, but not every laboratory does this
A few more points:
B6 deficiency itself raises (because transsulfuration slows), especially after a high-protein mealSevere B6 deficiency causes a small-red-cell (microcytic) anemia, because the first enzyme in making heme uses PLP; it does not cause megaloblastic anemia (that is folate and B12)The most-used measure of B6 status is plasma PLP concentration: in adults, > 30 nmol/L is generally considered adequate and < 20 nmol/L suggests a shortfall
Mechanism · How one tool can have four uses
B6's four jobs look unrelated: making amino acids, making neurotransmitters, handling , and breaking down glycogen. Yet they share one coenzyme. The reason is the unusual way PLP works: it does not touch the bond that is going to break. It first grabs the amino acid by its handle and then lets that bond loosen on its own.Step one, latching on. An amino acid carries an amino group (the α-amino group). PLP reaches out with its own aldehyde group and docks onto it, and the two form a covalent intermediate called a Schiff base. The amino acid is no longer a free-floating molecule; it is tethered in the enzyme's active pocket with its orientation locked.
Step two, pulling the electrons over. PLP's ring acts like a sponge that draws electrons toward itself. Of the bonds radiating from the amino acid's central carbon, whichever one faces this sponge is pulled loosest, loose enough to break.
Step three, the enzyme decides which bond breaks. This is the key: the decision does not belong to PLP; it belongs to the enzyme holding PLP. Whatever angle the enzyme holds the amino acid at, that is the bond facing the sponge:
If the bond on the amino-group side breaks, the amino group is lifted off and handed to another carbon skeleton: that is transaminationIf the bond next to the carboxyl group breaks, carbon dioxide leaves: that is decarboxylation, which is how neurotransmitters are madeIf a bond on the side chain breaks, homocysteine's sulfur is moved to a new position: that is transsulfuration
So the phrase one tool, four uses can be made more precise: PLP supplies the same handle and the same sponge, and the enzyme supplies the shape of the tool. This also explains why a B6 shortage rarely slows just one pathway. Several slow at once, because their shared part has been pulled out.
Glycogen phosphorylase uses PLP differently again. There PLP does not act on an amino acid at all. It sits permanently attached to the enzyme as a prosthetic group and uses its own phosphate group to help pass protons along, clipping one glucose off the end of a glycogen chain. Same molecule, different post.
Chapter 2
Neurotransmitter synthesis
PLP is a required link on several of the pathways that make neurotransmitters:
Gamma-aminobutyric acid (): the brain's main braking signal. Glutamate decarboxylase (GAD) turns glutamate into GABA, and that step needs PLPSerotonin: tryptophan first becomes , which then loses a carbon dioxide to become serotonin; that last decarboxylation step needs PLPDopamine and norepinephrine: DOPA decarboxylase (which needs PLP) turns levodopa (L-DOPA) into dopamine, and norepinephrine is then made from dopamine
What needs saying plainly: taking part in making neurotransmitters is not the same as B6 supplements improving mood or anxiety. These pathways are regulated at many levels, and no single nutrient sets neurotransmitter levels, except in a true deficiency.
Gamma-aminobutyric acid (): the brain's main braking signal. Glutamate decarboxylase (GAD) turns glutamate into GABA, and that step needs PLPSerotonin: tryptophan first becomes , which then loses a carbon dioxide to become serotonin; that last decarboxylation step needs PLPDopamine and norepinephrine: DOPA decarboxylase (which needs PLP) turns levodopa (L-DOPA) into dopamine, and norepinephrine is then made from dopamine
What needs saying plainly: taking part in making neurotransmitters is not the same as B6 supplements improving mood or anxiety. These pathways are regulated at many levels, and no single nutrient sets neurotransmitter levels, except in a true deficiency.
Clinical · Drugs that clash with B6
B6 is one of the B vitamins with the most drug interactions in clinical practice: some common prescription drugs quietly use it up, and others are interfered with by B6 in return.Drugs that can leave you short of B6:
Isoniazid (a tuberculosis drug): it binds B6's precursor so that it is excreted, and it blocks B6 activation. Long-term use easily causes peripheral neuropathy (numbness and tingling in the hands and feet). So during TB treatment, people at risk of neuropathy are usually given B6 as well to prevent it (commonly 50 mg a day), which is standard practiceLevodopa (L-DOPA, a Parkinson's disease drug): B6 speeds up the conversion of levodopa to dopamine outside the brain, so less of it reaches the brain and the drug works less well. People taking levodopa on its own should therefore avoid high-dose B6 supplements; the levodopa-carbidopa combination now in common use gets around this problemPenicillamine (used for Wilson's disease): it binds PLPCycloserine: it inhibits enzymes that use PLPHydralazine (a blood pressure drug): it binds PLPOral contraceptives and estrogen therapy: they make tryptophan-processing enzymes more active, which indirectly raises the need for B6 slightly
Drugs that B6 affects in return:
Phenytoin and phenobarbital (antiseizure drugs): in some reports, high-dose B6 sped up their breakdown in the liver and lowered their blood levels, so people with epilepsy should not take high-dose B6 on their own
What to do in practice:
If you take any of these drugs long-term and develop numbness or tingling in the hands or feet with no clear cause, ask your doctor to check your B6 status (plasma PLP)Adding B6 during TB treatment is standard careIf a drug-induced B6 shortage is suspected: a mild dose (10–25 mg/day) is usually enough to correct it, with the exact dose set by a doctor. Avoid > 100 mg/day long-term, which can itself cause sensory neuropathy (the US upper limit, the , is 100 mg)
Chapter 3
Helping muscles release glycogen
The glycogen phosphorylase in muscle is one of B6's less-mentioned but very concrete jobs: PLP is covalently bound to this enzyme as a prosthetic group and is an essential part of its ability to catalyze.
During exercise, muscle has to break glycogen down quickly to make , and that route depends on PLP.
That means B6 links three areas at once:
Protein metabolism (transamination)The nervous system (making neurotransmitters)Exercise metabolism (glycogen breakdown)
So by this mechanism, people who train hard and eat a lot of protein may need slightly more B6, but a varied diet usually covers it and extra supplements are not needed.
During exercise, muscle has to break glycogen down quickly to make , and that route depends on PLP.
That means B6 links three areas at once:
Protein metabolism (transamination)The nervous system (making neurotransmitters)Exercise metabolism (glycogen breakdown)
So by this mechanism, people who train hard and eat a lot of protein may need slightly more B6, but a varied diet usually covers it and extra supplements are not needed.
Myth · Does extra B6 boost athletic performance?
B6 helps muscle mobilize glycogen, so B6 supplements boost endurance is a common inference in the supplement market, but the actual evidence is disappointing.The mechanism is real:
Muscle glycogen phosphorylase does need PLP, as a permanently bound prosthetic groupExercise combined with a high-protein diet speeds B6 turnover, so needs rise slightlySevere B6 deficiency impairs glycogen mobilization and lowers exercise endurance
What the trials show:
Manore's 2000 review: most athletes' plasma PLP is already adequateIn athletes whose B6 is already adequate, extra B6 (10-50 mg/day) has no consistent evidence of raising maximal oxygen uptake (), strength, or enduranceCorrecting a deficiency helps; adding more once you have enough does not. That is the rise-then-plateau curve common in nutrition interventions (and further up, at toxic doses, it turns back down)
The B vitamins that really do affect performance:
B12 and folate: by supporting red blood cell production (the oxygen carriers)B1 (TPP): the entry point of sugar metabolismB2 (FAD): the electron transport chainBut all of these work from deficient to normal, not from normal to extraordinary
In practice:
People with heavy training loads and high-protein diets (> 1.6 g/kg): by the mechanism, B6 needs rise slightly (an extra 0.5-1 mg a day is enough, which an ordinary diet covers)The gaps more common in athletes are iron (in women and endurance sports), vitamin D (for those who train indoors or live at high latitudes), and too little protein; test first, then supplementB6 on its own has no evidence behind it for improving athletic performance
The reverse risk of long-term high-dose B6 in athletes:
There are case reports from bodybuilding and strength-training circles of neuropathy after long-term use of 200+ mg a dayTraining itself makes it easy to mistake the early numbness of neuropathy for post-training soreness, which can delay the diagnosis
Chapter 4
Limited evidence for PMS
B6 is one of the most-studied nutrients for premenstrual syndrome (PMS, the mood and physical symptoms that recur in the days before a period).
The proposed mechanism: PLP takes part in making serotonin and gamma-aminobutyric acid (), hormone swings across the menstrual cycle may affect B6 metabolism, and extra B6 might steady these pathways, but this has not been directly shown.
The evidence: a systematic review by Wyatt and colleagues, published in the BMJ in 1999, pooled 9 (RCTs) and concluded that B6 at up to 100 mg a day (mostly 50–100 mg/day) may improve overall PMS symptoms and mood symptoms. But the trials were generally of low quality, and the authors said the evidence was not strong enough for a confident recommendation.
Safety limits: the adult upper limit () set in 1998 by the US Institute of Medicine (IOM) is 100 mg/day (still used by NIH ODS). It is a ceiling, not a comfortable margin. In 2023 the European Food Safety Authority (EFSA) lowered its upper limit to 12 mg/day. With long-term use > 200 mg/day, there are case reports of sensory neuropathy.
Practical advice: 50–100 mg is already far above the European upper limit. If you want to try it, talk to a doctor first, watch 2–3 cycles to see whether it helps, do not take it long-term, and do not raise the dose on your own.
The proposed mechanism: PLP takes part in making serotonin and gamma-aminobutyric acid (), hormone swings across the menstrual cycle may affect B6 metabolism, and extra B6 might steady these pathways, but this has not been directly shown.
The evidence: a systematic review by Wyatt and colleagues, published in the BMJ in 1999, pooled 9 (RCTs) and concluded that B6 at up to 100 mg a day (mostly 50–100 mg/day) may improve overall PMS symptoms and mood symptoms. But the trials were generally of low quality, and the authors said the evidence was not strong enough for a confident recommendation.
Safety limits: the adult upper limit () set in 1998 by the US Institute of Medicine (IOM) is 100 mg/day (still used by NIH ODS). It is a ceiling, not a comfortable margin. In 2023 the European Food Safety Authority (EFSA) lowered its upper limit to 12 mg/day. With long-term use > 200 mg/day, there are case reports of sensory neuropathy.
Practical advice: 50–100 mg is already far above the European upper limit. If you want to try it, talk to a doctor first, watch 2–3 cycles to see whether it helps, do not take it long-term, and do not raise the dose on your own.
Clinical · B6 for nausea in pregnancy
B6 has one fairly well-established clinical use in obstetrics: treating nausea and vomiting of pregnancy (NVP, often called morning sickness).The American College of Obstetricians and Gynecologists (ACOG) 2018 practice bulletin gives it its highest level of recommendation, based on good and consistent scientific evidence:
First-line treatment for mild to moderate NVP: pyridoxine (B6) 25 mg every 8 hours, plus doxylamine (an antihistamine) 12.5 mg at bedtimeThe brand names Diclectin and Diclegis are combination tablets of B6 and doxylamine. A similar combination went on the US market in 1956 and was later withdrawn amid litigation; later research did not find that it causes birth defects, and the US FDA approved it again in 2013
Mechanism:
Why B6 eases pregnancy nausea is not clearIt looks more like a drug effect at pharmacologic doses than filling a deficiency
Clinical evidence:
Several randomized trials show that 25-75 mg of B6 a day eases mild to moderate NVPVutyavanich 1995 is a classic randomized trial (n=342): 30 mg a day by mouth versus placebo lowered nausea scores on a visual analog scale (p=0.0008); the drop in vomiting episodes did not reach statistical significance (p=0.0552)
Safety:
At 25-75 mg of B6 a day in pregnancy, no risk of birth defects has been foundAntiemetics such as ondansetron and metoclopramide have a few studies raising concerns about fetal heart development, with inconsistent results, so they generally come after B6
Severe hyperemesis gravidarum (HG):
B6 plus doxylamine is still the foundationIf that is not enough, treatment steps up to metoclopramide or ondansetron, plus intravenous fluidsCritical reminder: people with HG who receive IV fluids usually need thiamine (B1) 100 mg/day added. Giving a large glucose load without B1 can trigger Wernicke encephalopathy, and obstetric deaths have been reportedIf during pregnancy you vomit so much that you cannot keep even water down, pass very little urine, feel dizzy when standing, or are clearly losing weight, see a doctor promptly
In practice:
For anyone with bad nausea in early pregnancy, B6 25 mg every 8 hours is a low-cost, low-risk first step; talk to your obstetrician before starting itDo not confuse this with B6 for PMS (50-100 mg/day each cycle): the doses differ, and so does the mechanism
vutyavanich-1995-pyridoxine-nvp
Chapter 5
Water-soluble can still overdose
Among the B vitamins, B6 is the one that most needs this said clearly: water-soluble does not mean you cannot take too much.
Long-term high doses of pyridoxine can cause sensory neuropathy:
Symptoms: numbness and tingling in the hands and feet, loss of the sense of vibration, and not knowing where your hands and feet are with your eyes closedDose: in early reports, cases mostly involved long-term use of >500 mg/day; there are also individual cases at >200 mg/dayUpper limits: the adult upper limit () set by the US Institute of Medicine (IOM) is 100 mg/day; the one set by the European Food Safety Authority (EFSA) in 2023 is 12 mg/dayRecovery: things usually improve gradually after the high doses stop, but severe cases can take months to years to recover
Why can a water-soluble vitamin be toxic? The mechanism is not settled. One explanation supported by cell experiments (Vrolijk 2017) is that large amounts of pyridoxine itself occupy the enzymes that need PLP and end up suppressing B6's normal function, so it behaves like a deficiency.
In practice: do not buy high-dose products casually because B-complex vitamins are pretty safe, and above all do not stack a separate B6 bottle on top of a B-complex that already contains a lot of B6.
Long-term high doses of pyridoxine can cause sensory neuropathy:
Symptoms: numbness and tingling in the hands and feet, loss of the sense of vibration, and not knowing where your hands and feet are with your eyes closedDose: in early reports, cases mostly involved long-term use of >500 mg/day; there are also individual cases at >200 mg/dayUpper limits: the adult upper limit () set by the US Institute of Medicine (IOM) is 100 mg/day; the one set by the European Food Safety Authority (EFSA) in 2023 is 12 mg/dayRecovery: things usually improve gradually after the high doses stop, but severe cases can take months to years to recover
Why can a water-soluble vitamin be toxic? The mechanism is not settled. One explanation supported by cell experiments (Vrolijk 2017) is that large amounts of pyridoxine itself occupy the enzymes that need PLP and end up suppressing B6's normal function, so it behaves like a deficiency.
In practice: do not buy high-dose products casually because B-complex vitamins are pretty safe, and above all do not stack a separate B6 bottle on top of a B-complex that already contains a lot of B6.
Clinical · When numbness comes from supplements
Long-term multivitamin use followed by abnormal nerve sensations is a situation that is easy to miss in the clinic.A common picture (for illustration):
A woman aged 40-60 takes a daily multivitamin long-term (with several times the recommended amount of B6), plus a few B6-containing energy, mood, or nerve support blendsAfter 1-2 years: first tingling toes, then numb soles, then loss of position sense (unsteady when standing with eyes closed), and finally an unsteady walkRepeated doctor visits rule out diabetic peripheral neuropathy, B12 deficiency, alcohol, chemotherapy, and autoimmune causes, and everything comes back negativeThe key question: what supplements are you taking? List all the multivitamins and blends and add up the total B6It turns out she takes 200-500 mg of B6 a day (added up across 3-4 different products)
Mechanism: Schaumburg described this syndrome in the New England Journal of Medicine (NEJM) in 1983. The main victims are the neurons of the dorsal root ganglia (where the cell bodies of sensory nerves cluster). The blood barrier there is relatively leaky, so high concentrations of substances in the blood get in more easily. Once these neurons are damaged, the farthest nerve endings fail first, so symptoms start in the hands and feet. Exactly how high-dose B6 damages the nerves is not settled.
Dose and time:
Most cases: long-term > 200 mg/dayEspecially sensitive people: 50-100 mg a day over several years may also be enoughOnset: gradual, over months to years
Diagnosis:
Nerve conduction studies (NCS): sensory nerve action potentials (SNAPs) are reduced or absentPlasma PLP is very high (suggestive, but there is no agreed diagnostic cutoff)History: always ask about all supplements in detail
Outlook:
Caught early (< 6 months): after stopping B6, most people recover clearly within 6-12 monthsCaught late (> 1 year): lasting damage may remain, because dorsal root ganglion neurons that die cannot be replaced
Prevention:
Total daily B6: unless a doctor has prescribed a higher dose for something like pregnancy nausea or PMS, keep supplements combined to no more than 25-50 mg (the US upper limit is 100 mg; the European limit set in 2023 is stricter, at 12 mg)Read every supplement label: multivitamins, B-complexes, energy drinks, and all kinds of blends often contain B6, and it adds up easilyUnexplained numbness or worsening balance: show your doctor the full list of supplements; that is far cheaper than repeating the tests for diabetes or multiple sclerosis (MS)
References · 7
- National Institutes of Health, Office of Dietary Supplements. (2022). Vitamin B6 — Fact Sheet for Health Professionals. Fact sheet (updated June 16, 2023; Wayback snapshot 17 September 2026): plasma PLP is the most common status measure; PLP above 30 nmol/L has been the traditional adequacy indicator in adults, but the FNB used 20 nmol/L as the major indicator when it calculated the adult RDAs; the FNB halved the dose used in the underlying studies to set an adult UL of 100 mg/day (fact sheet). ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional
- Hellmann, H., & Mooney, S. (2010). Vitamin B6: a molecule for human health? Molecules, 15(1), 442-459. 10.3390/molecules15010442
- Hargreaves, M., & Spriet, L. L. (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9), 817–828. 10.1038/s42255-020-0251-4
- Wyatt, K. M., Dimmock, P. W., Jones, P. W., & O'Brien, P. M. (1999). Efficacy of vitamin B-6 in the treatment of premenstrual syndrome: systematic review. BMJ, 318(7195), 1375-1381. 10.1136/bmj.318.7195.1375
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck, D., Bohn, T., et al. (2023). Scientific opinion on the tolerable upper intake level for vitamin B6. EFSA Journal, 21(5), e08006. Sets the adult tolerable upper intake level for vitamin B6 at 12 mg/day, replacing the 2000 value of 25 mg/day. 10.2903/j.efsa.2023.8006
- Schaumburg, H., Kaplan, J., Windebank, A., et al. (1983). Sensory neuropathy from pyridoxine abuse: a new megavitamin syndrome. The New England Journal of Medicine, 309(8), 445-448. 10.1056/NEJM198308253090801
- Vrolijk, M. F., Opperhuizen, A., Jansen, E. H. J. M., Hageman, G. J., Bast, A., & Haenen, G. R. M. M. (2017). The vitamin B6 paradox: supplementation with high concentrations of pyridoxine leads to decreased vitamin B6 function. Toxicology in Vitro, 44, 206-212. 10.1016/j.tiv.2017.07.009