B vitamins: what each single one does and why the complex makes sense

You turn the tub in your hand. On the label there is a list that looks like alphabet soup. B1, B2, B3, B5, B6, B7, B9, B12. Next to it numbers with percentages, some of them at 100 and some at 4,500.

And you think what I think when I see that kind of label: why are there eight? Why are B4, B8, B10 and B11 missing? And why would anyone need 4,500 times the amount of anything?

The short answer about the gaps: what is missing today was once counted along and later sorted out again, because it turned out not to be a real vitamin. The longer answer about the percentages is what this article is actually about.

Because the eight B vitamins are so tightly woven together across two shared construction sites that a high single dose rarely stays with itself. That is what we are talking about here.

An orchestra, not a solo act

Picture an orchestra. The first violin matters. But if you hand it a microphone and turn up the amplifier, you do not get a better concert. You get a different piece.

That is roughly how the B vitamins relate to each other. They are not eight independent switches. They are eight voices in two pieces that are played constantly and at the same time.

The first piece is called energy metabolism. Every cell builds ATP from food, the energy currency of the body. Thiamine, riboflavin, niacin and pantothenic acid are not spectators in this, they are the tools. Without thiamine a central enzyme in sugar breakdown stands still. Without riboflavin there is no FAD and no FMN, two electron carriers of the respiratory chain. Without niacin there is no NAD, the molecule that shuttles electrons back and forth in hundreds of reactions. Without pantothenic acid there is no coenzyme A, and with that the entry of fatty acids into the citric acid cycle comes to a halt.

The second piece is called methylation. The body continuously attaches tiny methyl groups to molecules: to stretches of DNA, to messengers, to hormones, to fats. These tags are like sticky notes that tell a molecule what to do now. Folate delivers the material, B12 hands it over, B6 processes the leftovers, B2 keeps the handover enzyme in shape.

And now you know why every single B gets its own paragraph in this article, and why the interplay still counts most at the end.

The eight one by one: job, deficiency picture, food source

Now the part you probably came for. I go through all eight, each with the same structure: what it does, what a deficiency looks like, where it sits in food.

One note in advance that I find important: the classic deficiency pictures are end states. They describe what happens when almost nothing arrives over months to years. The early signs are far less conspicuous and therefore not good proof of anything.

B1, thiamine: the spark in sugar metabolism

Thiamine works as thiamine pyrophosphate in enzymes that turn carbohydrates into usable energy. It sits, among other places, on pyruvate dehydrogenase, the gate between sugar breakdown and the citric acid cycle. Nerve cells live almost exclusively on glucose. That is why a thiamine deficiency hits the brain first.

The severe deficiency picture is called Wernicke encephalopathy and is a neurological emergency. A review describes it as serious, often missed and at the same time treatable, with the clear recommendation to give thiamine intravenously immediately when it is suspected, because oral dosing is not enough to avert lasting damage. Risk groups are above all people with chronic alcohol use, after stomach surgery, with persistent vomiting or long malnutrition.

In practical terms: if someone suddenly seems confused, sees double or walks unsteadily, that is an emergency. Then you call the emergency number 112 and do not wait for an appointment. Thiamine for injection is a prescription only medicine and is given in a clinic or practice, never in self management.

In food, thiamine sits in whole grains, legumes, sunflower seeds, pork and yeast. White flour and polished rice lose a large part of it.

B2, riboflavin: the electron carrier

Riboflavin is converted in the body into FMN and FAD. Both are electron carriers that run along in the respiratory chain and in many other redox reactions. Riboflavin also keeps glutathione reductase going, meaning the enzyme that reprocesses your most important endogenous antioxidant after use. And it is the cofactor of MTHFR, the enzyme that carries out the decisive conversion step in folate metabolism.

A deficiency shows up classically on skin and mucous membranes: cracked corners of the mouth, an inflamed tongue surface, scaly redness around nose and eyes, occasionally light sensitivity. Sources are dairy, eggs, almonds, mushrooms and green leafy vegetables. Riboflavin is light sensitive, which is why dark milk bottles were historically no accident.

B3, niacin: the backbone of NAD

Niacin is the precursor of NAD and NADP. These two molecules are the shuttle buses for electrons in hundreds of reactions: in energy metabolism, in fat metabolism, in DNA repair, in the detoxification work of the liver. The body can also build small amounts of niacin itself from the amino acid tryptophan, roughly at a ratio of sixty to one.

The classic deficiency picture is called pellagra and is described with three Ds: dermatitis on light exposed skin, diarrhoea and dementia. In Europe it is rare today and appears mainly with severe malnutrition, chronic alcohol use or certain absorption disorders. Sources are meat, small fish such as sardine and herring, peanuts, mushrooms and whole grains.

B5, pantothenic acid: the quiet builder

The name comes from the Greek word for everywhere, and that describes the matter well. Pantothenic acid is a building block of coenzyme A and of the acyl carrier protein. Without coenzyme A no fatty acid enters the citric acid cycle, no cholesterol is formed, no steroid hormone, no acetylcholine.

An isolated deficiency is rare, because B5 occurs in almost all foods. It has been described mainly under experimental conditions and in famine situations, among other things as a burning sensation in the feet. Good sources are organ meats, eggs, mushrooms, avocado, legumes and whole grains.

B6, pyridoxine: the amino acid manager

Vitamin B6 is an umbrella term for pyridoxine, pyridoxal and pyridoxamine. In the body these become pyridoxal 5 phosphate, PLP for short, and this PLP is the cofactor of well over a hundred enzymes. It rebuilds amino acids, it stands at the beginning of heme synthesis, and it is central in the production of messengers: GABA, serotonin, dopamine, noradrenaline. It is also the cofactor of the pathway along which homocysteine is finally broken down.

A deficiency can show up with irritability, low mood, anaemia and abnormal sensations. And now comes the unusual part: an excess can produce very similar nerve signs. I give that point its own section further down, because it supports the thesis of this article more firmly than anything else.

B7, biotin: the carboxylase cofactor

Biotin is the cofactor of five carboxylases, meaning of enzymes that attach carbon dioxide to a molecule. These enzymes work in the new formation of glucose, in fatty acid synthesis and in the breakdown of certain amino acids.

A real biotin deficiency is rare and shows up with hair loss, scaly dermatitis around body openings, conjunctivitis and neurological signs. It can be triggered by inherited enzyme defects, by certain antiepileptic drugs, or by eating very large amounts of raw egg white over a long period, which contains a biotin binding protein. Sources are egg yolk, nuts, organ meats, oats and legumes, plus a contribution from gut bacteria.

B9, folate: the carbon supplier

Folate is the umbrella term for the naturally occurring forms, folic acid is the synthetic variant from supplements and fortified foods. Folate transfers single carbon building blocks and is therefore indispensable for the synthesis of DNA building blocks and for methylation.

A deficiency can lead to megaloblastic anaemia: the red blood cells can become too large because the cell nuclei cannot divide fast enough. In early pregnancy a low folate status is linked with neural tube defects. For folic acid before and during early pregnancy the evidence is therefore unusually good, and it is firmly anchored in the official recommendations. Sources are green leafy vegetables, legumes, beetroot, asparagus and liver. Folate is heat sensitive and water soluble, so long cooking costs a lot.

B12, cobalamin: the specialist with two jobs

In humans, B12 works in only two enzymes. One is methionine synthase, which turns homocysteine back into methionine and needs exactly the methyl group that folate delivers. The other is methylmalonyl CoA mutase, which sits in the breakdown of certain fatty acids and amino acids. The marker methylmalonic acid comes from this second job.

A deficiency can also produce a megaloblastic anaemia, and in addition something folate does not do: damage to the myelin sheaths in the spinal cord, known as subacute combined degeneration. It often begins with tingling, unsteady walking and disturbed vibration sense. Sources are exclusively animal foods and fortified products. Absorption requires stomach acid and the intrinsic factor from the gastric lining.

And now you know why the alphabet soup on the label bundles together eight very different professions.

The homeostasis point: when folic acid covers up a B12 deficiency

If you remember only one paragraph from this whole article, make it this one.

Folate and B12 share a junction in metabolism. The folate brings the methyl group, the B12 receives it and passes it on to homocysteine. If B12 is missing, the folate backs up in a form it cannot pass on. The technical term for this is the methyl trap.

Now comes the part that matters in practice. For blood formation, this blockade can be partly bypassed with a lot of folic acid. The oversized red blood cells can regress under folic acid. The blood count then looks calmer.

Except: the second B12 enzyme, methylmalonyl CoA mutase, cannot be replaced by folate. And it is exactly this enzyme that is connected with the myelin sheath in the spinal cord. Which means blood formation can recover while the nerve damage keeps running.

So that you have the full picture, the opposing position belongs here too. A German and Belgian group reviewed the literature from 1945 to 2017 and comes to a clearly more reserved conclusion. Their argument: the masking concept falls short, because the blood picture and the nerve picture of a B12 deficiency do not develop in parallel anyway. They trace the nerve damage in the old folic acid trials back to the fact that folic acid alone was given at the time, without replacing B12 at all. They consider the European upper limit of 1 mg folic acid per day to be safe. For context: one of the four authors works in the safety research department of a pharmaceutical company.

There is a third voice, and it goes in the other direction. A British neurologist proposed in 2016 that exactly this 1 mg upper limit be reviewed, because he sees signs of neurological harm already at doses between 0.5 and 1 mg daily in the presence of a B12 deficiency. He also proposes adding B12 to fortified foods. It was this very paper that prompted the opposing position above.

Three groups, the same literature, three weightings. I show you all three, because a body of data that points in only one direction has usually been filtered. For you, the same practical rule remains in all three readings.

The European Food Safety Authority reassessed exactly this question in 2023. Their result: the risk of a worsening of neurological symptoms with an existing B12 deficiency remains the critical effect on which the upper level is based. They kept the previous tolerable upper intake level of 1,000 micrograms of folic acid per day for adults. And they explicitly established that this amount also applies to the approved 5-MTHF salts.

For diagnostics this means in practice: the total B12 value in serum reacts late and fluctuates a lot. An analysis of 11,833 blood samples compared four markers against each other. Holotranscobalamin performed best overall, closely followed by methylmalonic acid and total B12. In women from 50 years of age, holotranscobalamin was clearly superior to the other markers. In men and younger women the authors found no relevant advantage over the simple B12 value.

And now you know why on the topic of folic acid and B12 I would rather measure once too often than once too little.

Vitamin B6: why a water soluble vitamin can indeed do harm

There is a sentence almost everyone has heard: you cannot overdose water soluble vitamins, whatever is too much goes out with the urine.

For a few B vitamins the sentence roughly applies in everyday life. As a rule it does not hold. High folic acid doses can cover up a warning signal, gram doses of niacin can burden the liver, blood sugar and uric acid, high dose biotin can distort lab reports, and for vitamin B6 there is a clearly defined upper level. Water soluble therefore does not mean without consequences.

For vitamin B6 the sentence holds least of all.

Too much B6 over a longer period can trigger damage to sensory nerve fibres. The picture typically starts at the feet and works its way up: tingling, numbness, burning, unsteadiness when walking in the dark. Motor function and strength usually stay intact. The tricky part: exactly these complaints make many people reach for a vitamin product first.

How can a vitamin that is needed for nerve metabolism damage nerves? A mechanistic review from Maastricht offers an explanation I find elegant, even though it is not proven.

The assumption: very high pyridoxine concentrations could inhibit the enzyme pyridoxal kinase, meaning precisely the enzyme that converts B6 into its active form. Fittingly, people with inherited defects of this enzyme develop a very similar axonal neuropathy. Because the blood brain barrier lets pyridoxine through poorly, the inhibition would mainly hit peripheral tissue. That would explain why the complaints start at the feet and the brain is spared.

Twelve milligrams. Please take one look at your products now. B6 does not only sit in the B complex. It sits in magnesium formulas, in nerve capsules, in energy powders, in sports blends, in cycle products. Four products with 10 mg each add up to 40 mg, and nobody has ever added that up.

And now you know why I feel uneasy when someone says vitamins surely cannot do any harm.

Biotin: the vitamin that can shift your lab report

This section is the most practically relevant safety note in the whole text, and almost nobody knows it.

Biotin binds to a bacterial protein called streptavidin as strongly as almost no other bond in biochemistry. This bond is so reliable that laboratory medicine uses it as a tool: in a great many automated tests, antibodies are labelled with biotin and captured via streptavidin.

As long as there is only a little free biotin swimming in the blood, that works. If someone takes high dose biotin, this free biotin competes with the test design. And then values tip over.

How high does the dose have to be for that to happen? It depends on the individual test and the manufacturer, so there is no simple threshold. What can be said: the usual small biotin amounts in a standard B complex are a different matter from the skin, hair and nail products with five or ten milligrams, and those in turn are a different matter from the high dose products with hundreds of milligrams.

How seriously this is taken is shown by a large neurological trial. In the SPI2 trial, 642 people with progressive multiple sclerosis received either 100 mg biotin three times daily or placebo. The clinical endpoint was not met, 12 percent in the biotin group versus 9 percent under placebo. What is remarkable is the side clause in the authors' conclusion: despite countermeasures taken specifically for this, the product led to inaccurate laboratory results in tests with biotinylated antibodies. They name that explicitly as a health risk in its own right.

By the way, because the question comes up almost every time: how well supported is biotin for hair? A dermatological review searched the literature systematically and found 18 documented cases in which biotin was used for hair or nail changes. In all of these cases there was an underlying disorder, for example an inherited enzyme defect or a brittle nail syndrome. For healthy people without a deficiency, the authors see no sufficient evidence.

And now you know why in the consultation I ask about the hair product before I interpret a thyroid value.

Activated forms: what methylcobalamin, 5-MTHF and P5P can honestly do

You see these words on the more expensive tubs: methylcobalamin instead of cyanocobalamin. 5-MTHF instead of folic acid. Pyridoxal 5 phosphate instead of pyridoxine. Riboflavin 5 phosphate instead of riboflavin.

The idea behind it is compelling, and I understand its appeal well. The body has to convert B vitamins into their active forms before it can use them. If you take that work off its hands, you bypass possible bottlenecks in the conversion steps.

How much of that can be supported cleanly? Less than the price premium suggests. But also not nothing.

Exactly this last point is a nice example of network thinking. In a small randomised trial with 91 people already on medication, all of whom carried the MTHFR 677TT genotype, systolic blood pressure after 16 weeks with 1.6 mg riboflavin was on average 5.6 mmHg lower than under placebo. For the diastolic value there was no statistically meaningful difference. A single trial of this size can show a signal, it cannot deliver proof, and the results apply only to this genotype and only on top of the ongoing medication.

An Irish observational study with 6,076 adults fits this picture: a poor riboflavin status could co-influence the genetic risk of high blood pressure in this genotype. Observational data show associations, not causes. No reason to change blood pressure medication.

And now you know why, before buying a more expensive product, I would rather know what the starting value looks like.

Covering needs or therapy: why the dose changes everything

Here lies a distinction that blurs completely on the shelf, and that changes everything clinically.

Covering needs is what you buy in the drugstore or online. Doses roughly in the range of the official intake recommendations, sometimes a multiple of them. Their purpose is to fill a gap. No therapeutic effect is to be expected from them, and they should not promise one either.

Orthomolecular or therapeutic high dosing is something else entirely. Considerably higher doses, used in a targeted way, limited in time, under medical supervision, with a baseline lab and follow up appointments. Only in this range can a nutrient behave like a medicine. With everything that belongs to that: effect, side effect, interaction, contraindication.

The B group supplies perhaps the clearest examples of this in the whole world of micronutrients.

I deliberately show you two examples in which the high dose did not work out. Not because orthomolecular approaches would fail in principle, but because that is the honest lesson from them: a plausible mechanism story plus a high dose does not yet add up to a clinical benefit. Sometimes it only adds up to more side effects.

And that is why at the beginning of every sensible consideration there is not the tub, but the plate.

Supply from natural, real food of good origin and good husbandry is always the first choice. Not out of romance. But because food delivers nutrients in a matrix of cofactors that a capsule does not rebuild. Liver, eggs, fish, legumes, nuts, leafy greens and whole grains together can cover the B group to a large extent. No supplement replaces nutrition, sleep, movement, sun and relationship.

And now you know why I find neither an anti supplement camp nor a pro supplement camp useful. The question is never whether, but what, how much, how long and why.

Who really carries a risk, and what sits behind it in the body

If you are wondering whether you belong to them, these are the four groups I meet most often in practice.

People living plant based without a reliable B12 source. A systematic review across 141 studies compared nutrient intake and status between plant based diets and mixed diets. Across the studies, B12 intake and status on plant based diets were generally lower, lowest in people living vegan. The paper was written by employees of a food company, which is worth keeping in mind while reading. A meta-analysis of 17 studies with 3,230 participants found, fittingly, higher average homocysteine values and lower serum B12 in vegetarian and vegan people than in omnivores. That is no argument against plant based eating, quite the opposite. It is an argument for closing the one point where it has a gap in a targeted way.

People on medication that can shift the nutrient balance. A case control analysis with 25,956 people newly diagnosed with a B12 deficiency and 184,199 comparison persons found, for proton pump inhibitors prescribed for two years or longer, an increased likelihood of a B12 deficiency diagnosis, with an odds ratio of 1.65. At higher daily doses it was higher. For metformin a connection to B12 has been described for years, and an analysis from two large trials and a care database additionally found an increased risk of anaemia, whose early time course speaks against B12 as the sole explanation. For hormonal contraception a review describes shifts in B2, B6, B12 and folate.

People with a relevant alcohol burden. Here thiamine comes first, followed by folate. Alcohol impairs uptake, storage and activation at the same time.

Older people. With age, stomach acid production and appetite go down, and both hit B12 hardest of all, because its absorption needs both.

Why this goes beyond tiredness shows up best when you look through four lenses at once. That is exactly what clinical psychoneuroimmunology does.

These four lenses explain why B vitamins show up in such different guides: with exhaustion, with mood, with migraine, with nerve complaints, with hair, with pregnancy. Not because they could achieve something everywhere at once. But because coenzymes that run along in hundreds of reactions can make themselves noticed in many places when they get scarce.

What has to be said honestly here: the hope of protecting the ageing brain with B vitamins via the homocysteine pathway has not been clearly confirmed in large trials. A meta-analysis of 11 trials with data on 22,000 people lowered homocysteine by a good quarter and still found no effect on cognitive function. A later, more broadly designed analysis of 95 papers with 46,175 participants found small positive signals, above all with long interventions and in people without dementia. So the picture is not empty, but it is also not what some advertising suggests.

And now you know why I would rather measure thoroughly once than supplement into the blue for a year.

Three levers you can put into practice today

No protocol, no brands, no dosing recommendation. Just three things that lie entirely in your hands.

And now you know why the most honest answer to the question about the best B complex starts with a question back: what are you actually looking for?

Frequently asked questions about B vitamins and the B complex

How this topic connects with the others

The B group rarely stands on its own. If you want to go deeper at one point, these paths lead further.