what are the signs of a B12 deficiency?

Low B12 announces itself in odd places. The whites of your eyes. Your tongue. The tips of your fingers. None of them are where you'd think to look, and most of them are easy to explain away as something else.

The NHS lists the signs of B12 or folate deficiency anaemia as: [9]

• shortness of breath or rapid breathing
• feeling weak or tired
• headaches
• palpitations
• problems with your vision
• a sore or red tongue, sometimes with mouth ulcers
• loss of appetite, indigestion or diarrhoea
• problems with memory, understanding and judgement

Low B12 specifically can also cause numbness, pins and needles, muscle weakness, problems with balance and coordination, and psychological changes. [9] Jaundice is not on the NHS list at all, which is part of why it catches people out.

what is vitamin B12?

A vitamin your body needs constantly and can't make.

B12, or cobalamin, does a handful of important jobs in the body. It helps form red blood cells normally, which is where the skin and eye colour comes in. It keeps the nervous system working normally, which is the nerves and the vision. And it helps reduce tiredness and fatigue, which is the symptom most people notice first and dismiss fastest.

It's also the one nutrient a plant-based diet genuinely can't supply on its own, which is a separate problem we'll come back to.

SUMMARY: B12 builds red blood cells and keeps the nervous system working. Your body can't make it, so it has to come from food or a supplement.

why can low B12 turn your skin or eyes yellow?

Because the red blood cells are being built wrong, and then falling apart.

B12 is part of how your body makes the DNA that new red blood cells need. Without enough of it, the instructions come out incomplete and the cells grow too large to leave the bone marrow properly. That's the condition called megaloblastic anaemia. [1][3]

The cells that do make it into circulation are fragile, and a lot of them break down early. When red blood cells break down they release bilirubin, a yellow-brown pigment, and when there's too much for the liver to clear, you see it in the skin and in the whites of the eyes. That's jaundice. [4]

So the pallor and the yellow tinge are two halves of the same problem: too few red cells to give you colour, and too much yellow pigment from the ones that broke.

SUMMARY: low B12 makes oversized, fragile red blood cells. Too few of them makes you pale; the ones that break down release bilirubin, which is what yellows the skin and eyes.

can low B12 affect your vision?

It can, and it's one of the rarer ways it shows up.

The nerve damage that low B12 causes isn't limited to your hands and feet. It can reach the optic nerve, and a clinical review describes optic neuropathy as an occasional finding in adults with B12 deficiency, causing vision loss that is symmetrical, painless and gradual. [1]

Painless and gradual is the important part, because it means there's nothing to alarm you into acting. In one published case a man in his sixties had months of declining central vision before anyone measured his B12; his sight improved after B12 injections. [7]

Although that's a single case, it tells us that the connection is real. But it doesn't mean that all vision problems are B12-related, or that a supplement fixes them all.

SUMMARY: low B12 can damage the optic nerve, causing gradual, painless vision loss. It's an occasional finding rather than a common one.

what else does a B12 deficiency feel like?

There are several signs, some of which are easy to miss.

Tiredness and weakness. Fewer working red blood cells means less oxygen moving around, and the result feels like just ordinary exhaustion, which is why it can often be ignored. [1]

Breathlessness and palpitations. Same cause, different symptom. When the blood carries less oxygen, your heart and lungs work harder to compensate, so you get out of breath on stairs you used to manage easily and feel your heartbeat race when you shouldn't. [1][9]

Memory and concentration. Low B12 shows up cognitively as slow thinking, patchy memory and difficulty holding attention, and the NHS lists problems with memory, understanding and judgement among the symptoms. [1][9] This one is easy to file under stress or age.

Balance. B12 damage to the nervous system can affect your sense of where your limbs are, which shows up as unsteadiness or clumsiness rather than anything painful. [1][9]

Pins and needles. B12 keeps the nervous system working, and when it runs short the classic sign is paresthesia: tingling or numbness, usually starting in the hands and feet. [1] Pins and needles on their own are extremely common and usually mean nothing, so on its own, this is a symptom that may mean nothing.

A sore, smooth tongue. Glossitis is inflammation of the tongue: red, swollen, painful, sometimes losing the rough texture as the small bumps flatten out. A report of four patients found that glossitis with long straight lesions appeared as an early sign, in people who had neither anaemia nor neurological symptoms yet. [5] A separate case describes a woman whose burning tongue was put down to burning mouth syndrome for months before a blood test found low B12. [6]

SUMMARY: fatigue, tingling in the hands and feet, and a sore or smooth tongue. Each is common enough on its own to mean nothing much, which is the problem.

how do you know if it's actually B12?

You get a blood test. There's no way to work it out based on symptoms alone.

These symptoms belong to a lot of conditions. Jaundice especially: a clinical review of how doctors approach yellowing notes that most patients who turn up jaundiced have an underlying liver or biliary problem. [4] Yellow eyes should get checked properly, so don't immediately assume a B12 deficiency.

In the UK, diet usually isn't the cause either. The most common cause is pernicious anaemia, an autoimmune condition where your immune system attacks the stomach cells that make intrinsic factor, the protein B12 has to bind to before it can be absorbed further down the gut. [3][10] Stomach surgery and some stomach conditions do the same thing. [10]

This is important, because someone who can't absorb B12 won't be helped by swallowing more of it. A real deficiency needs a diagnosis, and often injections directly into the bloodstream.

Then there's the timing. The NHS advises seeing a GP if you think you might be deficient, and says it's important to diagnose and treat it promptly, because while many symptoms improve with treatment, some of the damage a B12 deficiency causes can be permanent. [3] That is the one genuinely urgent thing to keep in mind.

And the symptoms don't always arrive in a set order. Of patients whose neurological problems were traced to B12 deficiency, 28% had no anaemia at all. [2] The NHS makes the same point: you can have the deficiency without the anaemia. [3]

One note on testing: a standard serum B12 test may not be reliable for people using spirulina, chlorella or certain other plant foods, because they contain compounds that look like B12 to the test, but don't do the same job. Blood counts can mislead too, since a high folate intake, which plant-based diets tend to have, can mask the anaemia. [8] 

For a more accurate result, ask for one of these instead:

• Active B12 - measures the usable fraction. NICE now accepts it as a first-line test.
• Homocysteine - under 10 is the target. Useful, though other things can push it up.
• MMA - the most specific of the three.

One thing worth knowing about MMA: a normal result is reassuring, but not conclusive. The usual threshold is set to catch clear cases, so a borderline reading can still sit under it.

SUMMARY: a blood test is the only way to know. Most jaundice is a liver matter, and most UK B12 deficiency is an absorption problem rather than a dietary one, so a real deficiency needs a GP assessment first.

how long does it take to become deficient?

Years, usually, which is exactly what makes it so easy to miss.

Your body holds a reserve. The NHS puts it at around two to five years' worth without being replenished, so a change in diet can take a very long time to show up as anything noticeable. [10] For comparison, your folate store lasts about four months. [10]

The Vegan Society's estimate is similar: in the absence of a dietary supply, symptoms usually take five years or more in adults, though some people run into problems within a year. [8]

That cuts two ways. It's a generous grace period, and it's also the reason people go years without knowing. If you stopped eating animal products at some point and never thought about B12 again, the absence of symptoms doesn't mean your levels are in the safe zone.

SUMMARY: the body holds around two to five years of B12, so a deficiency can run for years before symptoms show. Feeling fine is not evidence that you're topped up.

why isn't B12 in plant foods?

Because nothing that grows makes it. Only bacteria do.

The Vegan Society puts it in one line: B12, whether it reaches you in a supplement, a fortified food or a steak, comes from micro-organisms. [8] Plants have no use for B12, no machinery to build it and no way to concentrate it, so there's nothing for them to pass on.

Animals don't make it either. Cattle and sheep absorb B12 produced by bacteria in their own digestive systems, which is how it ends up in the animal rather than being made by it. [8] Meat and dairy aren't the source of B12, but the storage.

SUMMARY: only bacteria make B12. Plants can't make or store it, and animals are containers rather than producers.

how do vegans get enough B12?

Fortified foods or a supplement. Those are the only two reliable routes.

The Vegan Society is unambiguous about this: the only dependable vegan sources are foods fortified with B12 and B12 supplements. [8] Not algae, not fermented foods, not mushrooms, not unwashed vegetables. The plant foods that seem to contain B12 mostly contain analogues that don't work.

Their guidance gives three ways to get there: fortified foods two or three times a day totalling at least 3 micrograms, or a daily supplement of at least 10 micrograms, or a weekly one of at least 2,000 micrograms. [8] The reason the weekly number is so much larger than seven daily ones is that absorption is easily saturated, so the less often you take it, the more of each dose goes to waste.

For anyone not vegan and wondering where it turns up: meat, fish and shellfish, dairy, eggs, and fortified breakfast cereals. A diet with meat, fish and dairy in it usually supplies enough. [10]

But B12 deficiency isn't a vegan problem. Plenty of dedicated meat-eaters are short of B12 too, because the usual problem isn't what you eat, it's whether you can absorb it. Somewhere between 10 and 30% of older adults can't absorb the B12 in food properly, which is why the standard advice for the over-50s is to take it from fortified foods or a supplement, whatever their diet. Common medications like metformin knock it down too.

If you'd rather not think about it either, ours is vegan vitamin B12 as methylcobalamin.

SUMMARY: fortified foods or a supplement, and nothing else is reliable. At least 3 micrograms a day from fortified foods, 10 micrograms daily from a supplement, or 2,000 micrograms weekly.

methylcobalamin or cyanocobalamin?

Two forms of the same vitamin, with a slightly different molecular structure.

Methylcobalamin is one of the two forms your body actually uses. Cyanocobalamin is a synthetic form that your body converts into the active ones after you swallow it. Both raise B12; the argument is about how directly.

Ours is methylcobalamin, which skips the conversion step and is better absorbed and retained. 

SUMMARY: methylcobalamin is an active form; cyanocobalamin needs to be converted by the body. Both work, but methylcobalamin is the superior form.

the bottom line

B12 deficiency is quiet, slow and easy to mistake for something else, and the signs that give it away are ones you'd never connect: the colour of your eyes, the surface of your tongue, a tingle in your fingers.

If any of that sounds like you, the useful next step is a blood test.

If you're vegan and simply want to avoid the problem, that part is genuinely easy. Fortified foods or a supplement, taken regularly, and you're done.