Vitamin B12: what it is, what it tells you and why we test it

Vitamin B12 is needed by your nerves and to make red blood cells. Your body cannot make it: it comes from food, almost only from animal foods. If you run short, symptoms come late, which is why it is worth measuring in your blood.

Why we test it

B12 is needed by your nerves and to make healthy red blood cells. Your body cannot make it: it has to come from food.

Your liver keeps a store that lasts years. That is why a deficiency builds slowly, and symptoms come late.

If a deficiency lasts a long time, your nerves can be damaged, sometimes for good. That is why it is worth measuring even without symptoms.

What it tells you, and what it does not

If it is low, your body may be short of B12, even if you feel well.

If it is in the lower part of the normal range, that does not rule out a deficiency. The level in your blood is an imperfect guide: on its own it is not enough.

If it is high, a supplement is usually behind it. If you take none, the reason needs finding.

That is why it is read together with other tests:

Normal vitamin B12 levels

The lab decides what is normal. It is usually 197 to 771 pg/mL. Your own report may print different limits, because they depend on the method used.

A number in the lower part of the range does not rule out a deficiency. If you have symptoms, your doctor can ask for other tests, such as methylmalonic acid or homocysteine, to show whether your body is really short.

If your report gives it in ng/L, the numbers are the same. If it gives it in pmol/L, they are lower: compare your number with the limits printed next to it.

In the Rosa report you also see your optimal range and what to do to reach it or stay in it.

What your result means

Find the row for your result, in pg/mL. Below 200 should always be seen by a doctor, even if your report still marks it as normal.

If your vitamin B12 is high or low

Low vitamin B12: the causes

Usually too little comes in with food, or your body cannot absorb it.

Symptoms of vitamin B12 deficiency

They come slowly, often after months or years. They can affect the blood, the nerves or both:

In the blood, red cells often grow bigger than normal and MCV rises. These symptoms have other causes too, but when the nerves are involved it is not worth waiting: talk to your doctor soon.

Where vitamin B12 is found

In food, almost only in animal foods:

Vegans usually need a supplement: talk to your doctor. If your B12 is already low, your doctor decides the dose and for how long: sometimes tablets are enough, sometimes injections are needed.

What can skew the result

A few things change the number, or how to read it. It helps to know them first.

When to worry

Talk to your doctor if:

With Rosa, a vitamin B12 below 200 pg/mL is always flagged to a doctor.

Frequently asked questions

What does vitamin B12 do? It keeps your nerves working, helps make red blood cells and lets cells divide. Your body cannot make it: it has to come from food.

Can low B12 be a sign of cancer? Usually not: the most common causes are diet, some medicines and age. But if a deficiency has no clear reason, your doctor looks for the cause, including in the stomach.

Do vegans and vegetarians need to take B12? Vegans usually do, because plant foods contain almost none. Vegetarians get some from eggs and dairy, but often not enough. Checking it helps, and your doctor decides the dose and form.

Does metformin lower vitamin B12? Taken for years it can, and the risk grows with the years of treatment. If you take it, it makes sense to check your B12. Do not stop metformin on your own: talk to your doctor.

Can vitamin B12 and folic acid be taken together? Yes, they are often taken together when needed. But folic acid on its own can correct anaemia and hide a B12 deficiency while the nerves keep suffering. That is why it is worth checking B12 before taking it.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. Vegans, and people who have taken metformin or stomach-acid medicines for years, may need more frequent checks, if their doctor says so.

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