Vitamin B12 is used by the body to make new red blood cells, nerves, and DNA.
It's also used in a number of other everyday bodily functions, but huge numbers of people may not be getting enough in their diet.
The body doesn't naturally engineer vitamin B12 - like it does for vitamin D - which is why it's crucial that everybody eats enough B12-rich foods.
But if you are at risk of a deficiency, it can be difficult to even know.
Vitamin B12 deficiency symptoms: Seven 'sneaky' signs that may be overlooked (Image: GETTY Images)
There are a number of different tell-tale signs of a vitamin B12 deficiency, but they can be very "sneaky", according to Harvard Medical School.