There are many reasons why someone may not get enough B12 but the two most common are diet and pernicious anaemia.
Pernicious anaemia is the leading cause of B12 deficiency in the UK, according to the NHS.
It is an autoimmune disease that prevents the body from making intrinsic factor (a protein made by the stomach and needed to absorb vitamin B12 in the intestine).
The popularity of vegan and vegetarian diets may account for the majority dietary-related causes.
Vitamin B12 is found naturally in a wide variety of animal foods and is added to some fortified foods.