Vitamin B12 helps to make red blood cells, and keep the nervous system healthy, according to the NHS.
The vitamin is also used by the body to release energy from food.
Meat, salmon, milk and eggs are all packed full of vitamin B12.
Not getting enough vitamin B12 could lead to anaemia.
“Not having enough vitamin B12 is called vitamin B12 deficiency anaemia,” said medical website WebMD.
“This condition makes the body produce larger than normal red blood cells, described as megaloblastic or macrocytic, which don’t do their job as well.
“Vitamin B12 deficiency is risk for people who follow a strict vegan diet who don’t eat the major food sources of B12: meat, eggs and dairy products.
“Babies whose mums are vegetarians may have vitamin B12 deficiency.”
Symptoms of a vitamin B12 deficiency can include yellowing of the skin, or having a sore, red tongue.
Changes or loss of some sense of touch may also be a sign of the condition, along with feeling less pain and vision problems.
Persistent mouth ulcers, extreme tiredness and headaches are also symptoms.
Signs of the deficiency may be mistaken for early dementia symptoms.
Vitamin B12 deficiency is most common in older people, and affects about one in 10 people over 75 years old.
You should see a GP if you’re having symptoms of the condition, the NHS advised.
A blood test may be needed to confirm your diagnosis.
Some symptoms caused by the condition can be irreversible if left untreated.