B12 (also known as cobalamin) helps to maintain nerve cells and red blood cells. It is also needed to make DNA. It's common in fish, dairy products, eggs, meat, poultry. Hence the suggestion that vegetarians should consider taking a B12 supplement. Peopple with gastrointestinal disorders may also find it hard to break down opr absorb vitamin B12 in foods. Deficiency can lead to anaemia, fatigue, nausea, constipation, flatulence and loss of appetite.