What is the definition or description of: vitamin b12 deficiency?

LACK OF ABSORPTION: B12 deficiency can be due to a lack of absorption or intake from diet. Since B12 is primarily found in animal products (e.g., dairy, egg, meat, etc.) vegetarians are more at risk. A deficiency is suspected when rbcs increase in size (e.g., high MCV and/or MCH in cbc) and can cause megaloblastic anemia. Symptoms can include fatigue and neuropathy. It is best corrected with methyl B12 by injection.