Roughly, Vitamin B12 helps produce red blood cells. With lower amounts of B12 the RBCs that are being produced are forced to carry more oxygen (your body still needs the same amount of oxygen), so in order to do this the RBCs have to carry more oxygen which leads to the increase in size. This is a problem because they're not meant to be that large.
My husband had a b12 deficiency and went for shots until it got up, now he has to take 1000mg a day.
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