What causes my mcv to be low and rdw to be high? and how to prevent it?

Iron supplements: Low MCV points to iron deficiency. High RDW points to the body making blood cells at a higher rate than normal. An iron supplement might be a good idea, be it from a pill, diet or both. However, you need to check with the doc who ordered these tests to make sure there isn't something else going on.

Get workup: You may have that minor which is trivial, iron deficiency which is very serious, or something else.