Vitamin B12 is involved in many important functions. It helps make red blood cells and keeps the nervous system healthy. A deficiency can lead to serious problems, especially in the young.
Read more here.
Since I once suffereed from vitamin B12 deficiency and nearly died before the doctors found out what the problem was, I feel quite strongly about this. I know that in some countries food is enriched with B12, but not everywhere. If you’re vegan, please make sure you’re getting enough B12 one way or another.