Vegan and nutrition
While it is true that if you only eat plant food you can be at risk to develop vit B12 deficiency& if it's not treated you can develop anemia
also other neurological problems which could be irreversible- however, this is something you can prevent.
If you decide to become a pure vegan - you should take vit B12 supplement. You wont become malnourished if you become vegetarian, if you eat right.