There is a well known effect related to deficiency of vitamin B12 for a pure vegan diet. The only way to get the necessary amount of B12 vitamin would be to use "fortified" food which are in any case linked to non-veganism diet. The risks related to such deficit are very serious. Veganism is anyhow a good practice but I think a minimum percentage of non-vegan foods like meat, fish or honey has to be considered to have a better healthy condition.