The American Dietetic Association has said that "appropriately planned vegetarian diets are healthful, nutritionally adequate, and provide health benefits in the prevention and treatment of certain diseases."[9] However, poorly planned vegan diets can be deficient in nutrients such as vitamin B12,[61] vitamin D,[62] calcium,[63][62] iodine[64] and omega-3 fatty acids.[65] These deficiencies have potentially serious consequences, including anemia,[66] rickets[67] and cretinism[68] in children, and osteomalacia[67] and hypothyroidism[68] in adults.

http://en.wikipedia.org/wiki/Veganism#Precautions

Faut faire gaffe quoi.