Unfortunately, some vegans take this to mean that they can eat anything they want, so long as it’s vegan, and never give a thought to nutrition. This is an incredibly dangerous belief to have, as it can lead to nutrient deficiencies that can permanently damage your health. Luckily, it takes just a short amount of time to familiarize yourself with the main nutritional pitfalls on a vegan diet, and it’s easy to include foods and supplements in your diet that will keep deficiencies from arising.

This FAQ is not a place to offer comprehensive nutrition advice. Instead I’ll just tell you the main deficiencies to watch out for on a vegan diet, and then direct you to resources where you can learn more. The nutrients most often lacking on a vegan diet are: vitamins D and B-12, iodine, calcium, and Omega 3 fatty acids.

The VeganHealth.org website offers a background on each of these nutrients, along with recommendations to guard against deficiencies. Additionally, you may wish to read a book like Davis and Mellina’s Becoming Vegan, which will familiarize you with the essentials of constructing a healthy vegan diet.

Source: http://www.vegan.com/articles/faq/#nutrition%20issues