Here is what Registered Dietician Whitney English says about the importance of supplement B12 as part of a vegan nutrition plan:
B12 is only found naturally in animal products.
Despite what you may have read online, foods like seaweed, fermented soy, tofu, sourdough bread and sea vegetables, do not provide the active form of B12 that our body needs to perform its many functions like making DNA and red blood cells.
The only B12 found in a vegan diet is from fortified products such as nutritional yeast or plant milk.
Vegans absolutely must supplement with B12.
Here's what we say:
Studies have found that as many as 92% of vegans could be deficient in B12.
As a vegan, it's impossible to get B12 from whole food sources, with the exception of fortified food like nutritional yeast, where the supplement has been added to the food.
Fortified foods are an unreliable source of B12 though because of the difficulty of ascertaining precisely how much B12 you're consuming.