Well-planned vegetarian diets can be healthful and nutritionally adequate, while providing potential health benefits in the prevention and treatment of chronic diseases. In fact, the 2015-20 Dietary Guidelines for Americans includes the vegetarian diet as one of its highlighted healthy eating patterns.
But some nutrients require special attention when following this diet. Vegans, especially, may not be getting enough vitamin B12.
Vitamin B12 is needed for normal red blood cell formation and normal nerve function. The body needs only small amounts and can store it in large amounts. Therefore, a deficiency takes a long time to develop, maybe several years. Once a deficiency does develop, however, it results in irreversible nerve damage. Vegans need to pay special attention to this nutrient, which is not found in common plant foods.
Vegetarians consume dairy products or eggs daily and should get enough vitamin B12 in their diets. Vegans eat only plant foods, including fruits, vegetables, legumes (dried beans, peas, and lentils), grains, seeds, and nuts, and so have little or no vitamin B12 in their diets. They must obtain the vitamin through regular use of a vitamin B12 supplement or through food sources, such as nori, spirulina, chlorella, algae, unfortified nutritional yeast, fermented foods (such as tempeh fermented with beneficial bacteria), commercial breakfast cereals, or fortified soy beverages.
Source: Colorado State University Extension: extension.colostate.edu