One of the best choices you can make for your health is to eat a plant-based diet. At the same time, it also carries a serious and possibly fatal risk. The plant-based diet contains many of the vitamins and minerals you need for your health, but it is typically devoid of vitamin B12.
Vitamin B12, otherwise known as cobalamin, is important for producing red blood cells and DNA as well as working along with your nervous system1. It is found in animal products, such as fish, meat, poultry, dairy, and eggs. There are also other products that may be fortified with B12, including some plant-based milk and bread.
Two varieties of edible algae (Dried green (Enteromorpha sp.) and purple (Porphyra sp.) seaweed (nori)) have been found to have active B12, but other algae have inactive B12-analog compounds that have no apparent benefit in animal metabolism.
Some varieties of mushrooms and some foods made with certain fermentation processes have very small amounts of active B12.
Unfortunately, humans are unable to absorb the vitamin B12 produced in the colon because vitamin B12 is only absorbed in the small intestine, which is upstream of the colon.
Primates, indigenous societies, and prehistoric populations never got cobalamin deficiency because they didn’t have the advanced hygiene techniques that we have today, even if they were eating a plant-based diet. The food that they ate had enough bacteria on the surface to provide the vitamin B12 they needed. Humans typically clean and cook their foods, thereby reducing the vitamin B12 content in the plant foods they eat.
Many vegans and vegetarians are at risk of a vitamin B12 deficiency for that very reason. The longer an individual remains a vegetarian or vegan, the higher the risk. In one study, vitamin B12 deficiency affected 86 percent of all vegans2. Vitamin B12 deficiency is serious and can cause anemia, nerve damage, neurocognitive changes, and, over time, paralysis—all problems that you don’t need.
Perhaps the most reliable and easiest way to make sure enough vitamin B12 is being included in your diet is to use a B-Complex supplement.
https://www.ncbi.nlm.nih.gov/pubmed/22116706, Cobalamin deficiency, 2012;56:301-22. doi: 10.1007/978-94-007-2199-9_16
https://www.nature.com/articles/ejcn201446?foxtrotcallback=true, The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature, 06 July 2016
https://www.ncbi.nlm.nih.gov/pubmed/23782218/, Biologically active vitamin B12 compounds in foods for preventing deficiency among vegetarians and elderly subjects, 2013 Jul 17;61(28):6769-75. doi: 10.1021/jf401545z. Epub 2013 Jul 2