All the nutrients a human being needs for optimum health can be obtained from plant foods, with the exception of vitamin B12. However, the fact that vitamin B12 must be supplemented in a vegan diet doesn't mean the diet itself is inadequate.
All vitamin B12 comes from bacteria and is found in the soil as well as in the intestines of animals. At one time vitamin B12 was readily available in our soil and water supplies, and the minute amount that humans require was easy to obtain. Today our land and waterways are polluted, making it is essential that our produce is thoroughly cleaned before we eat it and our water is purified before we drink it, thereby eliminating any available B12.
The B12 that is produced in the intestines of animals gets incorporated into their tissues, which is why it's found in meat, fish, dairy products, and eggs. Humans also produce vitamin B12 from bacteria found in the large intestine (colon). But vitamin B12 is absorbed in the small intestine much higher up, rendering the B12 from the colon unusable.
It was once believed that sea vegetables and certain cultured or fermented foods, such as tempeh and miso, were good sources of vitamin B12. However, these foods contain what are called B12 analogs. These are B12-like compounds that have no vitamin activity and could actually compete with real vitamin B12 for absorption. Therefore, relying on these foods for B12 might actually increase a vegan's risk for deficiency.
Humans store large amounts of vitamin B12, and new vegans who have eaten B12-rich diets in the past may have enough in storage to last them several years. Most B12 deficiencies are due to absorption problems that are not related to diet. Nevertheless, it's reasonable to expect that B12 levels in many vegans will decrease over time as stores are used up. Also, B12 is absorbed less efficiently as we age, raising the risk for deficiency, particularly when diets are already low in B12.
Although overt vitamin B12 deficiency isn't very common among the vegan population, vegans do tend to have lower B12 levels. Indications of B12 deficiency may be subtle to severe, ranging from neurological problems to memory impairment, and these symptoms may be irreversible. Low B12 levels can also raise the risk for other health problems, and blood tests may not always be able to detect B12 anemia since certain conditions may mask early signs. Among health professionals there is very little disagreement that vegans need to supplement their diets with vitamin B12.
The easiest and most reliable way to add B12 to the diet is with vitamin supplements. Vegans can also include B12-fortified foods in their diets. Many breakfast cereals, meat analogs, and milk alternatives that have been enriched with vitamin B12 are excellent sources (check the labels), as are certain brands of nutritional yeast, such as Red Star Vegetarian Support Formula. The vitamin B12 that is used to fortify these foods is obtained from bacterial cultures, not animal products, and is vegan.