Vegetarians are at risk of not getting enough vitamin B12 in their diet, because very few plants provide any vitamin B12 at all. The Dietary Reference Intake, or DRI, for adult men and women is 2.4 mcg of vitamin B12 per day. Plants can help provide some vitamin B12, but usually not directly. This is because vitamin B12 is a byproduct of bacteria in the hindgut, but the amount of this vitamin produced from bacterial fermentation is still not enough to meet the nutritional requirements of vitamin B12, according the Vegan Society.
Sources of Vitamin B12
Protein foods are the primary sources of vitamin B12, such as animal meats and dairy products. This is why a vitamin B12 deficiency is often seen in strict vegetarians who do not consume these products. These people must rely on other food sources to meet their vitamin B12 requirements. Some plants contain trace amounts of vitamin B12, but plants are not reliable sources of this vitamin. According to "The New Oxford Book of Food Plants" by J.G. Vaughan and C. Geissler, the amount of vitamin B12 found in plant foods depends on the plant-to-soil ratio of microorganisms at the plant's root level. These include bacteria, fungi, molds and yeasts.
School of Thought
Beans from the soya plant are good sources of vitamin B12. The reason soybeans contain this vitamin is because of their protein content. According to the Mayo Clinic, vitamin B12 is bound to the protein in foods. This is a primary reason why plant-based foods do not supply adequate amounts of vitamin B12. Further, even when consuming protein-rich plant foods, such as legumes, your body might not be able to absorb the vitamin. Substances in the stomach called the intrinsic factor are what release the vitamin bound to protein. If the body lacks these substances, then the body will not benefit from any food source that contains the vitamin.
Other Plant Sources of Vitamin B12
Fermented soybeans and soy-based products, such as soybean milk, are the primary source of vitamin B12 coming from plant sources. Soy dishes include tempeh, tamari, tofu, miso and shoyu. Shiitake mushrooms also provide vitamin B12. The amount of vitamin B12 in these foods depends on factors that include bacterium present during fermentation, the region where the plants grow and the richness of microorganisms in the soil, according to reports published in "Plants: Diet and Health," produced by the British Nutrition Foundation.
People in Western cultures might define "plants" as vegetable, fruits or legume-producing foliage grown on the land. However, plants grown and used as food sources in Eastern diets include plants grown in the sea. Sea vegetables, such as arame, kombu, nori and wakame, are some of the sea plants that provide a significant source of minerals and vitamin B12. Unfortunately, sea vegetables are not popular food choices in Western diets, hence the notion in the West that plant foods are not significant sources of vitamin B12, according to Rebecca Wood, author of "The New Whole Foods Encyclopedia."