There are so many topics I want to cover regarding a vegan diet, among other things, and I hope to answer many of your questions. One of the questions I got was regarding vitamin B12, and I hope to provide a decent summary here. It is somewhat technical but I tried to provide some translation for my non medical/research readers. Of course this is by no means an exhaustively researched paper and there’s much more to know. My references are included for those of you who would like to read more in depth about this topic.
Vitamin B12, also known as cobalamin, is the most complex of essential vitamins. B12 is important for DNA synthesis in the cells; repair of myelin (a component important for the function of nerve cells), energy synthesis in mitochondria, and production of blood cells in bone marrow. B12 is synthesized by microorganisms in the gut of animals, therefore it is not found in plants unless they have been fortified either through soil or in production. Low levels of B12 are associated with neurological decline/dementia, neural tube defects in fetus of pregnant women with deficiencies, cardiovascular disease, osteoporosis and macular degeneration. Some of these symptoms and conditions are irreversible, highlighting the importance of consuming appropriate levels of vitamin B12.
B12 is a water soluble vitamin, meaning that the body will absorb a limited amount of what you consume and the rest will be excreted in the urine. For example, studies show that after consuming a dose of 1 migrogram (mcg), 50% was absorbed (0.5 mcg) compared to 5% of a 25 mcg dose (1.25 mcg). Some research has shown that consumption of 1.5-2 mcg is enough to saturate the receptors, meaning that any additional consumption will result in excretion therefore is of no benefit. The amount an individual absorbs varies depending on factors including age, the source of B12, and it is assumed that 50% of dietary B12 is absorbed in adults with normal gastrointestinal function.
B12 serum levels can be an indicator of deficiency, and although no specific value is determined, a sufficient range of B12 serum levels has been defined as anywhere from 200-500 ng/L. Some believe that it takes decades to develop B12 deficiency with low dietary intake, however deficiency has been shown to develop within 2-4 years of adopting a vegan or vegetarian diet. A study of young adults and the elderly in several countries determined that 32% of vegetarians and 43% of vegans were deficient in serum B12. Even among those who took a B12 supplement, 31% of vegetarians and 88% of vegans were measured to be deficient. A different study found that in a group of vegans only 11% were deficient, and only 19% of the group was using a dietary supplement for B12. These differences could be attributed to how strictly they set the guidelines for what qualified as deficiency, but as you can see it may be difficult to determine.
Animal foods such as beef, chicken, liver, eggs and dairy tend to be very high in vitamin B12. Here is a short list of specific amounts of B12 in common foods (100g = about 4 oz.): eggs (mostly in the yolk): 0.9-1.4mcg/100g, milk: 0.3-0.4mcg/100g, salmon/sardine/trout/tuna: 3-8.9mcg/100g, tempeh (fermented soy): 0.7-8mcg/100g, nori (edible seaweed wrap about 0.3g each): 32-78mcg/100g, fortified breakfast cereals: varied. This a very short list, of course there are other plant sources that have been found to contain vitamin B12, but I didn’t find them to be realistic in the amounts you would have to consume to get a decent amount of the vitamin, which is why I didn’t list them here.
B12 absorption can be negatively affected by diseases and conditions such as Crohn’s disease, anemia, elderly, pregnancy, and certain medications. If you have any of these diseases or take medications it is extremely important to consult with your doctor or a clinical nutritionist to determine that you are getting enough B12 in your diet, particularly if you are interested in adopting a vegetarian or vegan diet.
I am supplementing with an Emergen-C dietary supplement that contains 25mcg of Vitamin B12 as cyanocobalamin, among other things. According to the studies that I read I believe this is sufficient to meet my needs. I also eat tempeh fairly regularly now, so between the two I am not too worried about my serum B12 levels. However, when I go for my annual checkup with my doctor I do plan on asking to have this parameter measured just to be sure.
Thanks for reading, and keep the questions coming!
O’Leary, F. & Samman, S. Vitamin B12 in Health and Disease; Nutrients 2010, 2, 299-316; doi:10.3390/nu2030299
Pawlak, R., Parrott, S., Raj, S., Cullum-Dugan, D., & Lucas, D. How prevalent is Vitamin B12 deficiency among vegetarians? Nutrition Reviews Vol. 71(2):110–117; doi:10.1111/nure.12001
Watanabe, F. Vitamin B12 Sources and Bioavailability. Experimental Biology and Medicine 2007, 232:1266-1274. doi: 10.3181/0703-MR-67